The coldest air of the season has reached the Pacific Northwest, and some western Washington locations are already reporting precipitation in the form of ice pellets!
You can see the impact of the cold air on visible satellite imagery taken on Monday afternoon (see below). As frigid cold air from Alaska and northern British Columbia moves over the warmer water of the Pacific, an unstable situation develops that produces cumulus convection......towering cumulus clouds with brief showers.
You can see the cumulus showers on the satellite image (white blobs with clear spaces between them), and they are aligned with the wind direction (red arrows).
Why does cold air moving over water produce showers?
Because it creates a large lapse rate, a change of temperature with height, that causes the atmosphere to convect, with upward motion producing clouds and precipitation.
You could see the convective showers coming onshore late Monday afternoon, as viewed by local weather radars:
As the atmosphere cooled and showers moved in, snow started to fall in the mountains, such as at Hurricane Ridge at around 5000 ft in the Olympics.
Late Monday afternoon, the freezing level was about 1600 ft in the Olympics, which means the snow level was roughly 600 ft. It will decline further over the next few days.
During today (Tuesday) and Wednesday, very cold arctic air will push into southern British Columbia, with some of it leading into western Washington through the Fraser River valley.
The situation Wednesday AM is shown below, with the purple and white colors indicating the coldest air.
Wow. The Arctic will be getting very close to us.
A closer view on early Thursday AM (below) shows very cold temperatures over Washington...cold enough for lowland snow. The only thing missing will be precipitation.
The latest UW model forecast showing predicted snowfall totals through Thursday morning does show some lowland snow from Tacoma southward. Details uncertain, but there is a good chance that some lowland folks will see some flakes, with some localized minor accumulation.
Finally, expect particularly cold air moving southwestward down the Fraser Valley into Bellingham. This is illustrated by the ensemble of many high-resolution forecasts at Bellingham (below). Temperatures will drop way below freezing. Windy as well.
Some wind chills may decline below 0F. BRRR.
It snowed for about an hour last night in north Bellingham. With temps above freezing and warm ground, there was no accumulation.
ReplyDeleteCliff, a couple of weeks ago you made a post saying that we were past the point were we were unlikely to get any really cold weather. As a cycle commuter and messenger I have to point out that that posting seems to have been premature?
ReplyDeletethis is NOT really cold weather.
DeleteIt's all relative. But no, for this area it isn't that far below the average. Even so, I'm ready for spring. But then again, I'm ready for spring right after Halloween!
DeleteAs Cliff said, this is not really cold. Cold enough OVERNIGHT to allow some vestige of snow, perhaps on cars, but not much else. Daytime highs would be much too warm for snow. Now, if it were to drop into the teens/twenties overnight and say barely at freezing or colder during the day, THEN,
DeleteTo finish my statement in replying to Alistair. Then it'd be cold and then we'd have the issue of pipes freezing in older homes lacking insulation. Been there, done that, got the T-shirt.
DeleteThe cold we had late last week was really an overnight thing, and it warmed up to the low 40's during the day.
Thank you for the update, Professor! Winter is undoubtedly making its presence felt in Western Washington after a long absence of wintery conditions.
ReplyDeleteWe received 3+ inches of heavy wet snow just north of Lake Stevens last night, elevation 239’. Power went out with trees on power lines. About half of the snow survived the day and now the temp is dropping. Winter at last!
ReplyDeleteIt did not get as cold in Bellingham this morning as the actual forecast was calling for (mid-20s) much less the above UW 4-km Ensemble mean prediction which was considerably colder even than the actual forecast. It also has not been particularly windy.
ReplyDeleteKBLI reported a morning minimum temperature of between 28F and 29F and the minimum wind chill value was about 15F at just after 8AM.
Maximum sustained winds reached just 20-25mph and the maximum gust was only 38mph - well below advisory levels and quite tame compared to significant Fraser outflow events. The YWL-BLI pressure gradient peaked at around -14mb overnight - well below the -20mb threshold that generally instigates the classic "northeaster".
It is interesting how spotty the snow was. In Snohomish we had rain but a few miles to the north in Lake Stevens there was quite a bit of wet snow but when we went south towards Clearview there was no snow. So, the increase in elevation wasn't the only factor. In addition, we did not see the low temperatures predicted. Here in Snohomish, our temps barely got to freezing last night.
ReplyDeleteI think it was Thursday when I came out of work and I think we had ice pellets, then it was a mixture of rain and snow, mostly rain on the commute home. I think temps that day barely left the low 40's. This was in Kent, heading back to Tacoma.
ReplyDeleteNot untypical for this area February and March as we march towards warm weather, but it can be volatile nonetheless during this transition. Fall can be somewhat the same, though often far less dramatic (atmospheric rivers that cause massive flooding notwithstanding).