Wednesday, December 31, 2008

New Year's Forecast

Today is a typical post-frontal day, with unstable air producing showers and good orographic precipitation in the mountains. There is also a Puget Sound convergence zone enhancing precipitation north of Seattle and rain shadowing immediately east of the Olympics (see radar to see the precip and the surface chart to see the winds wrapping around the Olympics and converging over Puget Sound).
As I noted in my previous blog, it does not look like we will get a big windstorm...in fact, not much of windstorm at all. There will be a complex dual low system, with the first coming in tomorrow morning over or just to the north of us. If it does go north, we might get breezy, but no serious winds. The system is also weaker than the earlier model simulations. This system will bring steady rain to us tomorrow and lots of snow in the mountains. Take a look at the computer snow forecasts...we are talking FEET over the next 48 hrs (these images show 24-h snowfall ending tomorrow and Friday at 4 AM).

Now I don't want to get the TV stations too excited (although I do enjoy Jim Foreman's antics on KING TV)...but some of the models are suggesting the potential for snow after the second system moves through early Friday morning. 2-6 inches from central Puget Sound southward. This snow will depend critically on the trajectory and precipitation with the second system. Anyway, time to go to your local hardware store for some salt....just in case. And if the snow occurs it will be an interesting test to see of the City of Seattle and other municipalities has learned something from the past experiences.

MAJOR ANNOUCEMENT: The City of Seattle has decided to use salt for major storms!

Thanks to all of you that have expressed your opinions about this...I know that several City of Seattle staff members have been following this blog... Now they have to deal with lack of plows, the crazy pack it down rather than remove policy, and to get Metro to deal with snow events in a rational way. And hopefully they will fix their web site to deal with snow events...right now it is a disaster.

Tuesday, December 30, 2008

Windstorm Doesn't Look Likely

I just took a look at the latest model runs...and I am not expecting a major windstorm anymore. As noted earlier, there has been a lot of disagreements between models, even today when we are within 48hr of the event. Something is making it difficult for the solution to gel...and I think I can tell you what it is.

Take a look at the upper level forecast chart (500 mb--about 18K ft, solid lines give the height of that pressure surface above sea level) for 1o AM on Thursday. There are two upper level troughs (I have marked them) that are interacting to form the incipient disturbance. The phasing of these features is critical and this makes the development of a storm much more difficult...small errors in the speed of either trough will change the forecast. Furthermore, if they don't join, then you can end up with a complex, multiple storm that can't amplify effectively.

Take a look at the 48h forecast of sea level pressure...there are in fact two low centers. This is bad. I have been looking at windstorms for years...and my experience is this kind of multiple structure is the kiss of death. And the models know it.
So you can forget a Chanukah-Eve or Inauguration Day type storm event.
Tonight, a low center is moving way north of us over the Queen Charlottes...and rain associated with a trailing frontal band has reached our area (see imagery). If you look closely you can see the circulation over the southern part of the Queen Charlottes. The front assoicated with the low will be through by daybreak and then we will get into the cold,unstable air. Showers in the lowlands, breezy (winds 15-30 mph), cool, and lots of snow in the mountains. One final note...after the "storm" get through on Friday AM we could have some lowland snow showers, particularly in a convergence zone. But we can wait on that....

PS: Several of you have asked about good amateur equipment for observing the weather. I and others in the dept think highly of the Davis Vantage Pro equipment...really good value for the money...and high quality instruments. But anyone can start cheaply...a digital thermometer and rain gauge can be purchased for 30 dollars total. Add a cloud chart and it is nice starter for a budding young meteorologists.

Quiet Day

Well, another forecast problem. The NWS was going for rain today...and I can't figure out why. Anyway, they just shifted the forecast today to a dry one for most of region.

The late Thursday/Friday storm is still uncertain. The best model...the National Weather Service GFS (Global Forecast System) model still has it in the latest runs...but weaker. The NWS NAM model (less skillful in general) doesn't have it at all. Comparing the forecasts of major international modeling systems...there is a lot of variability...but most are going for a weaker event. In contrast, in December 2006 the various modeling systems were really locking in and showing substantial agreement this far out (roughly 66-72 hr in the future). In short, for a confident forecast I look for temporal consistency in the solution and consistency among major modeling systems.

So at this point, I would certainly back off in predicting a major event and wait until tomorrow to see if the model solutions lock on a common prediction. There is large uncertainty and the trend of even the most severe outlier (GFS) is to weaken the event.

Monday, December 29, 2008

A Poorly Predicted Wind Event

My profession did not do an adequate job today, when we had a significant, and poorly forecast, wind event. Both on the coast and here in the lowlands, wind gusts reached 50-60 mph near water and exposed locations, with the remainder of the area experiencing 30-40 mph gusts. The Evergreen Bridge had 40 mph sustained winds for two hours around noon. Lots of branches were downed and several thousand people lost power.
So what went wrong. Remember the low center I mentioned this morning? Well that low center was deeper (lower pressure) than analyzed and the predicted lower was stronger and went further north than our computer models suggested (the low was nearly going over us...that does not produce strong winds). This was a small scale low--which are essentially hard to forecast-- and clearly there was insufficient information to define its structure offshore. Computer models require a good description of the initial state (call the initialization) to simulate the future and this was clearly lacking this morning and before. This is one reason why I am pushing for a coastal radar...to give us a good description of what is happening in the near coastal waters. The radar would not have help yesterday's forecast...but we would have had a real chance to get it right last night or this morning...6-12 hrs ahead. So what grade would I give this forecast? Not a B I am afraid.. perhaps a D.

