Thursday, February 23, 2017

Why has California Been Wet and Washington State Cool This Winter

It has been a remarkable winter so far, very different in character from the previous winter seasons.  During the past 90 days, the southwest US, and particularly California and Nevada, have been MUCH wetter than normal.  Some locations have had over 400% of normal precipitation!  Western Washington has been a bit drier than normal.

For temperatures over the same period, Washington and Oregon have been much cooler than normal (particularly east of the Cascade crest), while California close to normal.

So what his interesting north-south pattern and why has it been so persistent?

Well, the proximate cause is easy.  There has been a persistent area of low pressure over the eastern Pacific west of northern California to Washington.  The following figure shows the anomaly (difference from normal) of the height the 500 hPa pressure surface (around 18,000 ft) for the past 90 days.   There is an amazing negative anomaly (purple color) signifying lower than normal heights (or equivalently low pressure).

Storm after storm has brought low heights/pressures along the northern West Coast.

Such a persistent trough or low along the northern West Coast causes the jet stream to be pushed south into California, instead of its normal position coming into the Pacific Northwest  (the jet stream, a current of strong winds tends to follow the outer periphery of the trough).   Troughs are associated with colder than normal air.  So with the jet moving south into California, CA get more precipitation (since the southern of the trough is associated with upward motion), while cold air is found over the Northwest.

But why is the trough found over the northern West Coast?   Good question.  If you look at the pattern of the height anomalies, you will notice a wave-like pattern, with alternating high and low heights.  This pattern is associated with something called Rossby waves in the atmosphere (they are names after a famous meteorologist Carl Gustav Rossby).   Think about throwing a rock into a big pond, with waves radiating away from the rock.

So what is the analog for a rock in the atmosphere?  What is disturbing the atmosphere causing waves to propagate over the Pacific Ocean and north America?

Lots of thunderstorms over the Maritime Continent- places like Borneo, New Guinea, Indonesia, and the adjacent islands.  

Here is a measure of the amount of thunderstorms that we can observe from space--called Outgoing Longwave Radiation  (OLR) for the same 90 day period.  To be more specific, this shows the anomaly (or difference) from normal.  Where values of OLR are low, there are more high clouds from thunderstorms.   You can see a big negative anomaly over the Maritime continent and SE Asia.
Thunderstorm are like rocks to the atmosphere, greatly perturbing the wind, temperature, and humidity fields.   That in turn generates atmospheric waves that can cause localized weather anomalies like we have seen on the West Coast.

But why is there a big thunderstorm anomaly with lots of thunderstorms over SE Asia and the Maritime Continent?    That is probably due to the La Nina of last winter, which wass associated with stronger trade winds that push warm water into the western Pacific.   And why is there La Nina?  Because of a natural oscillation in the tropical Pacific.

Enough questions!  But perhaps there is one more.  Will it snow over the lowlands on Sunday?   The latest model runs suggest some light snow is possible (see snowfall prediction for the 24 ending 4 PM Sunday).  Temperatures are marginal, but where there is some elevation and greater precipitation rates, snow may reach to near sea level.  More on Saturday.

The Northwest Weather Workshop

And don't forget...if you want to attend the big weather meeting of the year...the Northwest Weather Workshop on March 3-4 in have to register before.  The agenda and more information (including how to register) is found here:

Tuesday, February 21, 2017

La Nina is Dead. Can El Nino Be Far Behind?

I have sad news for all of you.

La Nina is dead.   Done. Finished. Terminated.   The final word came out a week ago, provided by the august U.S. Climate Prediction Center (see below).

And it appear that we have a better than even chance of moving back into the El Nino pattern again, a configuration that leads to generally less snow over the Pacific Northwest.

To track the oscillation between La Nina, Neutral conditions, to El Nino and back again, meteorologists follow the sea surface temperatures (SST) in tropical Pacific.  As I have noted in past blogs, this variation, also called El Nino Southern Oscillation (ENSO), represents a sloshing of the warm water in tropical Pacific.  When the water sloshes towards the east we have El Nino, when it sloshes to the west, La Nina.  You can think of the tropical Pacific as a giant bathtub.

