First Alert Weather Days next week; first winter snow chance in the lowlands

Well, it’s been nice while it lasted! I’ve been on my bike this month more than any winter month in the past. What a stable weather pattern with cold nights in the 20s and sunny afternoons up around 50°. Now that’s changing and it’s real clear that we’ll be flirting with low-elevation snow chances much of the first 7-10 days of February.

KEY POINTS

• Clouds arrive tomorrow, then a rainy day Friday will be followed by scattered showers Saturday and Sunday.
• Over the weekend, sticking snow should remain at/above 1,000′ in the I-5 corridor. But at the lowest elevations (including coastline) you may see snow in the air, especially Sunday, and splats on your windshield
• Scattered showers continue Sunday night through Wednesday, possibly tapering off quite a bit Tues/Wed.
• Expect spotty light accumulations of snow during the coldest parts of each day (overnight + morning commutes)
• There’s a good chance we get school delays and spots of snow/ice on SOME roads those mornings.
• This is NOT a widespread snow and “frozen city” event like last January, or February 2023.
• We don’t expect an “arctic freeze” (highs below 32° in Portland) through at least Tuesday/Wednesday

And here’s our current 7-day forecast. Notice I’m most interested in Monday-Wednesday mornings, thus the First Alert Weather Days (mornings)

Today we’ve tied the all-time (back to 1940) record for consecutive dry days in January. 17 this year! What a stretch of fantastic and invigorating weather.

This is not a record dry January though…we will likely end up with about 1/2 of normal rainfall.

WHAT’S AHEAD

Forecast models are in excellent agreement this evening; pretty good for 4-7 days out in time. They all agree a cold upper-level low drops down. The Euro model Saturday morning shows the cool westerly flow over us overhead, this is 500 millibars, around 18,000′

Between that time and Wednesday the cold trough drops south, then actually backs away from the coastline slightly toward Wednesday. This is the evening GFS model during those 5 days

In this setup we get scattered showers moving onshore through the whole period. We know from past experience that to get sticking snow to sea-level (or valley floor) in this pattern, we need 850 millibar temps (in °C) down to -7 to -9°. For guaranteed sticking down to ground level, you need -9 to -10. In a well-mixed atmosphere, that means Cascade pass temperatures are down around 20° (F). Here’s Monday morning on the latest GFS model

The red line is around -10, notice the coldest air remains west of us over the open ocean. The blue line is the limit of the cold/dry arctic airmass, it’s staying well north of us through Monday, although there may be some “leakage” of that cold airmass later Tuesday and Wednesday down into eastern Washington/Oregon. Models disagree on that a bit. That’s why at this point I’m not expecting a big freeze.

And the reason we’re just going for light showers in the 72 hours from late Sunday through Wednesday? Check out precipitation during that time…less than 1/2″!

The evening GEM model and earlier ECMWF were similar…quite dry during this period. In case you wonder how much snow you could squeeze out of 1/2″ rain? On average about 5″. So if you read that model literally (you shouldn’t do that), you could say “IF EVERY SHOWER WAS SNOW AND STUCK AT ANY TIME OF DAY, the maximum snowfall we could get during those 3 days would be 5″

Of course much of the time it’ll be above freezing so that’s not happening…you get the idea. This is why we keep talking SHOWERS instead of widespread snowfall early next week.

That’s it for this evening, the weather team will be recording a brand new podcast tomorrow evening; that should be out Friday morning at the latest. In these podcasts we take a deep dive into the forecast models and other weather geekery. We’ve got a brand new team member that we’ll introduce as well! You can find those here:

Then I’ll have a fresh blog post out Friday evening…likely before 8pm.

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