PS: the windstorm is still in the model forecasts...check this out for Friday morning--a 962 mb low to our north and and an intense pressure gradient over us...

Today's Rain and the Upcoming Potential Storm


Well it is back to normal around here. A modest Pacific disturbance is moving through this morning with moderate rain in the lowlands (see high resolution visible satellite image). There is a low center right off the coast...see the notch in the clouds off of Forks...that is where it is. If you ran an animation of the cloud images you would see the swirl (you can do so at (http://www.atmos.washington.edu/~ovens/loops/wxloop.cgi?ir4km_enhanced+12)

The associated front has made landfall...and you can the enhanced band associated with it just inland from the coast. After the front, the atmosphere destabilizes due to cold air moving in aloft and you can see the instability type clouds...cumulus type.. offshore. So we will transition later this morning to showers and sunbreaks.

Regarding the potential big storm on Friday...it is still there (see latest forecast output attached). Once features like this get within 100 hrs out, I really start to take them seriously. But there is still considerable uncertainty. Run after run now is giving a big storm, and multiple models are showing it as well, but they are still varying widely in timing, position, and to some degree amplitude. Some of the runs are predicting a low center of less than 970 millibars. Around here that is a serious low (although nothing special in the Gulf of Alaska). Someone around here is going to get it. But who? Sort of like a bullet being fired from a shaky rifle...something is going to get hit hard...but we are not sure where.

PS: Some people have asked for the placement of arrows on figures, etc. I just don't have the time to do that...this has to be a quick casual thing for me...or it will be too onerous to continue. I hope you understand. Also, some of you wanted a more interactive listserv or twitter function. If some of you want to do that, it is ok with me.

Sunday, December 28, 2008

The Human Element: Part I

The urban and local flooding prediction was dropped and for good reason. Other than some stopped-up drains there was little threat in the central Puget Sound lowlands since the temperatures were only warming up slowly and heavy rain was not forecast to occur.

This is a good example of over-warning...something that does happen occasionally. Why does it occur? Well sometimes it is due to faulty forecasts...not the case this time. Other times there is a tendency to try to cover a low-probability dangerous situation...better safe than sorry kind of philosophy. And then there are the psychological elements. Perhaps meteorologists are the last people you want forecasting the weather, because many of us love severe weather and subconsciously want it to happen. When you want something to happen....well you can guess the result. And lets face it...when one goes for a severe forecast, the media takes notice...another positive feedback for some. When I give my forecasting class I tell students they have to think like Spock or Data of Star Trek...put the emotions aside..put this is more easily said then done. Anyway, meteorologists are human and you have to factor that in. Certainlywe are no worst than economists...and I think you can argue that our track record is a lot better. I mean A LOT better.
How can you get into the head of a meteorologist to decide whether you believe their forecast? Well, for me you have this blog and my Friday 9:50 AM segment on KUOW. For the NWS, they have very useful forecast discussions...something a few of you mentioned in the comments. The lead NWS forecaster on each shift pens a description of what she or he thinks is happening and will happen and why. A great product. The only issue is that there is often some technical jargon in there (like CWA--Coordinated Warning Area--and some weather terminology you might not be familiar with like 1000-850 mb thickness). But 50-75% should be understandable. The forecasters also identify themselves and if you read them frequently enough you learn about their "quirks." (I won't say any more or will certainly get into trouble) You can find the forecast discussion on the web....for the Seattle office: http://www.wrh.noaa.gov/sew/get.php?wfo=sew&pil=AFD&sid=SEW
I find it useful to read the discussions of neighboring offices (e.g., Portland) as well.

There are other useful discussions to look at. The Northwest Avalanche Center does a mountain weather forecast and discussion...and these individuals are very skillful and knowledgeable about mountain weather: http://www.nwac.us/forecasts.htm

Scott Sistek of KOMO TV--a past UW Atmos graduate and the chief behind the scenes weather staffer at that station--has a wonderful weather blog (http://www.komonews.com/weather/blog). Great information from a very insightful guy. King5 weathercasters also do blogs and some of them have substantial backgrounds (Jeff Renner, Sharon O'Donnell, and Rich Marriott all have professional degrees in the field).

And of course you can watch the TV weather broadcasts--where you often hear some of the logic and uncertainty underlying the forecast.

In any case, there is valuable information regarding the confidence and nature of the forecast that you can glean from the above sources. It pays to be an intelligent consumer of weather information.

PS: A great example of a Puget Sound convergence zone on the latest radar:

Saturday, December 27, 2008

Perhaps I should not mention this..

I try not to hype the weather...and one can't be sure about forecast model output more than 4 days out...but....the predictions of some the computer models for the end of the coming weak are extraordinary and possibly severe.

We start with possibly a minor windstorm on Wednesday and perhaps a little snow. The a very powerful windstorm strikes the area on Friday..as strong as the Chanukah Eve event. And then back to snow.

There is, of course, a lot of uncertainty that far out and perhaps I shouldn't have mentioned it...but now two modeling cycles have shown it. Anyway, we need to watch this carefully...if these storms are still forecast on Tuesday...then we should prepare.

