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