Super Icy

I normally bike to work every day, rain or shine.  But not today.
When I went outside to walk my dog, I started to slide.  Later when I tried to drive my car to the UW, it started to slide and the traction warning went on.

The ground was really iced up in places right now and there are two possibilities of the origin:  freezing rain and cold rain on a freezing surface.

Right now the National Weather Service has a freezing rain advisory out for the south Sound and around Puget Sound away from the water:

Freezing rain occurs when rain falls from an above-freezing layer into a surface layer that is cold enough to supercool the rain to below freezing.  When it hits a cold ground the rain freezes immediately you can get freezing rain.   But according the temperature observations above Seattle (see below), there is no real evidence of much as subfreezing layer above the surface.

There may have been more of a cold air layer at low levels over the south Sound and along the eastern slopes, so classic freezing rain might have occurred there.

But I think it is a bit more complicated that just freezing rain.  I  suspect what has occurred in many locations (like Seattle) is that with clear skies last night, the surface cooled to below freezing (see min temps below).  Then cold rain came in this morning and froze on the cold surface.

So be careful for a few hours until the ground warms up.

Update:  there were a number of bicycle accidents this morning on Seattle's Burke Gilman trail due to icy conditions.

Split Personality: The Future of Winter 2016 over the West Coast of North America

During the past week or so, there is only one way to characterize the atmospheric circulation over the west coast of North America:  split flow, with much of the active weather going north and south of the Pacific Northwest.

What is spit flow, what does it do to West Coast weather, and why do we think our weather will have split flow personality for the rest of the winter?

All of this will be revealed in this blog.

So what does a split flow look like?  Here is the forecast 500 hPa weather map for 1 PM Saturday (at roughly 18,000 ft, black lines are height lines, winds shown by the barbs).   Winds are parallel to the lines and strongest when the lines are closest together.

There is a trough of low pressure out in the Pacific with a single current of strong winds north of Hawaii.  But east of the trough the winds spit into two currents, the strongest going into Alaska and the weaker into California.   Winds over the Northwest are light.   This is split flow.

Weather systems tend to follow the stronger winds and they get torn up by the split.  The result is that most of the real weather is going north and south of the Northwest:  into Alaska and California.

Here is an example from last Sunday, January 3 at around 2 AM.  Take a look at the infrared satellite image below.  You can see a weather system moving into California and a large system out over the Pacific, south of the Aleutians.  Clouds over SE Alaska.  Nothing over us.

And guess what the upper atmosphere (500 hPa) flow pattern shows?  You guessed it...a split flow!

You better get used to split lows.   Strong El Nino's tend to favor them after January 1 and the flow pattern the last few weeks has been like this very often.

How do we know this?   Because if we average (or composite) the upper level flows for other strong El Nino years, that is the pattern we see.  Here is the deviation of the strong El Nino average from normal (called the anomaly) for 500 hPa.  The purple shows a large negative anomaly (must deeper than normal low pressure offshore).   Such a trough offshore results in the split flow pattern.

The latest 10-day precipitation forecast from the NWS GFS model suggests a continuation of this persistent El Nino split flow, with lots of precipitation in California. (see below).  Lucky devils.

Finally, a few folks might be interested in the weather that the Seahawks will experience on Sunday.  Here is the latest Weather.com forecast for Minneapolis.  Game time around noon.  If they are lucky, the temperature will rise to zero by the time game starts.  Below 5F the entire game.  But it will be sunny and winds will be modest.

Heat Surge in the West, While Cold Stratus Dominates Eastern Washington

I was running in the UW Arboretum during lunchtime and I learned rapidly that my running partner and I were overdressed.   With seemingly springtime temperatures and full sun, it felt like April, not early January.

When I got back to my department, I was surprised to learn that the temperature on the top of my building had surged to 55F! (see plot below)

And as this additional plot shows, we enjoyed the warmest temperature in at least a month (4 weeks are plotted below)

At the same time, eastern Washington was stuck in cold temperatures and low clouds.  The visible satellite image at noon Wednesday clearly showed the story.
Some delicate cirrus over the Puget Sound basin, clear over the western slopes of the Cascades and socked in with low clouds east of the Cascade crest.