Remember...this is a neutral year...neither El Nino nor La Nina...and the most severe snow and wind events...when they do happen...do so in such years.

Transition Day

The temperatures in nearly all of western Washington are now in the 40s and warmer southwesterly flow has removed the persistent cold air. Take a look at the Seattle profiler winds and temperatures for the last day. 24h ago there was weak SE low-level winds and temps around 0C (32F) in the lower atmosphere...now there is fairly strong SW flow and temps of 6-7C. The melt-out has begun in earnest. Many primary and secondary roads are passable, with a little slush in places.

A cold front is now making landfall on our coast...you can see that by the switch to NW flow on the coast (see map). General precipitation will drop back in the lowlands...although it looks like a Puget Sound convergence zone will form north of Seattle this afternoon (that usually occurs with NW winds on the coast). However, this is an ideal pattern to get big snow in the mountains. In fact, the heaviest snows often fall there after cold front passage when winds are more westerly. Why? Winds from the west have a stronger wind component up the mountains then prefrontal winds from the south. Also the air behind our cold fronts are cold and unstable...with lots of cumulus-type clouds. Such instability clouds really blossom as they are forced to rise by terrain.
The next several days promises one wet system after another and more seasonal temperatures. The media has been pushing the flooding issue, but I don't think that lowland flooding will be a serious threat this time...assuming everyone clears their drains. Behind the cold front, temperatures will fall and the melt off will be much slower than in 1996..the last major melt-off flood event.

Friday, December 26, 2008

Evening Update


Temperatures have only slowly warmed, but now most locations have temperatures in the mid to upper 30s and have switched to rain (see map for current temps..the top left number at each of the weather stations). You will also see the strong southeasterlies over NW Washington and the central WA coast....interestingly enough these winds are forced by the Olympics (my book has a whole section on this). Slow warm up is good....minimizes the chances for local flooding. The air will warm up overnight and by the morning serious melting should be occurring.
We have a serious rainshadow going on now from Seattle northward. Another good thing that will work against urban flooding. The model's suggested this and they were right.

With large amounts of snow in the mountains, I90 is now closed for avalanche work.

Books Now Available Locally Again: Several you have been unable to get my weather book...but now supplies from the new printing are coming in to town. University Book Store in Seattle received hundreds of copies today and are available for either in store purchase or mail order. More coming this week. Other local book stores should be getting more as well before the week is up.

Plant Damage, Snow, Rain, Melt Out, and Avalanche Danger


Walking to the bus stop this morning I saw increasing evidence of damage to plants and trees. Large branches broken off by the weight of snow, bushes bent over to the ground or broken. At the UW, the main road was blocked because of heavily leaning fir tree...snow loading had done the trick. Anyway, I suspect many of you will find quite a bit of broken or bent vegetation around your homes and in our parks.

A major weather system is now approaching rapidly...with a warm front tonight and cold front tomorrow morning) and it should be precipitating over the western interior around lunchtime (see satellite and radar images). It is raining on the coast and Quillayute turned from rain to snow.

So what will happen here as it begins to precipitate? Temperatures are generally above freezing at the surface and the Sand Point profiler indicates the freezing level is at 1000 ft. The snow level (the level where all the snow is melted) is usually about 1000 ft below that....sea level in this case. So we may initially see some wet snow at higher elevations (above 300-500ft), and very, very wet snow and rain near sea level. You all know whether you are in the high or low elevation category.
During the afternoon the cold air will hold in place at low levels, but this evening it will be replaced by warm, southwesterly flow and strong winds. You WILL be able to tell when the warm Pacific air hits tonight. Attached is a time-height cross section showing temperatures (C), winds, and humidity forecast over Seattle. You can see the 0C freezing line move nearly straight up in the evening. The heights are in pressure...850 mb is about 5000 ft, 700 mb about 10,000 ft.
This system will be wet, but not a pineapple express. Seattle, partially rain shadowed, will have less than .5 inches....minimizing the threat of urban flooding from the combination of melting snow and rain (but please clean out your street drains to make sure!). Temperatures tomorrow will get back to normal...into the mid-40s and most streets should become passable by late tomorrow.

The mountains will get several feet of snow...and then we come to another issue....avalanches. The snowpack is not large right now, but it has a number of weak embedded layers. With massive snowfalls on top of it, a large avalanche threat will exist by tomorrow...so be very careful if you are skiing out of controlled areas. Even better is to avoid such locations.

And finally...there will probably be strong southeasterly winds over northwest Washington waters tonight as the front approaches.

Thursday, December 25, 2008

More snow tomorrow?

This event just doesn't end. Tonight temperatures will drop enough that wet roads...and particularly where the water is not deep..can freeze. I just walked home over some previously wet roadway..which is now frozen. Another issue today was flooding of some streets where slush and snow have clogged the drains. I dug out mine and released a huge amount of water that had pooled up. In 1996 my house almost flooded from such a stopped street drain. So if all of us tended to nearby street drains, a potential source of flooding could be avoided.
There are some snow showers in the south Sound now...but nothing serious. Tomorrow, a strong Pacific weather system will move in and cold air will be in place. Before sufficient warming can occur there is a good chance of lowland snow. It will turn to rain, but the switchover could take a few hours. Precipitation from this system should start sometime after 1 PM tomorrow.
This will be a wet system, capable of dumping several feet of snow in the mountains. At this point it does not look like it will produce heavy rain in the lowlands...just our normal light to moderate intensities.
Finally, I would like to thank all of you for sharing your weather observations. Such reports are not only helpful to the other readers, but are valuable as a research aid, instructing me and others about the substantial variations that can occur.

