A Perfect July 4th Weekend in the Pacific Northwest

With an extreme heat watch over much of the central and northeast U.S., including temperatures reaching near 100 F (see below), it is good to know that the weather over the Northwest will be near perfect over the holiday weekend.

Temperatures will rise into the mid-70s in Seattle on Friday through Sunday, while temperatures in eastern Washington will reach about 90F.  

A forecast map of surface air temperatures around 8 PM on Saturday, July 4,  shows the mild temperatures in western Washington, with reasonable warmth in the Columbia Basin.     Cool in the mountains.

No worries about precipitation on July 4th, as illustrated by 3-h precipitation totals ending 11 PM that day (below).

Obscuring clouds on Independence Day?    Here is the cloud prediction for 5 PM Saturday.   Mostly clearly, except for high terrain in the Olympics.

To make things even better, the cool/wet conditions this week have made regional fuels less flammable, reducing fire risk this weekend (when all kinds of crazy fireworks activity will occur).   See the predicted ERC and Fuel Moisture values below.  There are no reds (serious risk) and only some marginal values (orange).

  But please be careful.  

Wind and Wildfire

When it comes to Northwest wildfires, many only consider precipitation and temperature.

But for most Northwest wildfires during the summer, another element is more important:  strong winds.

Recent grass and range fires in eastern Washington are prime examples of their "windy" origin.

For example, yesterday, there was a grass fire near Winthrop (see below).  Strong winds were crucial.

Another grass/range fire (the Peterson fire) surged rapidly this week in Klickitat County (below).  Strong winds again.

A few weeks ago, large range fires (e.g., Juniper Dune, Tule Rd) were also forced by strong winds.

In a map of recent eastern Washington wildfires, virtually all were associated with strong winds (graphic courtesy of the Washington State Department of Ecology).

More than climate change warming, strong winds are the essential ingredient for the large grass/range fires in the region.  

In fact, cool/wet spring weather can worsen wildfires in our region, since wet/cool springs enhance the amount of vegetation, which inevitably dries out during our typically arid summers.  That means more fuel.

This week was a great example of strong winds, associated with cooler temperatures, leading to rapidly growing range/grass fires.

Consider the winds at Ellensburg from June 22 to today (below).  Sustained winds are shown in blue and gusts with black dots.

A large acceleration of the winds started on June 26, with some gusts reaching over 50 mph!

As the winds increased, temperatures greatly COOLED at this location (see below).

Want to be impressed with how strong the winds have been this week, east of the Cascade crest?   

Below are the maximum gusts yesterday (Sunday) over eastern Washington.   

Wow.  Many exceed 50 mph, and some reach over 60 mph.

Why are there strong, fire-supporting winds when temperatures cool?

When cool air moves into the region from the west, it causes pressure to rise west of the crest, since cold air is denser and heavier.  That causes low-level pressure to rise.   Even if some cool air gets over the Cascades, the depth of the cool air is greater west of the crest, allowing the pressure to rise more to the west.

To illustrate, there is a sea level pressure analysis (solid lines), sustained surface winds (wind barbs), and low-level temperatures (color shading_ at 5 PM Wednesday.   Cooler temperatures to the west and a strong pressure difference over the Cascades.  

A day later,  eastern Washington had cooled, and much stronger winds had spread over the Inland Empire.

This weekend, temperatures will warm, and the winds along the eastern slopes of the Cascades should weaken.