Flash Heat Wave Hits the Puget Sound Region

Last night you could have comfortably walked around in a tee shirt and shorts in parts of Puget Sound Country.   Temperatures so warm, that would have had to go south to Cabo San Lucas, 1800 miles to the south to equal them.

Conditions similar to Seattle Last Night Were Found Here

A flash heat wave, where some locations around Puget Sound climbed to 66F for a few hours. A heat wave that broke the all time record for that date in several locations (Sea Tac hit 62F). For example,  considering the roughly 3/4 century of records at Sea-Tac airport there have only been three warmer January days: 64° Jan 15, 2018 and  Jan 20, 1981 and 63° Jan 25, 2015.   

Sounds unbelievable?  Let me show you the goods and explain what happened.

Here are the high temperatures yesterday around Puget Sound (click on the image to make it big).  66F around North Bend, located in the foothills of the Cascades.  Around 60F from Seattle southward to Tacoma.

At Sea Tac Airport, the temperatures spiked over a few hours (see plot below).   Our regional trip to Cabo didn't last long.

And there is more. The spike in temperature was associated with a surge of dry air, with relative humidity dropping to around 30%...something shown by the observations on top of the atmospheric sciences building at the UW (see below).  Time is in UTC--0000 UTC 4 January is 4 PM Friday. Look at the third and fourth panels.   Temperature rose to about 60F, dew point plunges to around 30F and relative humidity to 30%.

OK... so what happened?  

A strong Pacific Front was approaching and leading the front there were powerful southerly winds that brought warm air up unto our region.   Below is a plot of conditions at around 5000 ft (850 hPa) on Friday at 4 PM. Dark orange and red are warm temperatures.  The solid lines are heights of the pressure surface.  Winds are parallel to the lines and the closer the lines are together, the stronger the winds (wind barbs are also shown).

So warm air was flooding northward into region due to this well-defined front.  This is confirmed by a plot of winds and temperatures above Sea-Tac Airport over the past day (temperatures are in C), height is in pressure (850 is about 5000 ft), time (in UTC) is on the bottom axis.  A time-height cross section.   Wow... strong southerly wind descended to near the surface last night, with very warm temperatures (16C or 61F) at the peak.

 But why the boost to 66 F in the Cascade foothills and why the low humidity?

The front was associated with a trough of low pressure and as it barreled in, and offshore pressure gradient (pressure decreasing from east to west) developed over the Cascades.  This accelerated air from the east (easterly flow) that produced a zone of downslope flow.  This easterly, downslope flow brought air down to the surface, forcing it to warm further by compression.  Such downslope air is generally dry.  Why?  because moisture levels are usually lower aloft and because the warming air has the capability to hold more water vapor.

The foothills, well positioned to experience the descended air, ends up the warmest location.   The UW high resolution model, in fact, predicted this (see below).  Warm conditions also hit southeast Washington as warm air descended the Blue Mountains of northeast Oregon.

The irony of all this is that we are about to move from the frying pan into the cooler.  But before I blog about this, I am heading to the food store to stock up on bread and milk 😀.  I learned my lesson last time.