10 AM Update


It is snowing fair hard in Puget Sound...and it appears that the snow is being focused by a weak Puget Sound convergence zone (check radar). The computer models have this feature (see graphic) and predict it will hold in through roughly 1 PM. So it ain't over yet. You can find sun today (see satellite image)...sun is shining on the coast and some places near Olympia (won't last long though in the latter).

Snowing Again


I sit here, watching large snowflakes fall in the darkness outside my window. This is the last snow day in the lowlands for a while I suspect.

The radar shows the story (see image): moderate to heavy precipitation north of Seattle that is bringing wet snow, with some rain in warm locations near water. Outside my house it is collecting in a slushy layer, but some of the WSDOT cams show accumulation on some roads.

This precipitation is associated with an upper level trough over the region...which will move out later today. You can see the clouds associated with this precipitation on the latest infrared satellite photo (see image)--the roughly east-west band over the northern portion of the state. And there is nice low circulation off northern Oregon. These showers will lessen during the day,

The big change is going to occur later tomorrow and Saturday. The atmosphere is about to radically reorganize itself. For nearly two weeks we have had a large scale configuration with a big ridge (high pressure) over the eastern Pacific and northerly, cold flow moving southward into our area (see figure). But that will change...the flow will become more east-west ("zonal" in the business) and that will bring in much warmer and moister air (see figure). That means a return to our familiar rains late Friday and Saturday. By late Saturday I suspect most roads will again be passable.

Hopefully, the last week has taught us something about future preparations for ice and snow and rational changes will be made in how we deal with it. We narrowly avoided a terrible tragedy, with two buses nearly falling on to I5--clearly there would have been serious injuries and deaths if it had. Anyway, this will be my last editorial on this issue!

Wednesday, December 24, 2008

Christmas Eve Update




I think we will see some more snow tonight...particularly at higher elevations. Temperatures warmed a great deal during the day, and lower elevations (less than 500 ft) generally switched to rain. Some of you at higher elevations did have snow most of the day...although it was generally the wet, sloppy variety. Anyway, the freezing level is starting to drop due to cooler air moving in from the NW and normal nighttime cooling. Right now the snow level is between 500 and 1000 ft and will drop during the night and during intense showers--so even the lower elevations will see snow showers by morning.
Looking at the satellite picture, another disturbance is making landfall...associated with an upper level trough and cool, unstable marine air (see image). You will note that the clouds are not continuous like the system earlier...because it is made up of convective showers. The radar shows the showery nature well (see radar). Because the winds in the lower atmosphere are from the SW, there is a clear rainshadow north of Seattle and extending towards Everett...greater amounts of snow will be north and south of shadow, of course.
Anyway, the snow will not be uniform....because of the showery, hit or miss nature of the precipitation...so some of you will get a few inches and others nothing. This should be the final snow chapter of this week. And most of us will be happy to be able to get around again--my family is running out of groceries! And I hope all of you have a good holiday season and a good new year.

10 AM Update


Wet snow is still hanging on in central Puget Sound...but precip has switched to rain in parts of NW Washington (e.g., north Whidbey, Bellingham) and in the south sound near Tacoma. The profiler at Seattle Sand Point shows temps near 32F in the lower atmosphere...classic melting pattern (see figure, height is in meters, temp in C--really something called virtually temp which is .5 to 1 C above the actual temperature). Above 800 m I would not believe it. Times are in GMT--17 GMT is 9 AM.
A place that can remain snow and really get more accumulation is the Kitsap...so if you are in the lower hood canal area...be ready.
The character of the precipitaiton will change in next few hours from steady to more showery as the firs disturbance moves through. We will probably warm enough to make the showers rain below 500 ft...but if there intensity is enough snow can come back.

Snow Removal (if you don't like this stuff don't read it!)

Lots of you have commented about the lamentable snow removal in the city. Anyway, if I was the "snow czar" this is what I would do:

1. Acquire and be ready to use salt on the roads.
2. Triple the number of snowplows and use steel blades instead of the rubber ones. These units can be placed on city trucks. If necessary, contract with the private sector for more equipment, as they do in the eastern U.S. for snow emergencies.
3. Change the snow removal strategy to the immediate removal of snow off primary and secondary roads so we don't end up with thick, chunky ice we have today. Don't let it accumulate and freeze.

4. Work with Metro to establish a rational snow strategy. This would include moving buses to routes where they won't be endangered until the roads are cleared. Thus, you don't take so many buses off the roads or abandon them, but redirect them strategically before they get stuck. Have MORE buses on some routes (such as major routes in and out of downtown).

5. Make sure Metro's online bus tracker web pages are robust enough to handle the load. It is critical that the population knows where the buses in real time. We have the technology to do this--they just haven't sized their computer servers properly. And the bus tracker software should be made more prominent...not hidden as the current approach.

On the edge




It is snowing now in much of the western interior, except for light rain around Tacoma, with rain on the coast. The temperatures aloft are right on the edge. Over Seattle, the temperature is 32 F for the first few thousand feet above the surface. In the business we call such a situation "isothermal" and that temperature is being dictated by the melting of snow falling from aloft.

