Sunday, September 29, 2019

Early Snow Hits the Northwest

With cold air and plenty of moisture, an early snow has blanketed parts of our region....with convective cells even bringing temporary icing to Portland and vicinity.  Blizzard conditions with winds gusting to 50 mph and feet of snow are hitting parts of Montana.

Some of the images this morning are downright...chilling.

Mission Ridge, at 4500 ft above Wenatchee,  looks like it received just under a foot by 8 AM this morning.


Paradise on Mount Rainier (around 5000 ft), perhaps an inch.


Stevens Pass at 4000 ft-- a dusting, but much more just above.


What is more notable perhaps is Spokane, which beat its daily and month snow records with a snowfall of 1.9 inches.

Here is the National Snow Analysis at 11 PM last night (in two pieces).     The higher elevations of the Cascades (above 4000 ft) got whitened, with Montana getting huge amounts on the eastern side of the Rockies due to upslope flow. There is a lot more snow now and even more later today.



The cold unstable air that swung around the low center brought some heavy showers in parts of the west, with a particularly strong cell covering the ground with soft hail in Portland, turning the ground white (see picture taken by Justin Sharp).


The radar image at 4:13 PM, when this bedlam was occurring, was impressive, with intense precipitation (red colors) around Portland.


Winds?  You bet.   With a large pressure difference down the Fraser River Valley and air jetting out to the southwest, some areas around Bellingham had gusts to 30-40 mph, with one gust hitting 43 mph.


And the northerly and northeasterly flow attending the low center, FINALLY caused the BLOB to release it warm grip from the region, with our low temperatures falling below normal.  Examining the temperatures during the past 12 weeks at Seattle (compared to the normal highs and lows), Seattle for the first time in that period had its low temperatures drop to well below normal. 


Finally, there will be plenty of new snow falling today, as shown by the UW WRF total snowfall over the next 24 h. The ridges near the eastern slopes of the Cascades will get hit hard and parts of Montana will get a foot or more. 


The heavy September rainfall, plus this rain-snow event, is a boon to our water resources going into the fall.  All Northwest wildfires have been extinguished.   But there is a sad note  regarding this wet/cold bounty:  my poor tomato plants are turning yellow and dying.   Will the green tomatoes turn red?  Time will tell.

Thursday, September 26, 2019

Arctic Express Coming to the Northwest

If you live in western Washington, you might want to check that your heating system still works.

If you live in eastern Washington on the slopes of the Cascades, you might want to make sure you have a snow shovel.

If you live in western Montana, you might want to get your chains ready and stock up on food.

An unusually early and intense Arctic express will hit the region this weekend.  And our days of Blob warmth will be over for a while.

Let me start by showing you a stunning image from the NOAA/NWS Climate Prediction Center (CPC)--their 6-10 day forecast for temperature.  Specifically, it gives the probabilities of below normal (blue) and above normal (red) temperatures. 

Wow--- my colleagues in NOAA are quite sure that below-normal temperature will reign along the entire West Coast as well as the northern Plains States.


The action starts late Friday.

A huge upper level ridge develops in the Pacific, resulting in northerly flow over the western U.S., with a strong trough of low pressure/heights moving into the Pacific Northwest (see upper level-- 500 hPa map for 5 PM Friday below).  This pattern will not only bring Arctic air south over the Northwest but will isolate the Northwest from the warming impacts of the Blob (the region of warm water over the northeast Pacific).   As a result, western Washington will experience much colder minimum temperatures than observed during the past few months.



Now the details.  Here is a forecast map for 8 AM Saturday, showing sea level pressure (solid lines), lower atmosphere temperatures (color shading) and surface winds.  There is an intense pressure change (gradient) near the international border, with cold temperatures behind---this is commonly called the Arctic Front.   Low pressure, associated with the upper level trough, is centered over SE Washington.


By 5 AM Sunday morning, the cold air and large pressure gradient has pushed south. Eastern Washington, particularly to the east of the Cascade crest, get a piece of it.  Montana gets half the pie...with a huge pressure gradient--which means very strong winds will accompany the cold air.


Western Washington will escape the precipitation because the low is too far inland and we will be in the easterly (dry) descending flow.   But Bellingham and NW Washington will get very windy, as shown by the forecast for 10 AM Saturday.  Wind gusts could get to 35 knots (about 40 mph) from Blaine-Bellingham to over the San Juans.  Even in Seattle, winds could breezy and from the north.