The high temps on Wednesday reflected this (see below).  Fifties to mid-fifties around Puget Sound and extending to the coast, thirties to the east.

Ironically, the warm in the west and the cool, cloudy east are caused by the same feature:  a strong low off our coast, with higher pressure inland.  The forecast map for 1 PM Wednesday at around 5000 ft (850 hPa) is shown below with heights (like pressure), temperature in color shading, and winds.  At this level, the low is causing southerly flow aloft that has moved warmer air above the Northwest.

So why isn't it equally warm on both sides of the mountains?  Let's look at a lower level, viewing a map with sea level pressure, surface winds, and temperatures at a few thousand feet and at the same time.  Similar to 5000 ft, but with noticeably higher pressure and modestly cooler air east of the Cascades.

Enough hints...lets go through it.   Cooler air was trapped in the basin of eastern Washington (see terrain map)

That cool air, which was saturated enough for lots of low clouds, was in no rush to get mixed out between there was an inversion (temperature increasing with height) at the interface between cool and warm air. See the vertical sounding at
Pasco, WA at 1210 PM to show this.  HUGE inversion with 60F temperate at 5000 ft and low 30s near the surface.

Inversions are very stable and are difficult to remove.

West of the Cascades, there was offshore easterly flow that increased midday Wednesday as the low moved into position, while higher pressure held east of the Cascades.  The time-height cross section (see below) of winds and temperature above Seattle Tacoma Airport today showed this (time increases to the left and are in UTC/GMT).   Heights are in pressure (850 hPa is around 5000 ft).   The easterly flow helped move the warm air down to the surface, with downslope warming due to compression, zipping the temperatures up a bit.

Why were the warmest temperatures limited to Puget Sound, the central coast, and a few locations on the western Cascade foothills?   Because that is where the warm easterly flow aloft was able to surface to the ground.

It looks like essentially dry conditions for Washington State through early next week, with high temps in the lower to mid 40s.  Canonical El Nino pattern.

You want lots of rain?  Go to California.  Want very, very cold air?  Go to Minneapolis with the Seahawks.

California's Water Situation Improving

The drought is over in Washington and Oregon and now California is getting relief.  Major relief.

In some ways, the improving trend was expected.  This is a very strong El Nino year and such years often bring heavy precipitation to California, particularly after January 1.

Let me start by showing you the forecast total precipitation for the next two weeks from the National Weather Service GFS model. Much of California gets 5-10 inches.  As Darth Vader would say:  impressive, most impressive.

The UW WRF model, which generally is superior to the GFS over terrain shows 2-5 inches over northern CA during the next 72 hr alone.

And the official Climate Prediction Center 8-14 day precipitation probability is emphatic with above normal precipitation over California.

Looking long term, the NOAA CFS seasonal model is going for a very wet January through March over California.  Classic El Nino pattern

The good news is that all this precipitation will fall on a state in which the situation has already started to improve. The latest snowpack measurements show the Sierra Nevada snowpack is well above normal.

And the GRACE satellite soil moisture measurements show most of California wetter than normal, with the exception of the far SE side of the State.

But with all the good news, one must consider that California's reservoirs are quite low as a result of a multi-year drought--roughly 50% of normal, as shown by this summary of the major reservoirs below.  California has a very large number of reservoir and immense storage capacity.   During the next few months, substantial rains should greatly increase the reservoir storage.  I suspect this will be the turn around winter for CA and the end of the several year drought.  But it may take several years to fully recover.

The approaching period of heavy rains has emergency management folks worried about California flooding and landslides.  The LA Times has a big story on this:

Finally, it should be stressed that the large scale circulation patterns over the western U.S. are radically different this year than last. For example, we have not seen much evidence of the persistent West Coast ridge of the last few years and the BLOB is gone.   There is no reason to expect a repeat of last year.

Yes, It's Snowing in the Lowlands!

Well....there are flakes outside my window now and many stations in western WA are reporting light snow.

Quite honestly, our forecasting models had the timing of this band off by at least 6 hours....it came much earlier than expected.   With cool, dry air in place, the precipitation either started as snow or rain turning into snow.   Here is the latest radar (1:18 PM)....generally light.