You can see the incoming system in the satellite picture (image) and the precipitation in the radar picture to the right. The rainshadow(or in this case snowshadow) in the lee of the Olympics is clearly visible over Sequim and northern Whidbey Is. The lack of coastal and SW Olympic radar coverage are clear as well. Really frustrated we can't see what is coming in on the radar! The lack of a NWS coastal radar is similar to the city's lack of salt and snowplows...a reconsideration is needed.
The forecast from last night is still on track at this point...with snow for a few more hours and then change to rain as a stronger system comes in this afternoon. But the snow will be close...only a few thousand feet above us...and if our models and analyses are off....well, you know what could happen! In any case, the mountains will get large amounts of snow..which is good for our water supplies and those looking for some recreation over the holidays.

Tuesday, December 23, 2008

Snow versus Rain



Tomorrow represents a classic problem for local meteorologists in winter...snow versus rain. Many winter events are on the margin around here because warm air from off the ocean is always close. Later tonight and tomorrow AM we will initially see snow.. in fact, take a look at the latest satellite and radar imagery....a disturbance is making landfall and associated snow showers are moving into the western Strait and Forks is reporting light snow. In fact, snow appears to be coming in a few hours early..and that is good. But after that the models suggest that by midday the lowland temperatures will be too warm (into the mid 30s) for snow below 300-500 m and we will see rain as a stronger disturbances (an occluded front) moves in. But before it does so, we could get 1-2 inches of snow. However, my colleagues at the National Weather Service have a different interpretation and their latest discussion suggests they believe it will be all snow tomorrow--with 2-4 inch totals. So it will be interesting to see what is going to happen.

Take a look at the model total snow forecast ending at 4 PM to check out your local area based on the latest model run. And clearly the mountains will be getting plenty of new snow.

Snow 101

Surface temperatures don't tell the entire story about snow...and snow can fall when surface temperatures are above freezing by a few degrees (although wet snow). It takes about 1000 ft of above-freezing temperatures to melt snow completely into rain. Thus, the snow level is about 1000 ft below the freezing level. And whether the temperatures will be cold enough can be difficult to determine. For example, if the air is relatively warm, but dry, then evaporation can cool the air down sufficiently to get snow....at least until the air becomes saturated. Another issue is precipitation rate. Precipitation around here almost always starts as snow if you go high enough. When the snow hits warmer air below, it melts and cools the air as it does so. (It takes energy to melt ice!). The heavier the precipitation the more melting and cooling...and if you have heavy precipitation, the snow level can be driven to the surface. So this makes the problem harder...you not only need to be able to predict the incoming air temperatures, but you have to get the humidities and precipitation rates right as well.
You can see why meteorologists need a happy hour once in a while.

Stunning Pic and Snow


Last night didn't get quite as cold as expected because of the development of low clouds during the evening...but still cold enough to turn everything to ice and keep it that way. There is a stunning picture this morning, which I have attached. This is a high-resolution visible image from the NWS geostationary weather satellite at 35,000 km above the surface. According to aircraft reports this cloud has a base around 5000 ft and a top at 7000 ft. You can see the major mountains sticking throught it...like Rainier, Adams, and Baker....and this are clear shadows from the mountains on the cloud below. Would be wonderful to fly out and see this....assuming the plane was able to take off!
Other than roadway ice...not much happening today. The computer models are consistent on the approach of a Pacific system (see satellite picture)...which should bring snow to our region after midnight. There should be an inch or two by the end of the Wed. morning commute.
At this point it looks like temperatures will warm up enough by midday that that the snow might turn very wet or even to rain at the lowest elevations...but we are right on the edge. And then later in the day another system moves in--a stronger one associated with a Pacific occluded front. Now this storm will have lots of precipitation and if it were snow...then we would have a very major event. Right now, the models are suggesting the most of the precipitation from it will be rain at low elevations (below 500 ft)...but just barely. If the models are off or some of the model physics is error (like its tendency to mix out cold air too quickly) then we could have a major event. So right now...best estimate at lower elevations is snow starting midnight to 4 AM and continuing until midmorning...leaving a few inches. Then warming a bit and a let up in the precip, with the change to wet snow or rain miday. Then later in the afternoon the precip will pick up and probably turn to rain at low elevations and wet snow above roughly 500ft. But there is uncertainty in this. And knowing about uncertainty is in itself valuable information. Meteorologists like myself sometimes are very sure about the forecast, and other times we know there is substantial uncertainty...or a wide range of possible outcomes. A major challenge is learning how to quantify this uncertainty and then communicate to users like yourself....we have a major project to do this at the UW...including meteorologists, statisticians, psychologists, and others. The grant challenge in weather for the next ten years or so.
Getting back to the forecast.... on Thursday...another system... a tight, low pressure center moves southeastward....but the latest runs are suggesting that it will go far enough south to spare the Puget Sound region. Portand will not be so lucky. Will have to watch it. And on the weekend a warmer, wetter system is on tap.

Monday, December 22, 2008

Update II

The more I look at the forecast models, the more snow I see. Well, at least those who like a white holiday won't be disappointed.

As I mentioned earlier, tomorrow morning will be icy...but no new accumulations. It was bad enough when I took the bus home from the UW---buses sprawled on roads, abandoned on the sides. One almost hit a pole while I watched. It will be worse in the morning at some locations. (I better watch myself..I will end up sounding like a TV reporter!)