Snow?  You bet.   The temperatures aloft will be cold for this time of the year.  Here is the forecast for 850 hPa (about 5000 ft), with the colors showing temperature and the sold lines showing heights (like pressure).  Frigid (below -6C) air over northern eastern Washington, Idaho, and Montana, associated with strong easterly flow.


This means snow.  Snow accumulation through 5 AM Sunday (below) show several inches of snow on the eastern slopes of the Cascades, and immense amounts (feet) upstream of the Rockies. Spokane will probably see snow flakes.  I suspect there will be some daily temperature and daily snowfall records broken in some locations during the next few days.


Tuesday, September 24, 2019

Major Snow Event in Montana and Alberta, With Light Snow in Our Mountains

Getting some light snow in September over the higher elevations of the Northwest is quite normal, but what is about to happen over western Montana and southwestern Alberta, will be unusual--with several feet falling at some locations on the eastern side of the Rockies during the next week. 

And here in western Washington we will get a taste of much cooler temperatures and a dusting over our high terrain.

Regarding the first date of snow, here is a wonderful graphic from NOAA/NWS (link ) providing the earliest first date of snow.   Lots of locations with September and even late August dates in western Montana, eastern Oregon, the high terrain of Idaho, and the spine of the Cascades.


So now that the shock of potential snow is over, let's take a look at the latest forecasts for total snowfall over the next week--examining the forecasts of the European Center model and the UW WRF modeling system (below).

They are very similar...HUGE snow dump of 2-3 feet on the eastern sides of the Rockies.  And a few inches to a half foot over the higher elevations of the Cascades.  Eastern Oregon will get a thin veneer of snow.



The forecast temperatures at Seattle from the European Model show the abrupt change ahead.  Mid-60s for a few days, but then highs of only around 55F on Saturday and Sunday, with a slow warm up after that.   Rain late Thursday and more on Saturday.   No snow for us!

But the story in Spokane is different.  The bottom drops out there. A 20 degree drop in the high temperatures, with minima declining to below freezing.  And snow is forecast there.

Missoula is something else.  25-30F drop, 9 inches of snow, minima below 20F. 

The cause of all this?  An usually deep upper level trough that will move into our region from the Northwest (see 500 hPa upper level map below).  Wow.  I get shivers looking at it.



Sunday, September 22, 2019

Early Cold and Snow Heading for the Region

Would you like to see some snowflakes this week?  Anyone living in Washington State will be able to do so with a short drive, as unseasonally cold air will be moving in.

In fact, if this was November or December I would be warning the Mayor to get the snowplows ready....but not in September.   Seriously, the large scale atmospheric configuration is a dead ringer for the one that provides lowland snow.

Let's "warm up" by looking at the latest 5-day forecast for Sunrise on Mt. Rainier from the Weather Channel folks.  High of 35 on Thursday, with snow showers.  High of 30F on Friday with snow showers.


Turning to the latest forecast from the European Center...here is their seven day snow total forecast for the next 7 days.  Some light snow in the WA Cascades, but nearly a foot in BC.  But take a look at the Rockies... lots of places getting a foot...and a few favored locations more.  And even the lower terrain in Montana gets a snow dump.  This is an unusually early snowfall for that region.


So what is going on?  The key point is that atmosphere is moving into a cold configuration for the Northwest, with a big ridge offshore, northwesterly flow over the West Coast and a sharp trough moving south into our region.  The classic snow/cold pattern for us.   But let me show you.

Let me start by presenting the upper level  (500 hPa) heights (like pressures) over the Pacific Ocean.

5 AM Friday...bit ridge (high pressure) in the middle of the Pacific, with a trough over us.  Cold and moist.


Two days later.....another trough...even stronger...but same general pattern.


Wet-Tember will get even wetter...and colder. 

Here is Seattle the Weather Channel 10-day forecast (which is very good by the way), which shows only a high of 57F on Friday. And fifties for the end of the forecast period. The normal highs this time of the year are in the upper 60s, so we are talking about being 10 degrees below normal.  Not good.    I suspect we have very active winter ahead.





Friday, September 20, 2019

Wet-Tember

This is turning out to be one of the wettest Septembers in a number of years--and most locations in western Washington already exceed their normal monthly rainfall--by a lot.