To get a handle on what the sloshing is up to, meteorologists follow the SSTs for a few areas in the tropical Pacific (the Nino 4, Nino 3.4, Nino 3 and Nino 1+2 areas).  The figure below shows their locations.

The general approach is to track the SST anomalies (differences from normal) for these areas, with Nino 3.4 being probably the most popular. When Nino 3.4 has a warm anomaly greater than +.5C we have El Nino, an anomaly of -.5C or less, La Nina.   Near zero, a neutral or La Nada period.

Here are the latest graphs of SST anomalies.  Viewing the Nino 3.4 graph, you can see we had a weak La Nina for a while, but now temperatures are slightly on the warm side---we are in neutral conditions and probably have been for a few months.

What about the future?  The Climate Prediction Center provides a nice graphic of  past and predicted SST anomalies from many different ENSO forecasting systems (see below).   Forget La Nina.  Some of the model stay in neutral territory, but half move toward El Nino.

 The main U.S. seasonal model, CFSv2, is run as an ensemble (many forecasts).  Although there is a lot of uncertainty, most runs are heading towards El Nino.

Digesting all of the this information, the Climate Prediction Center is going for neutral conditions continuing into spring, but equal chances of moving toward El  Nino by midsummer, and a tendency towards El Nino by fall.

 Let me stress that ENSO forecasts made mid-winter have been notoriously bad.  Skill for the upcoming year increases substantially by summer.

One thing you can be sure about:  with our reservoirs in decent shape and a near normal snowpack, the water situation in the NW looks excellent for this summer. And with California soaked this winter and its reservoirs filled, vegetable/fruit prices should be more modest than in past years.

Sunday, February 19, 2017

More Threatening Weather at the Oroville Dam

11 AM Monday Precipitation Totals

Some of the terrain about the dam has received over 4 inches already.


The latest forecasts are worrying regarding the heavily damaged Oroville Dam in California.  And I am surprised there is to so little talk by CA state officials and the media about the danger.

During the past few days, with little rain and active drainage of water, the dam level has been reduced substantially (see image)

But central California and the Sierra Nevada mountains are about to be hit again...quite hard, with a strong atmospheric river (see moisture plot for 1 AM Monday).  The red and white colors shows high column integrated water vapor values.  Aimed right at the dam.
The forecasts show substantial precipitation with this atmospheric river.  Here is the accumulated precipitation for the 24h period ending 4 PM Monday from the excellent NCAR high-resolution ensemble system (the ensemble mean). 4-8 inches over the Sierra.

But that is the ensemble mean or average.  But there is a chance it could be more, so here is the ensemble maximum, the ensemble member with the highest amount of precipitation.  Much more (30-50% more in some locations)..and a serious problem for Oroville Dam.

The 48h precipitation total ending 4 AM Tuesday from the GFS model projects 5-10 inches in the mountains.

And the European Center model has the same idea, with 5-7 inches

Looking at the NCEP SREF ensemble system at Blue Canyon, about 15 miles away from the dam,  shows an ensemble mean of 7 inches, with a range from 5.5 to 13 inches.  Great confidence for a major rain event.

Hydrological forecasts by the National Weather Service, project a rapid rise of the Feather River, the main feed into the reservoir behind the dam, with the flow quadrupling from around 7.5 thousand cubic feet per second to around 32 thousand.

This is a dangerous situation and, as always, there is uncertainty with our forecasts.  Why is there so little discussion of the rapidly increasing precipitation?

And if you want to read an interesting blog about an unusually strong cold front that hit Hawaii, check this out:

Saturday, February 18, 2017

Will Seattle Beat the All-Time February Precipitation Record? Almost Certainly Yes

Sunday morning update:  SEA how has 8.01 inches,  1.10 inches below the record Feb total.

