A Fall Front Approaches

September is a meteorological transitional month in the Northwest.   The jet stream starts to strengthen and move south, while we begin to feel the effects of the first autumn-like fronts of the season.

The climatology of precipitation in Seattle illustrated this.  The first shows the probability of getting a real wetting:  0.10 inch or more.  The driest period is roughly July 10 to August 10.  Things moisten a bit through the end of September, followed by the "great wetting" in October.

A modest Pacific front is now approaching and will reach our region tomorrow.

Below is a satellite image this (Saturday) morning.  There are some low clouds along the coast, but the front is offshore and moving eastward.  A weak disturbance is found behind the front, followed by post-frontal showers.

This satellite image reveals that the air behind the front is not very cold--which is typical for mid-September.  Cold air is associated with deep, cumulus activity, which is absent.

How do I know this? The infrared satellite image at the same time (below), which measures the temperature of the cloud (cold is white and high), indicates relatively warm/shallow clouds behind the front.

Now, let's look at the latest UW WRF model forecast for precipitation (color shading) and low-level winds with the front.  

This afternoon, the front is offshore.  There is a large wind shift with it!

The front makes landfall about 11 PM tonight. Rain on the coast.

Tomorrow morning around 9 AM, precipitation reaches the western lowlands and the western slopes of the Cascades.   Some showers form over eastern WA and Oregon.

As the Pacific air wraps around the Olympics, a Puget Sound Convergence Zone will form over the north Sound, with precipitation north of Seattle

The predicted precipitation total for his event is shown below. A moderate wetting over western Washington and Oregon, with light rain east of the Cascades.  

This event will substantially reduce wildfire activity around the region.