Here is an impressive figure-- the percentage of normal precipitation for September 5- Sept 18th.  A lot of our region has gotten more than 400% of the normal rainfall for the period.  Northeast Washington has over 800%.  In contrast, southern California is drier than normal...but they don't get much this time of the year anyway.

Here is the rainfall so far this month at SeaTac Airport (purple) compared to normal (cyan).  The current time is shown by the blue arrow.  SeaTac is about 1.5 inches above normal and even if we didn't get a drop more, it would end up roughly 2/3rds of an inch above normal.


But we will get more....starting with Sunday.  Take a look at the latest European Center forecast for accumulated precipitation through the end of the month (below). Not dry...with several inches in the mountains and wet conditions stretching over Puget Sound (due to a convergence zone).   Vancouver BC will be soggy.  The coast--wet.


All this precipitation will begin the refill process for our reservoirs and dams, and our streams are already at and above normal levels (see below).  Good for fish.



So purchase some slug bait, get out your rain jacket, and make sure you have an umbrella handy.

Wednesday, September 18, 2019

The Weather Outlook for the Rest of the Fall and Winter

The question many of you are asking is:  what kind of winter do we expect?

The question you should also be asking:  do such forecasts have much skill?

Well, I can give you a forecast, based on the best tools we have.  But the forecast skill is not as good as one would hope...as it always true beyond two weeks.

For the next one and a half month, probably the most skillful tool is the extended range European Center ensemble mean forecast--which projects out 46 days.  This forecast now goes out through 1 November.

The EC forecast for precipitation anomaly (difference from normal) through 1 November is for wetter than normal over most of the Northwest, with a few exceptions--well offshore and over southern Vancouver Island.


Temperature anomaly?  Slightly warmer than normal offshore, which makes sense with the BLOB still there..and warmer along the coast.  But pretty normal over most inland areas.


This forecast seems very reasonable to me.   

For the longer term, it is useful to look at the El Nino/La Nina situation.  Remember that El Nino is associated with warmer than normal sea surface temperatures (SSTs) in the central and eastern tropical Pacific.  La Nina is when the water in that area is cooler than normal.   Neutral (or La Nada) situations are when the SSTs are within .5C of normal.

Here is a plot from the NOAA Climate Prediction Center (CPC) of the sea surface temperature anomalies (again difference from normal) for some well-defined areas in the tropical Pacific (a map of the areas is found below).  The Nino3.4 area is the one most commonly discussed.  Time increases to the right. 

You can see we have gone from an El Nino period to a neutral period---with the sea surface temperatures cooling off a lot over the summer.



A wide variety of statistical and full physics models are run to predict the temperatures in the Nino3.4 area. The results of these predictions made last month suggest that we will end up slightly warmer than normal-- in other words, a neutral state.

And the most recent probabilistic prediction of the NOAA CPC folks is that a neutral situation is most probable, with a bias towards the warm side.


OK, the best bet now is that we will be in a neutral situation in the tropical Pacific. 

So what does that mean for us this winter?

The bottom line is that there is no reason to expect an anomalous winter in our area.  Starting with a warm Pacific and the modest warming associated with increasing greenhouse gases, going for modestly warmer than normal temperatures here in the Northwest (normal being the conditions of the past 30 years), seems reasonable.

No reason not to buy an annual ski pass if that is what you like to do.  No reason to expect any kind of drought situation.    Good weather this fall for prescribed burns to clean up our forests (WA DNR take notice!).

Does such a forecast have a lot of skill?  Te El Nino/La Nina connection with our weather can explain  perhaps about a third of the year to year variability of our weather.   Not as much as we would like, but still useful.





Monday, September 16, 2019

Why have our low temperatures been so HIGH this summer?

Have you noticed something meteorologically very strange this summer?

The overnight low temperatures have been very warm for pretty much the entire summer.

I can demonstrate this in several ways.    Let's start with a plot of the temperatures at SeaTac Airport for the last 3 months, with the average highs (purple) and lows (cyan) shown.  Mama mia!  The observed minima were above normal of most days--often by 3-6 F.   Sometimes even more.


A plot of the difference between the observed and normal temperatures for the last 90 days shows the same thing (see below)--but with a few wrinkles.   The coastal zone is all warmer than normal, with some places 3-5F above normal.  But go inland and many areas have normal temperatures--some even below normal temperatures.  Why is that?