A Pacific occluded front will approach the area on Wednesday morning. Snow will start after midnight and it will certainly be snowing during Wednesday commute time. But as the occluded front makes landfall it will bring warming aloft and increased onshore flow...the question is what that will do to precipitation type. My colleagues in the National Weather Service believe that precipitation will remain snow. I am not so sure. I think there is a good chance we will see a change to rain or VERY wet snow during the late morning and early afternoon. Cooler temperatures will follow and there will be a switch to snow later in the afternoon. Accumulations by the end of the day--1-2 inches at low levels near the Sound, 2-4 inches at higher elevations.

The fun doesn't stop there. A low center from the Gulf of Alaska moves in Thursday morning...hugging the coast. Having a low over SW WA is a classic major snow threat....it can draw down cool air over NW WA and the Sound...and drive moist air over it...occasionally with large snowfalls. Take a look at the forecast situation for Thursday at 4 AM (plot)...not a perfect position, but close. Several inches of new snow could easily occur over the lowlands...and maybe more.

If you want to want to get an idea of snow totals over the area...I have included below the 24-h predicted snowfall ending 4 PM Wednesday and Thursday.




More Snowplow Musings:


A number of you comments on my "editorial" regarding a lack of snowplows in Seattle. A few additional ideas:

1. Although we have less events than say Denver, Minneapolis, and NY, ours are more serious.
First, we have the hills, which GREATLY magnify the problem. Second, our snow often falls on relatively warm ground and starts out as slush, which then hardens later into very slippery ice. Third, our snow and ice are generally relatively warm (just below freezing) and thus are more slippery.

2. We have a major snowfall (like this year) every 5-8 years and significant snowfalls 4-8 inches every other year or so. Each year we generally have few 2-3 inch snows, but some years only get a dusting.

3. I checked around on Ebay...snowplow blades are availabe for trucks for around $2000. Ok, this is Seattle....lets make it $10,000. So we could get 50 blades for 500,000 dollars--half of what those fancy automated bathrooms cost.

4. I am no economist...but image if the city had 3x more snowplows and could keep the roads in far better shape. What is that worth? Tomorrow the UW is closing for a second day. Image the cost of that in lost productivity...certainly tens or hundreds of thousands of productivity lost. What about the loss of business to retail? Any reasonable analysis should reveal that better snow removal capabilities would be a extraordinary investment.

Evening Update

I will be talking about Wednesday's potential storm later, but one thing is certain...there will be serious icing conditions in a few hours. Today temperatures got above freezing in most of the region and there was considerable sun. The unplowed roads became wet and slush and tonight many will freeze. We have clear skies that will promote good radiational cooling to space. Temperatures will drop into the lower 20s for most and into the teens in the normally cooler spots (away from water, in valleys or low areas). Snow really promotes nighttime cooling since snow is an excellent radiator in the infrared (that point and the fact that snow is a good reflector of solar radiation is why most record low temperatures occur when snow is on the ground).

So tomorrow morning may be a real problem with perhaps more troublesome conditions than this morning in some locations.

I am going to look at potential for snow from the upcoming storms...and it clear we have two shots at it...Wednesday and Thursday. But tomorrow, other than icy, should be a benign day with no new accumulations.

Noon Update and Editorial

Well...it looks like more snow is on the way. I just got a look at the most high resolution forecasts available in our region (the 4-km WRF-GFS if you like to impress your friends!) and a collection of other forecast models used to give some idea of forecast uncertainty. As this point they are suggesting that another batch of snow will began in the early morning hours Wednesday, snow through 9-10 AM, turn to rain, and then turn back to snow later in the afternoon. This would be a slushy mess.

Right now, temperatures are getting above freezing...making things nice and slippery. Did you know that the colder ice is the less slippery it is? When ice is below freezing, there still is a layer of liquid water on its surface...that is why it is slippery. The colder it is, the thinner that liquid layer is and thus the ice becomes less slippery...particularly as the temperature drops below 25F.

Editorial

Seattle is really not ready for snow! You would think that instead of reducing the number of buses radically as they have done, they would reprogrammed cancelled routes using the buses for the streets they have cleared. No way. There appear to be less buses on key arterial routes. I have given "Snow Talks" at Sea-tac Airport for years and the staff there always brags they have more snow removal equipment than the entire city. I can believe it. The City claims they have 25-30 vehicles with plows. Well, if they do, it is certainly not enough...and major routes (such as Sand Point Way in NE Seattle) are completely unattended. It is true that having extra plows for city trucks are not free and that snow events like this are unusual. But the economic loss of allowing the city to be crippled by such modest snows is substantial...and major decisions (like the cancellation of schools last Wednesday when no snow fell) are made in the context of such poor snow removal. Buses are important when cars have problems and the bustracking web site of Metro should a key tool. Unfortunately, they didn't size their server well...so it was completely overwhelmed this morning. And they hide the bustracking software on their web site as well. Last week I was in NY and they had a minor snowstorm. Plows were everywhere almost immediately...including secondary and tertiary roads. Certainly, we can do better. Global warming will not be the answer to this problem...at least not for 50 years.

