Thursday, March 8, 2018

Spring "Heat Wave" Coming to Western Washington and Oregon

During the past month, western Washington temperatures have generally been at or below normal, as illustrated by the temperatures at Sea-Tac Airport (see below, purple shows the normal high, blue shows the normal low, actual temps are red).  Our high temps have not pushed above normal, but our low temps have dropped below normal on many days.
This cool situation can be traced to the La Nina-like upper air pattern with high pressure offshore and troughing (low pressure) over the Northwest.

But even with this cool weather there have been signs of spring.  Days have gotten longer and the sun is higher in the sky.  Daffodils and other early bulbs are pushing up and flowering (see below)

And, of course, the surest sign of spring is found in front of many supermarkets:

But this weekend the situation is going to change in a big way.  Southerly flow will bring warm air up from the south, and this warmth will be enhanced by low-level offshore (easterly) flow, which will add downslope warming.

There will be no precipitation over the weekend.  Western Washington temperatures will rise into the mid-50s on Saturday, mid-60s on Sunday, and upper 60s on Monday, with some hot spots getting even warmer.   

Let me show you.  Here is the WRF model surface air temperature forecast for 4 PM Saturday.  Warm from Everett to Portland and to the southern WA coast, with highs between 56-60F for everyone, and 60-64F east of Seattle and around Portland.

 But that is nothing.  Here is the forecast for 5 PM Sunday (remember Daylight Savings Time starts that day). 60s everywhere, with upper 60s along the eastern slopes of the Cascades and nearby lowlands, and along the coast.  Look closely at the winds--the warmest spots are associated with downslope easterly flow.  Sunday is going to be amazing.

But Monday will be even more amazing, with temperatures getting up near 70F for many of you....too bad it will be a work/school day .  Here is the surface air temperature forecast for 2 PM Monday. Wow...upper 60s to low-70s from Olympia to Bellingham

Now before we get too excited, let's check on the uncertainty in these forecasts by looking at some ensemble output, which displays the predictions of many forecast runs.  Here is the "plume" display of 21 National Weather Service GFS model forecasts of surface air temperature at Seattle.  (the model runs started at 10 AM today, Thursday).  Progressive warming is found in all the runs, with the highest resolution member (blue) line showing the warmest temperatures (around 67F on Monday).  This makes sense because higher resolution results in higher amplitude downslope warming.

So stock up on sunblock and sunglasses, and pull out those short-sleeve will need them in two days.

For those of you who enjoy some irony, our warm weather is connected to cool, wet weather hitting central  CA next week.   Our warm southerly flow is connected with a developing, very large trough that is extending southward in the eastern Pacific (see upper level map for Monday at 5 PM).   Strong southwesterly flow will head into California, producing lots of rain at lower elevations and snow for the Sierra crests.   Actually good news for them--they need the precipitation.


Aram Attarashany said...

So does this mean we’ll be setting up the AC and fans well before summer starts?

Steve said...

NO!!! We barely had winter!!!!!

tjc said...

I believe you meant "Daylight Saving Time".

John Marshall said...

I wouldn't worry about premature summer. Whatever March, April and May may show us (probably a mix of everything but snow), there is always June Gloom waiting.

That last blast of often month-long cloudy is designed to make us look forward eagerly to the end of the first week of July, when, like magic, the clouds go away completely and Summer actually starts.

Gotta get the big blocking high well established over the Pacific to combine with the on-shore thermal low circulation to push the weather away from us.

But these little teases of Spring, and later teases of Summer, are so very enjoyable, if for no reason other than that we know they won't stay around long. Not before July, anyway.

Tree lover said...

What is on the radar tonight? Migrating birds again?