April 13, 2021

Heat Wave!

 Find your sunglasses.  Stock up on sunscreen.  And get your shorts and tee shirts out.  You will need them.   A Northwest spring heatwave is about to begin.  

The start to spring has been chilly and damp, but that will be a distant memory by this week.   Consider the latest ensemble forecast (running the model many times, each a bit different) for the NOAA/NWS GFS forecast system (see below). Steadily rising temperatures from the low 60s today to the mid-70s on Saturday and Sunday.

The ensemble members are all very similar, which means we should have confidence in the prediction.  

I know your next question.  What about the highly skillful European Center forecasts?  Here they are (below).  Highs of 74 and 73 on Saturday and Sunday, followed by a cool down next week.


What do we owe this turn to torrid conditions?  

A very high amplitude upper-level ridge of high pressure over the eastern Pacific.   The forecast of upper-level (500 hPa, around 18,000 ft) conditions at 5 PM Wednesday, shows the upper-level ridge extended into BC, with troughs (the L's) on both sides.  This is known as an OMEGA block, because it looks like the Greek letter omega.  Very persistent.


And by Friday afternoon the ridge of high pressure amplifies right over us.  Such high-pressure areas are associated with sinking air (therefore cloud-free) and warming conditions aloft.  At the same time, there is easterly or offshore flow at low levels, which isolates us from the cool Pacific Ocean.


To really warm you up, let me show you the predicted air temperatures just above the surface (2-meters).

The forecast for Wednesday at 5 PM, shows temperatures rising into the mid-60s on both sides of the Cascades, with moderate northerly and northeasterly winds


By Friday afternoon at 4 PM the story is very different, with temperatures surging into the 70s over portions of western Washington.  Note that western Washington is warmer than eastern Washington because of sinking and compressional warming over the western slopes of the Cascades.


Temperatures are even warmer on Saturday (note, this a lower-resolution forecast) and the winds are weaker as well, so it will seem warmer still.


It will be marvelous.

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Announcement

The agenda for the Northwest Weather Workshop is up....please check it out here.
The meeting will be on a half day (morning of May 1) and will be online.  Anyone interested can attend and we recommend you register if you want to be on our mailing list.


April 11, 2021

Frosty Morning and an Extraordinary Land Breeze

Much of the state experienced a hard freeze last night, a combination of unusually cold air and clear skies, which allowed the earth to radiate heat to space.  Plus, generally light winds that lessened the mixing of warmer air down to the surface.

As a result of these ideally chilly conditions, a number of daily minimum temperature records were broken on both sides of the state.  

A map of the minimum temperatures last night (Saturday night/ Sunday morning) shows teens over NE Washington (and a 9F over northern Idaho), with low 30s and 20s across western Washington.  


Note the cold temperatures along the Washington coast....we will get back to that in a second!  

Proximity to water has a major impact on temperatures, something illustrated by a close look at Seattle and vicinity low temperatures last night (see below).  Temperatures only dropped into the mid-30s near Puget Sound or Lake Washington, but dropped to the mid-20s over the eastern suburbs of Seattle.  Classic in these situations.


But what really caught my eye this morning was not the frosty temperatures around the region, but the formation of one of the best land-breeze situations I have seen in years.

But first some land breeze 101.

Below is a schematic of sea breezes and land breezes.   A sea breeze occurs when the land becomes warmer than the water in a coastal region during the day, with low-level air moving inland.   Sea breezes cool the coastal zone.   Land breezes are the opposite, occurring when the land cools below the temperature of the nearby water.  In that case, air moves from land to water.

Western Washington and Oregon rarely get land breezes.  

Why you ask?  

Because our water bodies  (like the eastern Pacific and Puget Sound) are relatively cool (around 50F all year round) and the land often doesn't cool below the water temperature, particularly during the warm half of the year.

This morning we had a land breeze of note, one you will be telling your grandchildren about.  The winds at 7 AM showed the offshore-directed, easterly (from the east) flow along the coast (see below)



And the visible satellite at this time along the Washington coast was amazing (below).  You see the clear zone along the coast and the line of clouds offshore?  

Blame the land breeze.  The land breeze extends offshore and is converging with contrasting (northerly) winds offshore, resulting in low-level convergence (air coming together at low levels) that produces upward motion and thus clouds.   

You have really good eyes?  Can you see the low clouds in the Chehalis River valley with a small tongue of river stratus jetting out into Hoquiam bay?


And if you want a little help seeing what I am talking about...check out the next image.  The breeze winds are shown by the blue arrows, the Chehalis cloud jet by the orange arrow, and the offshore clouds at the edge of the land breeze indicated by the red arrow.



To show you how far weather model technology has come, let me show you the forecast for 8 AM from the high-resolution (1.3 km grid spacing) WRF model run by my group at the UW.  

The first image shows the wind speed near the surge by wind bards, with wind speed indicated by colors (white, then blue, then green, then orange).   You can see the jet of winds coming out of the Chehalis River and the expansion of the land breeze offshore, where it converges with northerly winds.


A forecast of the winds and temperatures (color shading) at the same time is found below, with white and dark green being the coldest temperatures. You can see the cold air from the land pushing over the ocean, warming as it moves westward.


Impressively, the forecast model cloud forecast, predicts the low clouds in the Chehalis Valley and even the line of clouds paralleling the coast offshore.

Enjoy the sunny week ahead.






Heat Wave!

 Find your sunglasses.  Stock up on sunscreen.  And get your shorts and tee shirts out.  You will need them.    A Northwest spring heatwave ...