The Break


Yesterday was a significant snow event for the region with the lowlands receiving 3-8 inches in general, on top of the considerable snows of the past week. The last week or so has turned into the most snowy, cold period since the great event of the last week of December 1996. Take a look at the temps at Sea-Tac compared to normal (graph). Our HIGH temps have been below the normal mins (blue line) for nine days now. However, 1996 made what we are going through look like child's play...many locations had nearly two feet on the ground after two major storms...my house at 23 inches on the ground on Dec 29th and earlier that week the worst ice storm in 50 years hit the southern Sound. The snow was followed intense warm rain that melted the snow causing flooding and landslides.

For the next two days we will be in a break from the action. This morning a few light snowshowers remain (see radar), mainly over SW Washington. These will lessen during the next few hours and the remainder of the day and Tuesday will be generally dry. The low center is now moving south of us and this will pull some cooler air back into the region (see plot of model pressure and temperature at 7 AM)
Another strong Pacific system will be moving towards us Wednesday, but temperatures will be warmer than the last go around...which will place us in the familar snow turning to rain, marginal temperature situation that local meteorologists love. I will look at the storm in more detail later in the morning.....

Sunday, December 21, 2008

5 and 9 PM Update



There has been 1-3 inches of additional snow this afternoon, with many large flakes. The origin of this afternoon's snow is a band rotating around an offshore low (see sat pic--you can see the clouds swirling around the low). As clear from the radar..this band is associated with light to moderate precipitation. The band is over western Washington right now and will be moving through by mid-evening. After that there will be a break, followed by additional snowshower bands overnight that will be swing around the low. Total new accumulation today could reach 4-6 inches in some locations.

Not everyone has been getting snow..there has been rain on the coast. Portland has had snow, but rain is falling to the south in Eugene.

Monday will be cool (highs in lower to mid 30s), but generally dry. We should stay with below normal temperatures for several more days. As a result, there is a good chance this will be a white Christmas (a white Chanukah is a sure thing....since it starts tonight!). By the way, a white Christmas only occurs 7% of the time.
...and did I mention there is the potential for more snow this week? In particular, there may be serious accumulations on Wednesday.

More Precipitation Today

Lets examine what happened yesterday and then move to today's situation. Although the forecasts were not perfect, they did get a great deal correct. Looking at a range of reports (and I very much thank those of you who provided snowfall amounts), there was roughly 3.5 to 8 inches for the communities adjacent to central Puget Sound, while 6-15 inches occurred over Kitsap and neighboring westwide communities. Less snow--2 to 6 occurred over the east side. So the amounts in Seattle and immediately north and south were right on and the predicted east-west snow gradient was apparent, but was less than forecast (the Kitsap had less and the eastside had more than predicted). This is consistent with wind evolution as we will see.

Now lets talk about the winds. First, this was never going to be a general windstorm...from the first it a downslope wind event on the western slopes of the Cascades and a gap flow event over the Strait. The strong winds verified in the Strait and there were strong winds near the western foothills...sustained 30-40 mph, with gusts to 70. But the strong winds were not as extensive as our best local models suggested...even when we had a historic pressure difference across the mountains. That difference...17.3 millibars between Seattle and Yakima...was the highest I have seen in 30 years. The winds in Snoqualmie pass were fierce and closed down the pass. Anyway, the weaker easterly winds contributed to higher snowfall on the east side of Puget Sound and less snow on the western portion. I could go on about this...but my colleagues and I will try to understand this forecast error.

One issue I didn't mention was the freezing rain. During the event, warmer air was moving in aloft, while cold air was maintained at the surface. The temperatures several thousand feet up got above freezing, resulting in rain falling into the cold air below--particularly south of Snohomish County. The result was freezing rain as the rain was cooled as it fell into colder air below. Thus, there is a crust on the beautiful fluffy snow we had last night.

Several of you commented about the nature of the snow last night. Most of you are used to the large, dendritic crystals that fall when temperatures are near freezing...our usual situation. Last night you got to enjoy the type of snow they get in colder climates.

What about today? More precipitation is coming. Another system is now moving into our area (see satellite picture) and the radar shows the precipitation moving in from the north and northwest (see radar). Rain, occasionally heavy, has moved into the coast...and Shelton has reported snow and ice pellets.

I REALLY wish we had a coastal weather radar to see the details of this approaching system...it would very much help (see my link on the right about this issue). This afternoon and early evening, the precipitation will spread over us...and the big question is: will it be snow, freezing rain, or rain? The problem is that temperatures are now marginal for snow..in fact there are above freezing temperature above Puget Sound now. Temperatures will begin to cool aloft this afternoon and temperatures can be cooled by the melting of heavy precipitation. If the moisture falls as snow, we could have 3-4 more inches. The NWS is now angling towards snow and the computer models suggest a good chance of snow over Kitsap (which there will upslope on the Olympics) and light snow north of Sea Tac. But we are on the edge and there is considerable uncertainty. At this point, I would lean towards wet snow this afternoon north of Seattle-Tacoma, with a "wintry" mix to the south. But there is considerable uncertainty about this.