And the latest visible satellite image shows the cloud band.  Still clear on the coast.

I really don't expect more than a dusting in most Puget Sound locations.  There was considerable more around Portland (1-2 inches), as suggested by this cam image from Portland:

Here is the National Weather Service "weather story" on the snow.  There was much more south and east of Tacoma (as much as 1-1.5 inches) than over Seattle.

Offshore Flow, Eastern Washington Cold Pool, Declining Air Quality, and California Floods

We are in the midst of a world-class period of easterly flow over our region, with powerful winds downstream of gaps, a stubborn pool of cold air in the Columbia Basin, crystal clear mountains, and ironically, declining air quality in places.   A radical shift from the persistent clouds and precipitation of December.

The key element of other weather the past few days has been strong easterly flow aloft.  Such strong offshore flow typically produces fine visibility (as long as you are not embedded in low-level fog) along the western Cascade slopes and this morning is no exception (see below at 7:30 AM from the Space Needle cam).   Below that is the view at noon Friday.

Why are the mountains so crystal clear under easterly flow?   Because the air sinking down the western slopes warms and dries as is descends, so no clouds and low relative humidity.  Low relative humidity prevents the growth of water-loving (hygroscopic) particles.

But why is there easterly flow?

We start with a large-scale upper level atmospheric configuration with high pressure to the north and and an unusually persistent low to our south.  This figure shows 500 hPa (roughly 18,000 ft )heights, winds, and temps forecast for 4 AM Saturday morning.  The wind barbs show strong easterly flow (from the east) over our region.

Closer to the surface, the easterly flow is revved up by the large difference in pressure across the Cascades, with higher pressure to the east.   Here is the seal level pressure and low-level temperatures at 4 AM this (Saturday) morning. The black lines are isobars (lines of constant pressure).  A very large pressure difference across the Cascades (about 10 hPa), which drives easterly flow down gaps, such as the Columbia River Gorge.  Why the big difference in pressure?   The big driver is temperature, with cold dense air (blue colors) trapped east of the Cascades.

To show this temperature contrast, here are the minimum temperatures the last 24 hours across our region.  Lots of single digits in eastern WA, and even some below zero, while twenties were dominant over western WA.

 What about wind?  Here are the maximum gusts for the last 24 h (ending 7 AM).  Very strong winds over the western side of the Columbia Gorge (51 mph) and the western side of the Strait of Juan de Fuca.  Air is accelerating down the pressure gradient in these gaps.  And you will also notice some strong winds along the eastern slopes of the Cascades.

But the sunny weather has a subtle down side:  our air quality is starting to decline.   Clear skies allow the earth to radiate infrared energy to space, thus cooling the surface and low-level air.  The easterly flow, and associated warming, is stronger aloft.   The result is the development of inversions, where temperature increases with height.  And inversions lessen vertical mixing and result in declining air quality.  You can see the changes by looking at the vertical temperature soundings at 8 AM, once a day, for the last several days at Seattle (see below).  Today (black line) there is a decided inversion.  Not so a few days ago.

Puget Sound Clean Air Agency has burn bans for Snohomish and Pierce Counties.  The air quality map (green is good, yellow is marginal, orange/red is poor) shows the situation.  Why is air quality better in a line from North Bend to Seattle?  Because of stronger winds pushing westward out of Snoqualmie Pass!

But changes are in the air.  A system will approach on Sunday, bringing clouds and rain to Oregon (but will hold off until Sunday night in most of Washington).    But nothing heavy.  Splitting systems.  Classic post New Year's situation under a strong El Nino.  But California is going to get hit, very hard.  Here is the GFS model total precipitation forecast for the next 10 days.  Biblical levels of rain.

2016 Northwest Weather Workshop On March 4-5, 2016

The Northwest Weather Workshop, the big regional meeting on Northwest weather will take place in Seattle on March 4-5, 2016.   At this meeting we talk about the latest advances in understanding our local weather.  There will be a special session on the OLYMPEX project.  It is open to all.   For more information go here.  

If you want to give a presentation, abstracts are due on February 1.

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