The North American Monsoon is About to Start: Part of the Northwest Will Get A Piece of It!

 Late June or early July is the traditional start time for the North American (or Southwest) Monsoon, during which moisture streams northward from Mexico into the southwestern U.S., bringing substantial showers to Arizona, New Mexico, Nevada, and sometimes Oregon, Idaho, and eastern Washington.

See the schematic below.  Such monsoons are often associated with an area of high pressure over Colorado and northern New Mexico, which draws moist air from the Gulf of Mexico into the American Southwest.

In a few days, the monsoon will get revved up, and some of the wet action will reach portions of the Northwest.

Let me show you!

Here is the predicted precipitation total over the next 10 days.  Some of the Mexican moisture pushes into Arizona and Nevada, and a small amount will extend into eastern Oregon.

The official NOAA Climate Prediction Center 6-10 day forecast is for wetter-than-normal conditions over the southwest U.S. The far southeastern portion of Oregon gets a wet taste of it.

As expected, this monsoon moisture will be driven by an upper-level high over the 4-corners area that will move moisture into the southwest from the southeast (500 hPa --about 18,000 ft map for each Sunday morning is shown below)

Here is the predicted precipitation for Tucson, Arizona.  Very wet!

There will be some good thunderstorms in Arizona if you enjoy big boomers.

This is not a particularly hot pattern for the Northwest, and Seattle will stay moderate in temperature (see below).

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Announcement

I will hold a special online Zoom session at 10 AM on Saturday for Patreon supporters. Will talk about the latest forecasts for the remainder of the summer and answer your questions!

Cooler Than Normal Water Off the Northwest Coast

One of the reasons Northwest weather is so temperate in summer is the cool coastal waters off our coast.

And this year, we start the summer with surface waters cooler than normal.

This is illustrated by the sea surface temperature anomalies (differences) from normal (below), with blue indicating cooler than normal water.

Below is a map of sea surface temperatures off the U.S. West Coast (°C).   The purple and blue colors indicate the coldest temperatures.   You will need a sweater to enjoy a coastal stroll!

Why such cold water along the coast? 

One reason is the upwelling of cold water from below, as illustrated by the figure below. Such upwelling is forced by northerly (from the north) winds along the West Coast.

Importantly, these northerly winds have been stronger than normal for the past month.

Why? 

Because of stronger-than-normal surface high pressure offshore of the northwest.  This is demonstrated by the figure below, which shows the sea-level pressure anomaly (difference) from normal over the past month.

The red area indicates a much higher-than-normal high-pressure area offshore.

Since winds move clockwise around high-pressure areas in the Northern Hemisphere, this implies enhanced northerly flow along the coast, something shown explicitly in the figure below (which shows the wind differences from normal of the past month).

With this persistent pressure/wind pattern in place, the current 5-day forecasts are for colder than normal conditions over western Washington, Oregon, and British Columbia (blue and green indicate colder than normal conditions)

Keep your sweaters handy!