March 31, 2021

A Cold Spring That Won't Go Away

 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.

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Announcement

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/
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12 comments:

  1. Perhaps our cool weather will deter those nasty Murder Hornets from progressing down into Puget Sound?

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  2. Yesterday, I encountered a few people who were quick to remark how nice the weather was. Of course I agreed with them but I also didn't have the heart to tell them that the temperature outside wasn't really much warmer than what we should expect for this time of year. I think the recent weather has lulled people into leaving their winter-tinted glasses on!

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  3. Wonder what's the correlation between La Nina and enhanced severe weather seasons back East? Have relatives in middle of Tornado Alley.

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  4. Correction: Other negatives: A delay in the swimming season, a delay in the flowers and vegetables, and gardening in general.

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  5. The other negative is my sad garden. The spinach and lettuce seeds I planted three weeks ago haven't even germinated.

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  6. This begs an interesting question - what percent of influence do the oceans have on land surface temperatures versus the atmosphere? Have the forecasted climate models adjusted over time based on additional years of observation of the ratio of the two influences?

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  7. Spring has not really has not even fully sprung yet where I live in Olympia. the woods still have the wintery look to them. much later spring compared to the last several years. I wont even try and growing cantaloupes this year.

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    Replies
    1. @JimmyT you still have plenty of time. I'm up on the north end of whidbey we rarely see temps higher than 70 in the summer, for melons and squash I don't even think about planting till the first week of May. Build them a simple cold frame/hoop house it'll help warm up the soil temps, they'll love you for it.

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  8. Cliff, so much of this post refers to the difference from "normal" Temperature, pressure, snow pack, rain fall, etc. How do we reach consensus on what is "normal"? Does that adjust over time, and on what cadence? How has "normal" changed over time?

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  9. Considering how much of the areas close to my home completely burned down last September, the later the dry and warm season begins can only be a good thing in the macro. Oregon in general needs a long and uneventful summer and fall wildfire season this year.

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