April 04, 2020

A Dry Five Days, With Welcome Warming, Thanks To Omega

Get out your sunglasses and put away your umbrella--the next five days should be bright and dry here in western Washington.

Who can you thank for this meteorological bounty?  The greek letter omega.

The total precipitation for the five days ending Friday at 11 PM from the European Center model is, well, zero over most of western Washington.
 Bizarrely,  all the precipitation will be going into normally much drier California!  Here is the California precipitation total for the same period.  Wow.  Much of California, particularly in the mountains, will get several inches.  While we are dry.

This extreme difference in precipitation and the persistence of the pattern this week is due to a highly persistent "locking" of the atmosphere called an "omega block."  To illustrate, here are the forecast upper level (500 hPa) heights (like pressure)  for Tuesday evening.  The solid lines are the heights and the colors represent anomalies from normal (blue is lower--a trough, orange is higher than normal--a ridge).

There is a ridge, with two adjacent troughs/lows to the SE and SW.    Doesn't it look like the Greek letter "omega"? (see below)
This omega configuration is very, very stable and can stick around for days (or even longer).  The Northwest is immediately downstream (east) of the ridge of high pressure/heights, thus we will have dry skies and warming temperatures.

Those poor folks in California are downstream of the low/trough, bringing them unseasonably cool temperatures and lots of rain.

Because of the ridge, temperatures will progressively warm during the week.  Here are the temperature forecasts from the European Center model for this week in Seattle.  Temperatures first rise into the fifties and then to 65F on Friday.  Can you imagine how good that will feel?

After getting through a March that was cooler than a  normal January, we deserve this.  For California, the cool/wet weather is good news--filling the reservoirs, upping the snowpack, and delaying the inevitable fires.


  1. Are we looking at the potential for a temperature inversion with this pattern?

  2. What great news, Cliff! I am going to soak up all of the sunshine I can get. Keep up the good work.

  3. It was 30.6F this morning in NW Bellingham - just a couple degrees warmer than the record low for the date.


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