The Second Great Northwest Heatwave of 2021

Today is nominally the first day of meteorological winter, but remarkably, a large portion of our region experienced one of the great heatwaves of all time-- by some measures the equal of the late June event!

Temperatures rose 20-30F above normal in many locations east of the Cascade crest today, with highs reaching into the lower 70s at a number of locations.   Check out this map of high temperatures today in eastern Washington (below, click on it to expand).  Lower 70s in Pasco and Richland, near the Columbia River, and along the eastern slopes of the Cascades from Yakima to Omak.  People there could not believe it was December.

ALL-TIME December high-temperature records were set in Wenatchee, Ephrata, and Omak and dozens of stations enjoyed daily records (for December 1).

Now, this may seem surprising, but the deviations of the temperatures from normal today (20-30F above normal) were comparable to those experienced during the great June heatwave.   (Don't tell the Seattle Times--it will be front-page news tomorrow).

To illustrate, here are the temperatures at Yakima, WA since June 15th, with the normal highs and lows shown by purple and cyan lines, respectively.  You can see the extreme June heatwave, deviating about the normal maximum by over 20F.  A blue arrow shows the deviation from normal.

But look at the current heatwave on the right hand side. 

Just as extreme!  Today's temperature deviated from normal just as much above normal (the blue arrows are identical in size).  Amazing.

Pasco, in the Tri-Cities, shows the same thing.  Today was JUST as unusually warm as the late June heatwave.

The extraordinary situation today was noticed by the Spokane NWS office, noting that today was equally as impressive as June.

But Why?

You are naturally asking why we are so very warm today.  Just as in June, several meteorological pieces had to come together.  Just different ones.

We started with an atmospheric river north of us, with our region in the warm air to its south.  Furthermore, there was associated strong westerly (from the west) airflow approaching us.  An analysis of the situation at 4 AM this morning at approximately 5000 ft (850hPa pressure level) shows the situation, with the blue arrow indicating the wind direction.  The reddish colors represent warm air.

In fact, the temperature at this level (around 5000 ft) above Salem, Oregon was at record-breaking levels for the date (see plot below, red line shows the record, the black dot was the observation at 4 PM today)

But more pieces had to be right to get the crazy warmth east of the Cascades.

With winds from the west, the air was forced to descend along the eastern slopes of the Cascades.  Sinking air warms as it descends since it is compressed by the increasing pressure at lower levels.

Just like the warmth of an air pump after you inflate a tire.

Sinking air quickly dries out, with clouds evaporating (the warming causes the relative humidity to plummet).   Thus, there was a clear zone to the east of the Cascades, letting plenty of (weak) December sun in to warm the surface (see satellite image at 10:36 AM this morning.

Atmospheric river and a weather system to the north brought warm air to our shores.  Strong winds from the west developed.  A mountain range produced strong downslope flow.

It all came together to produce a historic heatwave.    And it is our little secret...

The last atmospheric river in the series. Will Seattle break the November precipitation record?

The third atmospheric river in a sequence will be directed towards our region tomorrow.

Examining the situation, I was impressed by the rate at which this atmospheric river will be moving moisture into our region.  

Specifically, the integrated water vapor transport (IVT), the product of water vapor and wind speed, is predicted to reach extraordinary levels tomorrow morning off our coast (see figure below).  Blue is high.  Purple levels are quite unusual, as is the great north-south extent of this moisture plume.   

It is also coming almost from due west, rather than from the southwest as was true of most of the moisture plumes this fall.

And the moisture/water vapor "river" of this event extends THOUSANDS of miles across the Pacific (see below).  Essentially from the Philippines to our door.

This event will bring substantial precipitation to the regional terrain of Washington and Brtish Columbia, and with the moisture plume's west-east orientation, will bring one of the most extreme rain shadow situations you have ever seen.   I will show you.

The precipitation for the 24 h ending 4 AM Wednesday is predominantly over southwest BC and Northwest Washington, with huge contrasts (see below).  

Virtually nothing over the lowlands from Olympia to the San Juans, as air descends the coastal terrain.  There is one exception...a narrow Puget Sound Convergence Zone feature over Seattle.  Precipitation increases over southwest Washington where there is less high terrain to the west...and thus less rain shadowing.

3-7 inches perhaps on the western side of the mountains of Vancouver Island and the Cascades of southern BC.  The North Cascades and Olympics get a piece of it.

But nothing east of the Cascade crest until the Rockies and Blue Mountains.

Super rain shadow event, while the potent atmospheric river is hitting southwest British Columbia quite hard.  There will be a dry break on Wednesday.

This has been a wet November, but we won't beat any records in Seattle.  So far we have 10.14 inches....very respectable, but 2006 had 15.63 inches.  We won't come close to that.

But we ARE winning the first-place laurels for September through November rainfall, for which we are slightly beating 2006 and some other years (see below).  No wonder lawns are full of moss.