A Superfront Will Reach the Northwest Coast on Wednesday Morning

Most weather fronts that reach the Northwest coast are generally wimpy.   

Only a slight change in temperature, a minor wind shift, and a modest change in humidity.  

Quite a contrast to the often strong fronts of the central and eastern U.S.

The reason for our generally unexceptional fronts is two-fold.

First, the fronts reaching our coast are generally aged, declining occluded fronts, compared to the prime, active fronts of the western Pacific.   Cyclones and fronts are driven by large temperature gradients  (changes of temperature over distance) and a strong jet stream, something that exists over the western Pacific, where cold air from Siberia meets the warm water of the Kuroshio current.  Not true off our coast.

Frontal Image Produced by Dall-E Machine Learning

Second, the temperature changes across our fronts are generally weak because they have traveled across thousands of miles of temperate ocean, which progressively warms the cold air following the fronts (see Pacific sea surface temperature below).   Temperature contrasts weaken as a front crosses the Pacific, and temperature contrasts, in turn, drive pressure and wind contrasts.  Thus, our fronts have unremarkable changes in these parameters.

The result:  wimpy fronts.

Tomorrow:   An Exceptional Front

But occasionally, conditions produce a strong, landfalling front on the Northwest coast and that will happen tomorrow (Wednesday) morning.  Let me show you.

A very strong front will reach the Northwest coast early next morning.

Below is the forecast radar image for 1 AM Wednesday morning.  You see the narrow, corrugated line off the coast (I put in a red arrow to help)?  That is the front.  Meteorologists call this a narrow cold frontal rainband, and it possesses intense precipitation and wind shifts.

Three hours later, the front will be on the coast (see below).

This front will be associated with a well-defined surface trough of low pressure and an abrupt shift in winds from southwesterly to northwesterly, as illustrated by the forecast sea level pressure and winds below at 1 AM.

The front will savage the coast and then be weakened on the lee (eastern) side of the Olympics, sparing Puget Sound the worst.

But Puget Sound will pay the piper later as strong winds push down the Strait creating another line of heavy rain and wind shift that will move into the Sound during the afternoon (see forecast radar image for 2 PM below).  A Puget Sound convergence zone will then set up around Seattle after that.

Why is this front going to be so strong?  Because an unusually strong slug of very cold air has been rapidly pushed southeastward out of Siberia and Alaska towards our region, drawn southward by a strong trough of low pressure.

Here is a forecast map of the temperature and winds Wednesday morning at around 10,000 ft (700 hPa pressure, blues and purples are the coldest temperatures).   You can really see the cold, arctic air pushing off our coast.  The Arctic will be paying us a visit!

The uber-cold air is evident in the visible satellite image this afternoon (below).  The mottled, pop-corn area indicates the cold air.

Interestingly, the temperature contrast with the front is stronger at 10,000 ft than at the surface--a real classic around here due to the moderating effects of the mild ocean waters.

Expect a wet/windy day tomorrow west of the  Cascade crest, with noticeably cooling temperatures.  I may not bike to the UW....