People love records.   And precipitation records are falling over the West Coast.

Here is Seattle, the record February precipitation is 9.11 inches, which fell in 1961.

As of midnight, Seattle Tacoma Airport had 7.85 inches so far this month.
Since then, Sea Tac got another .07 inches, so we are now at 7.92 inches or 1.19 inches behind the record.  A plot of actual (red) and average (blue) cumulative precipitation at Seattle shows the story...we are about 6 inches ahead of normal now.

Looking at the forecasts, we have a very good chance exceed this.   For example, here is the precipitation forecasts for Seattle from the European Center ensemble and high-resolution models.  1.3 inches for the ensemble and 1.7 inches for the high resolution forecast.   If true, the record is toast.

Or the UW WRF forecast for the 72 hr starting at 4 AM this morning. The purple color is more than 1.28 inches.... very close.  And there is more rain after that.

So the chances are very good that a new February rainfall record will be set at Seattle Tacoma Airport and a number of other western WA locations.

All this precipitation has caused our soils to become highly saturated, resulting in mudslides and slope failures, such as the one that closed down I-90 near Issaquah and another that closed I-5 near Woodlawn (see below).


Northwest Weather Workshop:  

The Northwest Weather Workshop, the big annual weather gathering in the Northwest, will take place on March 3-4, 2017 in Seattle (NOAA Sand Point Facility).   Everyone is welcome.  For more information and to register, go to

Friday, February 17, 2017

Northwest Weather Hits Southern California with the Most Substantial Storm in Years

Southern California is about to be hit hard with heavy rain of 3-9 inches and powerful winds.  Flooding and slides are inevitable.   But it will have a silver lining:  any talk of drought in California should be ended as the last below-normal reservoirs are filled.

Let's start with the 2-day precipitation totals (ending 4 PM Sunday) from the wonderful NCAR high-resolution ensembles (the ensemble mean--the average of the ten ensemble members are shown). Totals exceeding 3 inches covers a wide area from LA north, with some locations in mountains north of Santa Barbara getting 7-9 inches.  That amount of rain would be huge anywhere, but in dry southern CA that is immense.     For a number of locations, this will be the greatest rain event in years.

The reason southern CA is so wet, while the Northwest is relatively dry, is because the jet stream has been deflected way to the south.  This is illustrated by an upper level forecast map for 4 PM Friday, with the sold lines presenting the heights of the 500 hPa pressure surface (around 18,000 ft).  A deep trough of low heights (pressures) is found along the west coast, with the jet stream/strong winds heading into southern CA.

Equally impressive is the latest infrared satellite image (Friday AM).   A extremely moist frontal band is headed right into the LA Basin, which will bring both heavy rain and strong winds.  There is clearly instability (convection/thunderstorms) embedded in the cloud mass.

The latest radar image from Vandenberg AFB shows extensive moderate rain over the region, with the yellow/orange areas being the most intense.

And now we have the problem--southern CA is ringed by high terrain and as the moist flow climbs the barriers, precipitation intensity is greatly increased (by 5-10 times is not unusual).  This air is relatively unstable, so there is also the risk of thunderstorms with locally intense rainfall.   The result is the potential for flash flooding and slides, with debris-laden regions that have recently burned of particularly concern.  The National Weather Service has a flood warning out for the Santa Barbara area and a flash flood watch extending down the coast. It is not going to be pretty down there.

Wednesday, February 15, 2017

For Some in California, the Worst is Yet to Come

After a few day break, California is about to experience a period of very wet weather, with the potential for flooding and slope failures in southern California and threats to the Oroville dam in the north.   For the 24h ending 8 PM Wednesday, the fire hose has been mainly directed to the Pacific Northwest, with some locations in the Olympics getting 5-7 inches of rain.  But substantial rain (greater than 2 inches) extends into the Cascades and down the coast to northern CA.

