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.

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A Fall Front Approaches

September is a meteorological transitional month in the Northwest.   The jet stream starts to strengthen and move south, while we begin to feel the effects of the first autumn-like fronts of the season.

The climatology of precipitation in Seattle illustrated this.  The first shows the probability of getting a real wetting:  0.10 inch or more.  The driest period is roughly July 10 to August 10.  Things moisten a bit through the end of September, followed by the "great wetting" in October.

A modest Pacific front is now approaching and will reach our region tomorrow.

Below is a satellite image this (Saturday) morning.  There are some low clouds along the coast, but the front is offshore and moving eastward.  A weak disturbance is found behind the front, followed by post-frontal showers.

This satellite image reveals that the air behind the front is not very cold--which is typical for mid-September.  Cold air is associated with deep, cumulus activity, which is absent.

How do I know this? The infrared satellite image at the same time (below), which measures the temperature of the cloud (cold is white and high), indicates relatively warm/shallow clouds behind the front.

Now, let's look at the latest UW WRF model forecast for precipitation (color shading) and low-level winds with the front.  

This afternoon, the front is offshore.  There is a large wind shift with it!

The front makes landfall about 11 PM tonight. Rain on the coast.

Tomorrow morning around 9 AM, precipitation reaches the western lowlands and the western slopes of the Cascades.   Some showers form over eastern WA and Oregon.

As the Pacific air wraps around the Olympics, a Puget Sound Convergence Zone will form over the north Sound, with precipitation north of Seattle

The predicted precipitation total for his event is shown below. A moderate wetting over western Washington and Oregon, with light rain east of the Cascades.  

This event will substantially reduce wildfire activity around the region.

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