January 04, 2020

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.


  1. Areas of Monroe had a light dusting of snow this morning. I was surprised to see as we had heavy rain throughout the night.

    1. Yes we woke up with wet snow on the grass, car. In Monroe.

  2. Hahaaa- love the last paragraph. Cant wait to see your blog regarding the changes ahead. Ive been watching this period creep into the extended forecast for a couple days now...

  3. Not exactly on topic but I am curious. I have noticed that most forecasts longer than 5-7 days move toward the averages for those dates. Can you say something about the real weather behavior, data and algorithms that might contribute to this? Are long range forecasts (like the month out daily temps and precip on Accuweather) essentially useless?

  4. GFS or Euro? It's cold either way but GFS is going crazy with the cold!

  5. Yesterday's spike lasted just a few hours here in Glacier, started climbing at 3:47 pm yesterday and peaked at 5:37 pm. It was all over by 6:32 pm. I do have the charts. Altogether, this event produced a deluge as cells passed over (1.13" precip for the 24 hrs thru 7 am) and Mt Baker Ski Area reported 17" new snow. Bye-bye snowpack? Not around here by a longshot. I had an hour of graupel this morning starting at 7:06 am that melted out to .10" "Coasters" need to get a grip on how different mountain conditions are from what's going on in the lowlands. Just one observer's observations.

  6. The warm air extended all the way up the Fraser Canyon in British Columbia. The temperature at Lillooet, BC, 180 km (113 miles) northeast of Vancouver hit 16.5 C (62 F) yesterday. The normal daily max for January there is 1 C (34 F).

  7. aeeeiii Prof Mass is seen buying milk and bread! Quick ye to the grocery! Fight to the death for the last wonderbread loaf [cue star trek fight music]

  8. So you basically just said the cold front is so strong, it drug the warm air up in front of it Like the ocean receding before the tsunami. Would that be about right?

  9. You've left us hanging! My phone's weather app tells me that snow is coming next weekend. (Your bread will be stale by then...) Please give us your thoughts: I love a good snowstorm so I can't help but get excited, even though I know it's too far out to get riled up (or buy extra milk) yet.

    I like to make sure that I have enough ingredients to make a big pot of chili when snow is in the forecast. The neighbors come over, we play board games, we potluck, and I make chili. Oh, and hot apple cider. Maybe I'll make sure I have enough apples in the crisper to make a hot apple cobbler....

    You can have your milk and bread. I've got bigger plans! :-) But will I need those plans? Do tell!

  10. Is Snotel a good source for snowpack? As of today, it shows Stevens Pass at 65% of average and Stampede Pass at just 32%.

  11. 49MPH wind gust in NW Bellingham this morning.

  12. I keep reading weather reports - dismal, dark, damp. And it keeps sun shining and blue skies. Wind is even dying down now. Friday, Saturday, Sunday. ps - not complaining!

  13. The rain shadow has been solidly parked over Kitsap and downtown for the most part for what seems a few years now. Everywhere else its business as usual other than Sequim. They have to get used to all the rain. BTW, you know what they call a place with endlessly sunny weather?



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