Now it is California's turn.  The U.S. GFS model forecast for the next 7 days is amazing, with portions of the Sierra Nevada near the Oroville dam getting 10-20

 inches of rain.  Very bad.  And heavy precipitation will hit southern CA as well, where flood and slide warnings are now in effect (see NWS warning page)

The European Center model precipitation for the same period (shown below) has a similar solution, with as much as 7-9 inches over the Sierra Nevada.

But how much uncertainty is there in this forecast? For Chico, CA (near the Oroville dam at lower elevations), the European Center ensembles (many forecasts) are all on pretty much the same page. (each line in the upper panel  \represents one forecast).  One event starting tomorrow, with the heaviest precipitation on Friday.  Then another event later Sunday and Monday.

The ensembles for Santa Barbara are also in agreement for a big event on Friday, with even heavier precipitation in the mountains.

This could well be the wettest storm of the year in southern California and damaging slides and floods are quite possible.

Tuesday, February 14, 2017

The West Coast Will Face An Extraordinarily Wet Period

Northwest Weather Workshop:  Open to AllThe Northwest Weather Workshop, the big annual weather gathering in the Northwest, will take place on March 3-4, 2017 in Seattle (NOAA Sand Point Facility).   Everyone is welcome.  For more information and to register, go to
Northern California is experiencing its wettest winter on record, with reservoirs full, the ground saturated, and many of the rivers at or near flood stage.

Oroville Dam, northeast of Sacramento, is on the brink of failure and downstream residents have been warned to evacuate.  Water is being released through the emergency spillway in a desperate attempt to save the dam.

During the past two weeks, immense amounts of precipitation has fallen over the West Coast (see image), with some locations in the Sierra and coastal mountains getting more than 20 inches of water.

Considering the last 60 days, large sections of California, Nevada, Utah, and southern Oregon have received 200-400% of normal precipitation, with limited areas exceeding 400% of normal.  Generally, Washington State is slightly drier than normal.

Folks, it is not over yet.  Although a ridge of high pressure temporarily has dried things out the last few days, the fire hose of rain will be returning on Wednesday.

Here is the forecast precipitation total for the next 9-days from the U.S. GFS model.  Nearly all of the Cascades, Sierras, and coastal mountains will get 5-10 inches of additional precipitation.   This includes the last hold-out from serious wetting, the mountains behind Santa Barbara.

The forecast for nearly the identical period from the European Center model shows a similar story.   Very wet conditions along the entire West Coast.

A number of rivers are now at flood stage in California (red dots on map below from the California/Nevada river forecast center) and they will be pushed higher during the next week.

There are a number of individuals and groups that talk about drought in California being a threat from global warming.  But heavier precipitation during major precipitation events during the winter is perhaps a more probable expectation, as atmospheric rivers become more potent as the earth warms. Warm air can "hold" more water vapor than cold air.

The atmospheric river that will approach the Northwest on Wednesday will be a strong one (see map, blue colors indicate very large water contents), and will then head towards California.  And more atmospheric rivers are to follow.

An historic wet winter and one that was not forecast last fall.  My profession has a lot to learn about seasonal forecasting, assuming it is possible.

Sunday, February 12, 2017

How much salt falls naturally out of the sky?

During the past several weeks, as snow has covered the roads of Washington and Oregon, salt is being used to prevent an ice layer from bonding to the concrete and to help melt the snow.  Clearly, salt can make travel far safer, prevent injuries and deaths, and keep our roadways functional. It can prevent a city from being crippled, like Seattle was in 2008 and Portland experienced last month.

But some folks wonder about the environmental impacts of roadway salt and the potential to corrode cars and other vehicles.   But to answer this question, one needs to start with another question.   How much salt is naturally in our environment, particularly salt that reaches the ground in precipitation or by dry deposition?

The answer is going to surprise you.  A lot of salt is falling out of the sky!  In fact, probably more than being spread by State and local departments of transportation.

