July 12, 2022

The Marine Air is Pouring Into Western Washington---But There is a Danger

 As I write this blog around 6 PM, marine air is surging into western Washington, with strong northwesterly winds pushing into the Strait of Juan de Fuca.

At Race Rocks, south-southwest of Victoria, BC winds just gusted to 48 mph and the latest UW WRF forecast model predicts strong westerly wind gusts, reaching over 40 knots, in the eastern Strait at 8 PM (see forecast map below).


We are now experiencing an onshore or marine push and the cool, dense marine air is moving in at low levels causing sea level pressure to rise.  Since the Cascades block much of the cool air from moving eastward, the pressure remains lower east of the Cascades.   

Thus, the result of the low-level marine air instruction is to create a large pressure difference (or gradient) across the Cascades, as illustrated by the sea level pressure forecast for 8 AM Wednesday (see below).   The lines are isobars of constant sea level pressure, and you will note a lot of them over the Cascades where pressure is changing rapidly.


The Danger

But there is an issue. As the cool air in the west deepens, some of it can push eastward across the Cascades, accelerated by the large difference in pressure.   The air then accelerates as it descends the eastern Cascades slopes causing strong winds.

And therein we come to the danger.  A popular index of fire danger created by several USDA Forest Service personnel (including Brian Potter of the Seattle FS office) is an index called HOT-DRY-WINDY (HDW), which basically is wind speed times a measure of atmospheric dryness.   The forecast of HDW for tonight (Tuesday night at 8 PM) is threatening, with high values over and downwind of the eastern slopes of the Washington Cascades (see below).

The green/yellows/reds are the most threatening.   This high hot-dry-windy index coupled with bountiful grass creates the potential for fire.   Folks need to be careful to avoid ignitions.


With cool marine air settling over the west, temperatures will be at least 10 degrees less on Wednesday and the weather this week should be near perfect west of the Cascade crest (dry and upper 70s to low 80s).  No unpleasant heat waves.

We suffered this spring...not it is payback time. 😁

4 comments:

  1. Are you saying that here in the Northwest we get punished one season and rewarded another? Or is your comment at the end of the post hinting that the current coolness is related to the cold rainy weather in April?

    ReplyDelete
  2. This year, the STP Bicycle Classic is returning after a two-year COVID hiatus. What's your prognosis for the weather on the ride starting Saturday morning? Will the riders get a tailwind down to Portland? Thanks in advance!

    ReplyDelete

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