Scott Sistek on his KOMO-News blog ran some of the numbers, finding that this summer we had 79 days in a row at or above 55F...absolutely SMASHING the previous record of 52 days set in 2013. 

The high minimum temperatures have made my tomato plants very, very happy--but why have we been so warm in the morning?

I think I know the answer--- warmer than normal sea surface temperatures over the eastern Pacific.   Also known as the BLOB--or in this case BLOB junior.

Here is a plot of the sea surface temperature anomaly (the difference of sea surface temperatures from normal) for the past 90 days.  You see the warm area off our coast?   A large area of 2C above normal--some a bit more?   That is the key feature.  2C is about 3.6F above normal....just in the neighborhood of how much warmer than normal our minima have been.

Now physically this all makes a lot of sense.    On most days, he air over the western WA lowlands was over the Pacific Ocean a few hours earlier, with the temperature of the surface air determined by the temperatures of the ocean's surface.

But there is more--the amount of moisture that air can pick up from the ocean depends on sea surface temperature---since warm air is able to hold more water vapor.  In fact, the amount of moisture air can hold goes up exponentially with temperature--that means REALLY quickly.  This figure illustrates this fact (saturation mixing ratio is the maximum amount of water vapor a sample of air can hold at a certain temperature).  So having air above warm water means the air can pick up more water vapor from the water.
Why does this matter?  Because water vapor is a potent greenhouse gas that warms the surface of the earth by reducing the loss of heat to space (sort of like an atmospheric blanket).  And a measure of the amount of low-level moisture in the air--the surface dew point--HAS been higher than normal this summer in western Washington.

So how can we really clinch this relationship between warm sea surface temperatures in the eastern Pacific and our minimum temperature "heat wave"? 

There is a way: we can play God using our WRF weather prediction model.  I asked UW Research Scientist Dave Ovens to do an experiment.  Redo an extended (7.5-day) forecast but replace the atuall warm sea surface temperatures with normal (or climatological) values of sea surface temperature.

Here are the results for a 7.5 day run verifying 5 AM last Friday--specifically the differences in surface air temperature between the current sea surface temperatures and normal sea surface temperatures.  Warmer air temperatures over ocean, which extend into western Oregon and Washington.  The Cascades seems to be holding back the influence from the interior.  Bingo.



Dew point temperature (again a measure of water vapor content) differences.... same story.  Much more oven the ocean, but with higher values getting into the coastal zone (and a bit farther in eastern WA). Bingo2.

So I think we know the culprit for our warm minimum temperatures...it is the area of warm water over the eastern Pacific.   A.K.A. the BLOB.

What caused the BLOB?  A combination of persistent high pressure that reduced vertical mixing in the ocean and southerly winds.  Essentially, a highly anomalous atmospheric circulation over the North Pacific.   That also can be inferred by the SST anomaly map shown in the third figure, since the north Pacific seems to be a big outlier.

Global warming could be making a small contribution.  A map of the surface temperature change from 1930 to now (from the NASA GISS site) indicates a warming of .2-.5C (.35-.9F) over the period.   Some, but not all, of that temperature increase might well be associated with global warming.



Saturday, September 14, 2019

Two Strong Fronts will Bring Unusual Amounts of September Rainfall

If the model forecasts are correct, this is going to be one of the wettest Septembers in recent memory.  And it is clear: wildfire season is over.

The first act starts tonight as a strong front moves in this evening.  The coastal radar is being overhauled--so I can't show you an ominous radar view.  But the latest infrared satellite image is impressive--particularly for September.


The UW high resolution WRF model 3-h precipitation forecast for 2 AM this morning show the front moving in.  And with the front, winds are going to pick up.

And then as a low pressure area moves towards us on Sunday, the precipitation will blossom, filling much of the state with moderate rain (see 3-h rain ending 2 PM Sunday).  This would be a lot for November.  And extremely wet for east of the Cascades.  


And then ANOTHER unusually strong front moves in on Tuesday (see below)


Now...are you ready to be meteorologically shocked?  Here is the accumulated precipitation total through 5 PM Tuesday.  Amazing.  Big areas of 2-5 inches.  Lots of rain in eastern Washington.


This September is already wetter than normal.   After the next week, the region will be thoroughly soaked.   A good time to plant grass seed or put in some bushes or other plants in the garden.