Update

I will be giving a postmordem of this storm and today's expectations around 10-10:30 AM... its not over...cliff

Saturday, December 20, 2008

8:30 PM Update


There has been some "hot" comments about meteorologists hyping the weather....please be patient...it is too soon to throw in the towel on this event...although there are clearly deficiencies in the model's solutions. The winds have not accelerated as much as forecast...yet the pressure different across the mountains is extraordinary...nearly as high as I have ever seen it...nearly 16 millibars across the Cascades (Seattle-Yakima pressure difference). The highest I have seen occurred on Dec 24, 1983 (17.1 mb) ..the great Enumclaw windstorm of the century when winds exceeded 120 mph in the foothills. Winds have increased aloft (see plot from the profiler here at Seattle Sand Point,heights in meters, time increases from right to left and is GMT--21/03 is 7 PM) and Sea-Tac is reporting gusts 26 kts and one of the individuals commenting to this blog reported 45 mph. At northbend, WA a NWS spotter reported sustained 35 mph, gusting to 45. But this event is not over yet. I think one thing all of you have learned from following the weather is that timing errors are not unusual...particularly ones of 3-6 hr.
Regarding the snow, it is now picking up as the main warm frontal band comes through (see image)...then there will be a break and the snow will pick up with the cold front band. I have about an inch at my house now.

5 PM Update

It is clear that the snow moved in in a few hours early...not an unusual timing error. A larger error is the easterly winds--which are substantially weaker than forecast by the high resolution models. They have picked up and will pick up further...but if they are weaker than expected that will increase the snow east of the Sound (since there will be less downsloping)...

6 PM...winds started to really increase aloft....and gusts increasing at surface sites...cliff

A little early

Very light snow has made it into SW Washington already....the models often start the light snow 2-3h late..and this an example of it. So light snow is now falling from Shelton to the SW and should get to PS by 2-3 PM.....the easterly flow will slow it up a bit...also as of 3 PM the easterly flow really hasn't revved up yet...slower than the model's suggested....but the pressure difference across the mountains has increased substantially.

The Storm tonight and Sunday

This is going to be a complex event and I am going to take some time to describe it...and Sunday may end up different than some of the media is describing. I think we can have substantial confidence what will happen through 4 AM tomorrow...after that there is more uncertainty.

Before first, I wanted to comment on the remarkable temperatures we are seeing in western Washington. This morning much of the area has had temperatures in the teens (16F at my house in north Seattle), with many getting into the single digits. One location Arlington got below zero (I can hardly believe it). Here are some examples:

Univ of Washington 19
Mountlake Terrace 13
Lake Forest Park 9
Woodinville 8
S Everett 8
S Whidbey Island 8
Marysville 3
Arlington AP -3

Look at the plot of Seattle temps against normal. Our high temps are less than the normal lows. This is certainly one of the most sustained cold periods in recent memory (please no comments on global warming!--not an issue with a short local situation like this).Another remarkable feature has been the extreme snow in eastern Washington...with records at Spokane and nearby sites.

Now to the forecast. As you can see in the infrared satellite picture at 9:30 AM, the cloud band of with approaching system has reached our coast. The computer models are all in agreement that snow showers will begin in the early afternoon on the coast and move inland, with snow reaching central Puget Sound between 4 and 7 PM. It will snow this evening around here...this is very certain. But before we talk about snow, how about wind?
The high resolution computer models are going for strong winds developing this afternoon on the western slopes of the Cascades, the Columbia Gorge, and the Strait of Juan de Fuca. I have attached some of this output for you to see (for 4 PM, 10 PM today, 4 AM tomorrow). The predicted easterly winds along the western Cascade foothills are some of the strongest I have seen, with sustained winds of 50-60 mph with gusts that could reach 70-80 mph. I suspect the model is overdoing this....excessive downslope winds is a frequent model error and looking at its predictions for right now...the observations are less. This will be a very major event...but I suspect sustained 35-45 mph and gusts to 60-70 mph will end up closer to the truth. Very strong easterly winds (sustained 40- 50 mph) will also occur in the Strait of Juan de Fuca. There will be power outages.

Next the snow. As noted above, there is little doubt this Pacific system will make landfall and snow will start in the Puget Sound lowlands sometime later in the afternoon or early evening. There is also little question there will be a large east-west snow gradient over lowlands with heavy snow in the Kitsap and eastern slopes of the Olympics (1-1.5 ft), moderate snow near the Sound (3-8 inches), and lighter amounts towards the western foothills (1-3 inches). As noted in yesterday's blog this difference is due to the easterly flow...the resulting downslope flow over the western Cascade foothills produces drying and upslope on the Olympics causing increased precip on its upstream side. Attached is the 24h snowfall ending 4 AM. You see this features and others...such as the heavy snow in the mountains...and substantial snow over the eastern Strait. There is also a rainshadow (or snowshadow) north of the Olympics. Since I think the easterly downslope flow is being overdone a bit...as is the drying effects, I suspect that the snow amounts over Seattle and the east side are being underdone in the model, and this could increase the snowfall by a few inches.
So here we are around 4 AM with lots of snow. The cold front associated with this system will move through in the morning and there will be increased flow from off the ocean...and as a result there will be some warming aloft. A secondary trough of low pressure moves through during the afternoon on Sunday with more showers. These will bring snow in the mountains, but what about the lowlands? That is where this gets complicated and more uncertain.
Looking at the temperature predictions it appears the central Puget Sound and NW Washington could stay as snow...albeit wet snow and we could end up with several more inches. The coast will switch to rain as will parts of SW Washington. I don't see a major freezing rain threat in the PS lowlands.
But let me be clear...I am less sure about the second half of the forecast than the first part.
Anyway, be prepared to see a notable weather show.