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7pm Monday...

As we head toward a big weather event, details start to appear in the last 2-3 days. We're just entering that time this evening and here's what I'm seeing...

A FEW KEY POINTS

  1. There is no guarantee we have a snowstorm coming later this week in the metro area. It's just a possibility for now. That's because at this point it's too far out to determine how much we will see, or whether it'll be a combo of snow + freezing rain. By tomorrow I expect us to know a lot more.
  2. We don't know yet whether it'll be a brief snowfall, or the city grinds to a halt for two days with a real snowstorm (Friday/Saturday).
  3. It's unknown whether snow will make it down into the Willamette Valley (Salem/Albany/Eugene)
  4. A snow storm is very likely in the Gorge and Cascades starting Thursday
  5. Models are still in some disagreement about how much moisture we get (how many inches of snow)
  6. I do not see a big arctic freeze (temps below 15-20) in the western valleys of Oregon.

WHAT'S NEW THIS EVENING

First, the GEM (Canadian model) has backed off the extreme cold solution it was showing for a few days. That's a nice change and brings it into line with other models. Second, the GFS model is all over the place, with operational runs showing almost all precipitation heading south of us...snowstorm for southern half of western Oregon but mainly dry and cold over the Portland metro area. A good reason to use ensembles. Those GFS ensembles are almost all wetter than the operational run, so I'll ignore the dry-ish and extra cold GFS for now. I'll assume we DO have significant precipitation on the way.

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Third, cold air arrival timing has adjusted slightly. Cold easterly wind arrives in metro area at some point Thursday or Thursday night. That means no freezing Wednesday night and we wake up to 40 degrees or so Thursday morning.

So at this point the first chance I see for travel-related issues in the Portland area would be later Thursday.

The ECMWF has been somewhat stable for about 4 runs now, showing a classic Gorge and Portland metro snow/ice storm Thursday evening through Saturday. An approaching (weak) surface low pulls the cold air out over us, but not very far south into the Willamette Valley. If this is the case, Salem to Eugene could be left with only rain or spotty freezing rain. And there could be part of the metro area that only gets freezing rain...no snow. This is not unusual in these situations. Look at the latest run for late Thursday night

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Less than 1/2 of the 18z ECMWF ensembles produce snow for Salem Thursday-Saturday, making the point that significant snow south of Portland is less likely than anywhere farther north or east. Sticking snow is far more likely in Seattle than Salem.

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One other way to tell this could be mainly a Gorge and metro event? Check out forecast snow levels from the ECMWF. This says beginning Wednesday the sticking snow level stays above 2,000' through the weekend???

ECMWF Snow Level From 850mb Temps LONG TERM.png

That's because we base this chart off of 850mb temperatures over Portland which typically works in our mild maritime climate. This says that without cold/dry air coming from the east, and deep enough to "fill the gap" in the chart above, snow won't make it down to sea level late this week. Hard to get cold air that deep in the Willamette Valley based on what I'm seeing on the ECMWF.

By the way, take a look at these numbers...the chance of getting at least 1" of snow in Portland beyond certain dates. 

Late Winter Snow Chance 1 or more PDX.png

We don't live in a snowy climate, and this shows how "rare" it becomes after early February.  You can't have too many numbers in a blog post!

That's it for this Monday evening. By tomorrow afternoon I should be able to make some significant calls for who will or will not get snow. Should be able to start looking at amounts too since we'll be just two days out.

Chief Meteorologist Mark Nelsen

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