Flash Heatwave and Wildfire Threat over Western Washington

 Tomorrow is going to be fascinating. 

 There is going to be a sudden, intense heatwave over western Washington, with temperatures jumping into the mid to upper 80s at many lowland locations.

And then much cooler on Wednesday.

This west-side heat will be associated with powerful easterly (from the east) winds over the Cascade crest and western slopes, which will provide a fire threat.

Consider the temperatures forecast by the UW high-resolution ensemble (many forecasts) prediction system for Seattle.  Tomorrow afternoon, around 4 PM (red arrow), most of the forecasts are for the upper 80s!

Let me show you some forecast maps of surface temperatures.

Tomorrow morning at 5 AM, the region is relatively cool, with the warmest temperatures on the lower western slopes of the Cascades.

By 11 AM, things are really starting to warm on the western slopes of the Cascades, Olympics, and coastal mountains, with some locations in the 80s!

But 5 PM is another...and very warm... story.  Western Washington will be MUCH warmer than eastern Washington, with much of the west in the mid to upper 80s.  Cooler near the water.

Why so warm WEST of the Cascades?  

Because strong easterly (from the east winds) will be descending the western slopes of the Cascades and coastal mountains, and descending air warms intensely by compression.

Take a look at the surface winds at 11 AM tomorrow, with stronger winds shown by yellow and orange colors. The western slopes will be gusty!

Warmer air can hold more water vapor than cooler air, so relative humidity falls rapidly as the air descends.  At 5 PM tomorrow, relative humidity will be below 20% in large swaths of western Washington and Oregon.

Strong winds and dry air can dry out surface fuels and stoke windfires.  In western Washington and Oregon, virtually ALL the big fires are associated with strong winds from the east.

An important tool for wildfire prediction is a parameter called HOT-DRY-WINDY (HDW), which combines wind and dryness.  

The forecast of HDW for tomorrow at 11 AM, with red and orange colors being high values.  Major areas of concern are north of Portland,  southeast of Seattle, and over the northern Oregon coast.

The National Weather Service has a red-flag warning out, which indicates the potential for wildfire on the west side of the Cascades.

The saving grace for this situation is that heat and dangerous values of hot-dry-wind will not be in place for long, and we had some rain this week.  

Still, folks need to be careful.