Extreme Heat Day 2: Triple Digits Continue, History-Making Highs on Monday

Update 9:45 AM Monday: Due to the extreme temperatures, dry conditions, and dry downslope winds, a Red Flag Warning is in effect through 8 PM Monday. It is posted below.

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Original Blog: Posted 9:30 PM Sunday

Sunday was the second day in a row of very hot temperatures across the Pacific Northwest. Below are high temperatures for the Central Sound area.

Highs of 100-105+ degrees were observed across most of the area. 

Notice how not all areas reached 100 degrees. Many areas hit triple digits, except for two…Tacoma and Federal Way.

While these two areas were close to 100 (generally 95-99°), the forecast of 100-105 degrees was too high. Why? North/northwest winds off Puget Sound. Even though these winds were relatively weak (10-20 mph), their origin over the Sound allowed for a few degrees of “cooling” in these areas.

However…it will all change on Monday, when the hottest temperatures ever recorded in the Puget Sound area are likely. 

This will happen for two reasons. The first is the thermal trough, and the second is the warming downslope winds, which will be influenced by the thermal trough.

First off…what is a thermal trough? Basically, it is low pressure generated by sinking hot air in the atmosphere, which forms low pressure. This is common in the Desert Southwest, but is rare for the Pacific Northwest.

In the UW forecast below for 5 PM Monday, the thermal trough is outlined in the rectangle.

Since the thermal trough is centered over the I-5 corridor, we will be cut off from any cooling air, and this thermal trough will draw hot easterly winds out of the Cascades, further warming the area.

Now the big question…how hot will it get? Below is the forecast for the area.

Monday:

Valley: 110-115°

Non-valley: 111-116°

The extreme nature of these temperatures is beyond anything ever recorded in Western Washington. To put it in perspective, the average high temperature for June 28th in Seattle is 74 degrees. The forecast high is 110 degrees, 36 degrees above average. The departure from average will make tomorrow the highest above average Seattle has ever been (previously 29° in April 2016’s heat wave). Additionally, Sunday night’s forecast low is 1 degree warmer than the average high. That is unheard of around here.

Then…after we roast in unprecedented temperatures, a large temperature drop will occur, due to the thermal trough moving across the Cascades, opening the door to cooler ocean air.

Below is a temperature profile for Puyallup from 4 PM Monday to 5 AM Tuesday, from the National Weather Service.

Temperatures drop from 112 to 68, 44 degrees, in 12 hours. Because of this marine influence, Tuesday will have highs in the low to mid 90s, hot still but nearly 20 degrees cooler than Monday.

Please continue to practice heat safety measures on Monday. That’s when you need to protect yourself the most. The temperatures in the forecast are historic, and call for us to protect ourselves against them. Please stay safe!

Monday will have a high HeatRisk (below) across Western Washington, with historic highs of 110-115 degrees.

All-time records will be set on Monday, and likely not broken for many years. I bet we will be talking about Monday’s temperatures for a long time.

One final note…due to the expected 90-95 degree temperatures on Tuesday, NWS Seattle has extended the Excessive Heat Warning through 11 PM Tuesday. The updated alert is below.