The coming weekend and the Christmas holidays start slowly | FOX 12 Weather Blog

9 p.m. Thursday …

We made it through the 2nd half of December, on average the coldest time of year in Portland. Technically, the coldest time of the year is just beginning and ending in mid-January. December has been a “blowtorch” this year across the United States so far. Almost the whole country was well above normal

It’s a lot of “hot” in 2/3 of eastern Oregon. I see Redmond running 6 degrees above normal. Fortunately, a model change happened last week as the models were advertising. The persistent and warm upper level ridge of mid-late fall is now gone and we see cold troughs descending from Siberia or Alaska in the eastern Pacific, then progressing into the northwestern Pacific. Peaceful. Or they sometimes just linger off. Usually this is a cool / wet model for us. At the moment we have a slight mass of air above us and this continues until Saturday.

Mark Jet Stream1.png

Another cold trough falls over Alaska for the next two days, then digs far offshore on Monday. This is one that some models (over 7 days ago) got us for a cold snap and probably snow. Instead, we get a very humid southwest flow overhead. Fortunately not excessively hot, but enough to drive snow levels up to the passes from Sunday to Tuesday.

Mark Jet Stream2.png

A cold front is moving south over the Pacific Northwest on Saturday and Saturday evening, bringing a nice series of heavy rain. But as the thalweg digs so far offshore, the front becomes stationary over Oregon, even retreating northward. The blocked front lets a LOT of rain fall. Fresh off the miniature train… 00z GRAF gives the Willamette Valley 2.00 ″ + of rain from Saturday evening to Sunday evening!

GRAF Rain Accumulation.png

The new GFS model gives some areas west of the Cascades over 3.00 ″ of rain… we will be watching this closely over the next 48 hours. There are also indications that we will have an “isothermal” atmosphere behind the cold front on Saturday evening and early Sunday. This means that even though the snow level has increased by about 3000 ′, a light wind and very high precipitation rates (large dollar-sized flakes) could allow the snow to stick much lower. Perhaps in the foothills of the Coast and Cascade ranges. That would be down by about 1,000 ′ or so. Everything should be perfect, but let you know that it is a possibility. Both GRAF and GFS suggest this could be the case. In fact, the euro 18z was also referring. We will see…

By next Thursday, the last day of our 7-day forecast, a cool low falls again in the Pacific Northwest.

Mark Jet Stream3.png

Back to the foothills snow and lots of cascading snowfall. Here is the 7 day forecast for Mt. Hood.

7 Day Forecast Cascades Mt Hood Government Camp.png

Then, on Christmas Day (at 9 days), a very large cold altitude trough settled just offshore. It is from the morning Euro run, but other models are similar

Mark Jet Stream4.png

This is the coldest mass of air you can get over the ocean offshore; snow showers to or very near sea level. This is a classic setup for heavy cascading snowfall and cold (mixed) showers in the valleys.

To sum up: At this point, I don’t see an outbreak of cold arctic air for Christmas week, or even as far away as Christmas Day, but snow at relatively low elevations COULD be in. cards in the next 8-9 days.

Enjoy the dry day of Friday! Chief Meteorologist Mark Nelsen

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