April 23, 2021

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A Cold Spring That Won’t Go Away

2 min read

 Today, was the first day this spring in which the temperatures got above normal in Seattle (see below, average highs and lows are shown by the purple and cyan colors).  

Yes, temperatures rose to an unimaginable 61F.  It felt very good, particularly since the last few mornings have been MUCH cooler than normal, with lows in the mid-30s.

The recent cool weather has not been limited to Puget Sound but extended over the entire West Coast, something illustrated by the figure below, which shows the difference from normal of the daily average temperatures for the past two weeks.  

Most of western Washington, Oregon, California, and the southwest states were more than 2F cooler than normal.   Chilling statistics

The cause of this icy reign?    La Nina and the associated and very persistent atmospheric circulation pattern it sets up.   This pattern is illustrated by the upper level (500 hPa–about 18,000 ft) weather map for 2 PM last Wednesday (March 24th).  

A high amplitude ridge of high pressure is offshore (the H is the graphic) and this produces strong (and cold!) northerly flow, chilling the West Coast.

To show how persistent this pattern has been, here is a map of the difference from normal (the anomaly) over the last week of the heights at 500 hPa pressure–think of it as the difference in pressure from normal at 18,000 ft).    Much higher than normal pressures (red colors) over the eastern Pacific and lower than normal over the continent.  That implies cool, northerly flow.  Very typical of La Nina years.

So what does the next month look like?  Below is the forecast from the European Center of the temperature difference from normal over the next month.

Yikes!  Colder than normal over the Northwest.

This cold air will foster late-season snowfall.     

We start with much above normal snowpack, with much of the Washington State 140% or more compared to normal.

With this base, the European Center snowfall forecast for the next month shows bountiful snow in the mountains

Late season skiing.  Check.  Plenty of water from snowmelt this summer. Check.  Lower wildfire threat at the upper elevations. Check.

The only negative will be a delay in the higher elevation hiking season.


The Northwest Weather Workshop, the annual gathering to talk about Northwest weather, climate, and major meteorological events, will take place on May 1, 2021. This year we will have a special session on the meteorology of the September 2020 regional wildfires. The meeting will be online. More information, the agenda, and registration information is found here: https://atmos.uw.edu/pnww/

noreply@blogger.com (Cliff Mass Weather Blog)

2021-04-01 03:58:00

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