December 01, 2021

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...


  1. The GEFS is looking favorable for lowland snow after 10 days out or so. Do you think we have higher chances of a White Christmas this year?

  2. This heat wave is much bigger in scope than the one in June. All time December monthly highs were not just set today in Washington,but throughout Montana and even North Dakota as well.Kalispell,Missoula,Helena,Great Falls,Dickinson,and Bismarck all tied or broke their monthly record.

  3. If there was a dollar paid retroactively for every day the temperature was "above average" this year, we are talking a good dinner out at a nice restaurant for two paid in full more than likely...

  4. GFS model on and EU model are showing +20 cm of snow for the 4th of December and the 6th of December. Can you comment on that???

  5. How is this year shaping up overall for average temperature? We've had a couple freak events, but overall it hasn't seemed exceptionally warm. Curious what the data say.

  6. Record breaking temperatures are not surprising if one considers the small spread between all-time and average. For example, the record high today in Yakima is 52F, the average is 43F, only a 9 degree difference between the two. Similar for lows: record 14, average 29. Similar for Seattle: high record 56, average 48; low record 23, average 37.

    1. On the other hand, that's what makes a 20-30 degree deviation from normal so anomalous.

  7. It was impressive, and I made sure to spend a little time outside in Richland. That said, there are a couple relevant details to add. First is that absolute anomalies in June do not necessarily occur with the same probabilities as those in December, so probabilistically, the events may not be equally unusual. In addition, the length of the heatwaves is important. This current heatwave has been sustained for several days like that in June although daily record highs were set 4-5 days in a row in June, which has not been achieved with this event. Second is that context for record breaking depends on the length of the data record. For example, Pasco had several days in the 1890s through 1920s that hit 70F but has had none since then, while many other stations begin their records much later with no days remotely close to 70+F. Third is that siting is also problematic. The Pasco station is currently heavily impacted by agriculture so summertime highs are commonly lower than at Richland Airport or anywhere closer to native vegetation with significantly greater dew points. This was the case for June 29 when Richland hit 118F with Pasco at 113F despite being at the same elevation 9 miles apart. The June event also impacted both sides of the Cascades with the west side getting most of the attention for good reason due to the amplitude of anomalies and societal impacts. While warm west of the Cascades, this event obviously wasn't similar in magnitude or impact, while east of the Cascades, single daily anomalies were similar in absolute magnitude between events, but I'm not sure how they (or multi-day anomalies) compare in terms of probability (data records are likely too short to address this well anyway).

  8. It felt downright balmy yesterday. Mid/upper 50s here with a dewpoint similarly high. Yuck.

    Bring on snowmageddon, please.

  9. One added ingredient was all the moisture on the west side leading to broad area of moist cooling of the air mass before it descended and warmed at the much greater dry rate. Great lesson on lapse rates. Not sure this event will get a rapid attribution report.

  10. "And it is our little secret..."

    We all noticed!
    Dry Side John

  11. Hi Cliff. I'll be interested to hear your take on this report-

    "The deadly heat wave that roasted the Pacific Northwest and western Canada was virtually impossible without human-caused climate change that added a few extra degrees to the record-smashing temperatures, a new quick scientific analysis found."

    1. I blogged extensively about this issue over the summer. This virtually impossible stuff was silly...

  12. How does the number of anomalous weather events compare to other years (record temperatures, record rainfalls, etc.)?

    I have been looking at the impacts of rainfall in Whatcom County and using some farmers' data sets (Valley Brothers, Rod Perry Clearbrook. of 50-100+ years. They show no prior month with rain fall above 14.5 inches and no prior 3-month rainfall above 23.5 inches. Nearly every gauge I look at in Whatcom has beat both of these records. The two I am looking at are in the NE part of the lowlands near Sumas, WA.

    The prior record on these was 14.5" in November in 2006 and 14.25 in 1995 and 13.47 in 1990 (the last major flood in Whatcom County). This year, gauges in the same area are 20.85" @ WSU Lynden.NE, 19.83" @ WSU Nooksack, 18.08 @ WA-WC-79 in CoCoRaHS, and 17.07" in WSU Sumas E.

    For the three month time period Sept-Nov, the farmer's gauges had never exceeded 23.5". This year, 33.77" @ WSU Lynden.NE, 32.32" @ WSU Nooksack, 29.64 @ WA-WC-79 in CoCoRaHS, and 29.3" in WSU Sumas E.

    The single day precipitation records at Clearbrook show only 3 days since 1903 with more than 4 inches of precipitation: 7.77 on 1-24-1935, 4.28 on 11-16-1918 and 4.2 on 5-31-1918. The nearest station to that site is the WSU Nooksack station; it recorded 4.59 on 11-14-2021, or the second largest single day precipitation since 1903 and the largest in any fall day.


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