One of essentially the most extraordinarily anomalies of Northwest climate is the tendency to get get very, very dry in late July.
I imply we’re drier than many of the jap 2/3rds of the U.S.. Drier than Phoenix.
And it occurs very, very quick. To illustrate, right here is the likelihood of getting at the least one-hundredths of an inch in a day at Seattle Tacoma Airport primarily based on previous climatology. An wonderful plummeting of precipitation likelihood from round 30% likelihood in a day for June to lower than 10% over the last week in July. From drizzly June gloom to late July solar.
What causes this speedy desiccation of the Northwest?
One huge adjustments is that top strain builds very quickly aloft throughout July. To present that, here’s a plot of the climatology of 500 hPa heights above Forks, on the Washington coast. You can consider this like strain at roughly 18,000 ft. The black line exhibits the median worth for every day. The heights/pressures go up very, very quickly in July to a peak in late July.
At the floor, the adjustments are delicate however important. In June, the east Pacific excessive strain space is centered off of California and lobe extends in direction of Washington State (see under). With excessive strain offshore and low strain onshore, cool, marine air is pushed into western Washington and Oregon–thus June gloom.
By July, the east Pacific excessive had strengthened appreciable and pushed northward. And there’s a delicate, however essential, change within the orientation and gradient (change over distance) of the strain strains, extra in keeping with northwesterly circulation than westerly winds. Westerly winds transfer in clouds and in addition end in convergence zones, with a lot of cloudiness in Puget Sound–so these adjustments assist scale back clouds.
Now constructing strain aloft is normally related to elevated sinking air. Let’s see if that’s taking place!
The subsequent figures present the vertical movement at round 18,000 ft for June and July. Sinking air is purple, orange, and inexperienced. Upward movement are the blues and purples.
June has upward movement inland and weak upward movement over western Washington.
But July has some important adjustments with downward movement extending over western Washington
Why can we care about this? Because sinking movement aloft causes warming and evaporation of clouds (clouds want upward movement to exist).
So it’s all type of delicate. Building excessive strain aloft ends in higher warming and downward movement that destroys clouds, whereas excessive strain extending northward ends in much less onshore westerly circulation however enhanced northerly (from the north) winds.
It all provides as much as a speedy drying in July and essentially the most spectacular climate within the nation. And I’ve a secret to inform you: it will occur this 12 months as effectively. Take a take a look at the most recent climate.com forecast under. Find your sun shades. Get your barbecue cleaned up and able to go…you have to it.