snow

11am Christmas Eve (Thursday)

Merry Christmas! It's almost here, although a much changed version from any other year...very quiet for obvious pandemic reasons. It's a dark time across the USA. That said, it HAS been nice to see bright sunshine the past two days.

I've been off the last 6 days, but will be back at work tomorrow evening through the middle of next week. We had quite a well-forecast soaking, warm temps, and gusty southerly wind. Now it's quiet except for dense fog in the Willamette Valley and strong easterly wind blowing in the Gorge.

A wet weather system moves inland Christmas Day. With cold air stuck in the Gorge, that means some of you will have a White Christmas! In this case I'll define that as seeing at least a dusting on the ground.

HIGHLIGHTS

  1. Nothing interesting happens through tomorrow morning, except lingering areas of dense fog south of Portland metro and easterly wind gusts 50-70 mph at the west end of the Gorge. Crown Point has gusted to 75mph this morning.
  2. Rain moves inland by late morning tomorrow; it'll be a gray, wet, & cool Christmas west of the Cascades
  3. As that moisture rides over cold air in place, snow begins falling early afternoon from Bonneville Dam eastward to The Dalles and down into north-central Oregon (Dufur, Maupin). Expect 1-5" Christmas afternoon through early Saturday morning in those areas. A White Christmas for Hood River, Stevenson, White Salmon, Lyle, & The Dalles. Least would be at freeway level and at The Dalles, most up around 1,000' and above. Expect there may be some snow on I-84 late tomorrow afternoon possibly through Saturday morning.
  4. Depending on how cold the airmass is, there could be the usual spots of freezing rain/sleet west of Bonneville Dam to hills above Corbett on Oregon side, and hills above Washougal on Washington side of the Gorge. Likely only up above 500-1,000'. East wind will increase further; gusting 60-80 mph midday/afternoon Christmas before suddenly turning light around midnight tomorrow night.
  5. A sudden surge of southerly wind should push up the coastline and Willamette Valley in the late afternoon & evening hours Christmas Day. Most models are relatively weak with the wind, but WRF-GFS suggest gusts to 40 in the valley and Portland metro area are possible around sunset and beyond Christmas evening. We will see...ECMWF isn't on board

A quick look at the "supporting documents". Sea level pressure forecast for 10am Christmas day shows a developing low offshore...

10amchristmasday.gif

Then at 4pm the low has strengthened a bit. Pressure gradient through the Gorge is up to 8 millibars = cold & windy there.

4pmslp.gif

Then at 10pm you see the surge of south wind and tight pressure gradient in the valley. It's important to note that this model is strong than others. Much weaker low would mean just light southerly breezes.

10pmslp.gif

This WRF-GFS model brings 70kt wind down to around 2,000'. Again, the strongest I've seen of all this morning's runs

7pmchristmasdayxsectionUAO2.gif

As for snow, ECMWF has been looking like this for the past 4-5 days! Several inches in Hood River and an inch or so at The Dalles.

ecmwf_totalsnow.png

That's because it's holding in a cold pool at 925mb tomorrow afternoon and evening. It also thinks Hood River and The Dalles only top out in the upper 30s today. If it gets well into the 40s (today), then it'll be obvious modeling has been a bit too cold.

ecwmf_925mb_4pmChristmasDay.png

The brand new IBM GRAF model (along with soon to be extinct RPM) thinks there will be no pool of cold air east of the Cascades and no snow anywhere near the Gorge. I think the GRAF missed the last snow event out there too.

GRAF_SNOW_12z (1).jpg

It's interesting that the 2.5km (very high resolution) HDRPS model (Canadian) brings plenty of snow to the central/eastern Gorge. Strange little no-snow zone around Lost Lake to Parkdale, apparently punching in a little warmer air aloft.

hdrpssnow.png

But just for fun, you can see what it's doing. Slightly warmer air overhead = sleet. Accumulation graphic here "fills in" those no-snow holes.

hrdps-continental-portland-sleet_total-8984000.png

The main message when looking at differing models/maps? You should never take any one image and say "that's going to happen". Far smarter to stick with ensembles, not just of one model but combine several of them together to get a general picture. In this case, "there will be some white tomorrow afternoon/evening and a little silver in spots east of Portland in the Gorge".

That's it for today. Enjoy your Christmas!

Chief Meteorologist Mark Nelsen

 

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