Update 3 on Northwest US Heat Wave Predictions

Reposted from the Cliff Mass Weather Blog

June 26, 2021

A One-Hundred Year Heat Wave Event Comes Into Focus

 As we get closer to the big heat event, powerful new forecasting tools are becoming available.  Tools that provide a higher resolution and more nuanced view of the extreme heatwave event that is about to happen.

One such tool is ultra-high resolution numerical weather prediction models.  My group at the University of Washington runs the highest-resolution operational weather prediction system in the region, with a grid spacing of 1.3 km.  High enough resolution to get many of the local water bodies approximately correct, as well as the impacts of our regional terrain features.

Let me show you the surface air temperatures predicted today through Monday…all shown at 5 PM. 

Today, Portland and the lower Columbia Basin surges above 100F and Seattle rises into the upper 90s. The kind of conditions we typically experience once or twice each summer.

Sunday afternoon is a much warmer story.  Portland is above 104-108 and the central Puget Sound away from the water is above 100F.

What about humidity?  Will the air be so moist that evaporation on your skin won’t be effective?  The answer is no.   
Here is the predicted relative humidity at the same time (5 PM Sunday):  most of the region will have relative humidity below 30%.  Sweating or wetting your skin will provide substantial cooling.  Very good.

And then there is Monday, a day that will be the warmest in the past century for portions of western Washington.
Just madness. If you are right on Puget Sound temperatures will be tolerable (80s), but go inland a few miles and temperatures will zoom above 104F.  Go inland a bit more,  temperatures will be above 110F.  I have provided a zoomed-in view for better viewing below.   I never expected to see such temperatures in my lifetime.
Eastside communities like Bellevue, Redmond, and Woodinville will be hit much harder than Seattle, Tacoma, and Everett.

Another powerful tool is high-resolution ensembles of many forecasts, which allow us to see the uncertainty in the forecasts.   

Here is the University of Washington high-resolution ensemble prediction for the temperature at Seattle Tacoma Airport, with the black line indicating the mean of all the forecasts (a good forecast in general).  Today (00/27) has the ensembles on the same page (the upper 90s).  More variations on Sunday, but nearly all above 100F (man about 104-105).  And on Monday the mean is around 110F, with a range of 100-115.   Yikes.

Now I have been somewhat fixated on high temperatures, but nighttime lows are also important because they greatly influence the quality of our sleep and ability to cool down our homes and apartments. 
The ensemble predictions above indicate a substantial increase in daily minimum temperatures, with temperatures on SundayNight/Monday morning only dropping to around 75F.
Wow…that is more than our typical highs this time of year (~72F).  It will be very hard to cool off before the super warm day on Monday.  We will be breaking major low-temperature records–the highest low temperature in history–at many stations.
The good news in all this?  Tuesday will be considerably cooler, but still way above normal.  More on that in my next blog.
When this is all over, I plan to do a detailed examination of this event in the context of global warming.