July 16, 2020

Why does the Northwest get crazy dry at the end of July?

One of the most extremely anomalies of Northwest weather is the tendency to get get very, very dry in late July.

I mean we are drier than most of the eastern 2/3rds of the U.S..  Drier than Phoenix.

And it happens very, very fast.  To illustrate, here is the probability of getting at least one-hundredths of an inch in a day at Seattle Tacoma Airport based on past climatology.  An amazing plummeting of precipitation probability from around 30% chance in a day for June to less than 10% during the last week in July.   From drizzly June gloom to late July sun.

What causes this rapid desiccation of the Northwest?

One big changes is that high pressure builds very rapidly  aloft during July.   To show that, here is a plot of the climatology of 500 hPa heights above Forks, on the Washington coast.  You can think of this like pressure at roughly 18,000 ft.  The black line shows the median value for each day.  The heights/pressures go up very, very rapidly in July to a peak in late July.

At the surface, the changes are subtle but significant.   In June, the east Pacific high pressure area is centered off of California and lobe extends towards Washington State (see below).  With high pressure offshore and low pressure onshore, cool, marine air is pushed into western Washington and Oregon--thus June gloom.
By July, the east Pacific high had strengthened considerable and pushed northward.  And there is a subtle, but important, change in the orientation and gradient (change over distance) of the pressure lines,  more consistent with northwesterly flow than westerly winds.  Westerly winds move in clouds and also result in convergence zones, with lots of cloudiness in Puget Sound--so these changes help reduce clouds.

Now building pressure aloft is usually associated with increased sinking air.   Let's see if that is happening!

The next figures show the vertical motion at around 18,000 ft for June and July.   Sinking air is red, orange, and green.  Upward motion are the blues and purples.

June has upward motion inland and weak upward motion over western Washington.
 But July has some significant changes with downward motion extending over western Washington

Why do we care about this?  Because sinking motion aloft causes warming and evaporation of clouds (clouds need upward motion to exist).

So it is all kind of subtle.  Building high pressure aloft results in upper warming and downward motion that destroys clouds, while high pressure extending northward results in less onshore westerly flow but enhanced northerly (from the north) winds.

It all adds up to a rapid drying in July and the most spectacular weather in the nation.   And I have a secret to tell you:  it is going to happen this year as well.  Take a look at the latest weather.com forecast below.  Find your sunglasses.  Get your barbecue cleaned up and ready to go...you will need it.


  1. Cliff,

    Wow! Can people have picnics in the park in Seattle now? I'm up for some good summer times.

  2. Oh thank the STARS! I really adore the warmth and sunshine!

  3. Looking for 80 degree plus highs in Bellingham using Bellingham airport records
    Last day that was 80' in 2019 was August 28 at 80'
    Next 80' day was May 9 82'2020 and May 10 81'2020, NON SINCE
    So we have gone 323 days and counting with only two days 80' or above
    We can use some sinking AIR

  4. noooooo!!! bring back the cool rainy days!
    it is too hot out!

  5. Would love to hear your take on this.

  6. Seattle transplant to Annapolis. Heat index has approached 108 each of the last 3 days. Love it! You can actually sit outside without a jacket at night. T-storms to boot, like right now.


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