Where is it coming from?  Breaking ocean waves!    The ocean is salty, of course, and when waves break or when wind produces spray, the air is filled with salt water droplets.  These droplets can evaporate, leaving small particles of salt in the air.   Here in the Northwest, salt particles can be easily blown inland by the

prevailing westerly winds and  brought back to the ground by precipitation or even dry settling (deposition).

To illustrate, here is a map created by the National Atmospheric Deposition Program, based on combining observations and models, that shows the wet deposition (by precipitation) of one component of salt, sodium ions.  (Remember,  from your basic chemistry that salt is sodium chloride, NaCl).   The red areas show large depositions (more an 4 kilograms per hectare).   And remember, a kilogram (kg) is 2.2 pounds and 1 square mile is 259 hectares.

You note there is more salt deposition near coastlines and particularly stormy coastlines.  That is why the Northwest is well salted but California is not.   You can see the effects of the Great Salt Lake as well.  Other years look very similar to 2015.

So based on such research it has been found that the Puget Sound lowlands get around 7 kg per ha per year of salt and the coast (closer to the breaking waves) receives about 30 kg per ha.

So there is considerable salt falling on the surface by natural processes, with little obvious problems for animals and plants.

OK, now let's have some fun, comparing the amount of salt falling on Seattle each year against the amount of salt resulting from roadway protection in the city.

Seattle encompasses 83.78 square miles.  Each square mile is comprised of 260 ha.  Thus, the city has 21,783 ha.    So if 7 kg  of salt falls per year per ha, then the total amount of salt reaching the ground in Seattle each year is 335,455 pounds of salt or 168 tons.

Mama mia!  We have a salty city!

Let's compare this total to how much salt is used to protect the roads for one snow storm, which seems to be the typical number per winter the last few years.  From a little digging, I believe that Seattle uses about 100 tons (200,000 pounds) for a single snow event--less than the 168 tons noted above.   But even I am off a bit, I suspect the bottom line is reliable:

The natural precipitation of salt from the ocean over Seattle is roughly equal to the salt spread for reducing icing during a single storm.  

Thus, I suspect roadway salt is not a significant environmental hazard. Furthermore, since the salt is spread over roads much of the salt goes into drains, some of which go directly to treatment plants and the Sound.  This concern is further reduced by the fact that Puget Sound is a salt-water estuary, with considerable salt content (although a bit less than the ocean, 2.9% versus 3.4%).

Roadway salt, spread during heavy snow periods, does not go into drains immediately, but rather over several days, thus reducing pulses of salt.    Furthermore, it is accompanied by a surge of melt water that dilutes its concentrations.  As an estuary expert I consulted stated:   "dilution is the solution".   You've got to love sayings like that.

It is interesting to note that the natural deposition of salt does not fall in a uniform way, but accompanies the major storm/rain events that also bring large waves and lots of sea spray.  I have confirmed this by looking at the weekly data available from the national deposition site.  Here is an example showing the weekly amounts of sodium from salt at La Grande, in Pierce County from January 2010 to now.  Lots of variability.

Thus, there are natural spikes in salt deposition, with only a few weeks each year providing much of the annual deposition.

I am not saying the salt on roadways is absolutely harmless, but that the risk is small compared to the alternative loss of life, injury, and economic damage. Like is about risk versus reward, costs versus benefits, and in this case the benefits of using road salt one or two times a year far outweigh the risks.

 Finally, modern vehicles have far better primers and paints, and thus are less susceptible to salt-associated corrosion.

Clearly, the environment implications far salt would be far greater for cities with a lot of snow that require many salt applications over the entire winter--such as Chicago or Buffalo.   Thus, Mayor Murray and the folks at SDOT can probably sleep soundly knowing the Seattle's occasional salt spreading is not undermining the environmental quality of the city.  A previous Seattle mayor learned the hard way the dangers of not using salt.

Acknowledgments:  I received guidance from Joel Thornton and Dan Jaffe, experts in atmospheric chemistry at the University of Washington.