Dry days continue through Saturday, but watching cold/snow possibilities for next week closely!
It’s that time of year...we are now in “primetime” for snow/ice west of the Cascades. The two months from mid-December to mid-February? It’s peak season for region-wide cold blasts or day-long ice/snow storms. It’s rare for either of those to happen outside of this time...west of the Cascades. Models HAVE been teasing us for at least 5 days, hinting that something MIGHT be up for early next week. Some weather apps or forecasts elsewhere would have you think some sort of big freeze or snowstorm is on the way. But, for now, I’m not seeing that. You’ll notice our FOX12 Weather App just has some mixed showers in it for Monday and no “freeze”. For now...
First, we are in a relatively pleasant period for the rest of the week. At one point (last week) it appeared a cold airmass would descend south on the east side of the Cascades, giving us a long period of very cold east wind. That has instead turned into just two days of “normal” gusty easterly wind blowing through the Gorge and east metro areas. That’s Thursday and Friday. At least that will clear out the fog/clouds tomorrow afternoon, leaving us with sunny days both Thursday and Friday. We’ll be staying dry through Saturday too. That’s due to a very strong upper-level ridge sitting over the eastern Gulf of Alaska. It’ll be there through Thursday, blocking all wet Pacific weather systems.
A change arrives this weekend, the ridge “retrogresses” or backs up. Then it amplifies northward into Alaska. That leads to a sudden southward plunge of cold polar/arctic air through western Canada,
By Sunday, this leaves very cold arctic air poised just north of the US/Canada border. This setup can bring snow down relatively close to sea level, but we don’t get a big freeze or guaranteed snow. That westerly flow overhead leads to light (rain) showers Sunday with onshore or southwesterly flow at sea level = mild.
What happens BEYOND Sunday is still up for debate, but models are definitely leaning toward WARMER compared to previous models runs the past few days. The main issue is that (as of this evening) no model is plunging that cold upper-low straight south into the western USA. That would give us a regionwide arctic air outbreak (2013, 2009, 1998, 1990, etc...). In fact, if the upper low doesn’t move any farther south than the position above, that means no arctic air makes it down into Oregon! Notice it still looks about the same on this model (morning ECMWF run) for Tuesday compared to Sunday; not much has changed since the cold low hasn’t moved.
We can get snow to sea level in this pattern, but that’s with onshore flow and we end up with something like we had last Christmas Day. Sloppy/heavy wet snow showers, especially on the hills. For the geeks, we need 850mb temperatures (Celsius) down around -7 to -9 for lowland wet snow in this pattern. Right now I see -5 to -6 Sunday (no snow), and -4 to -6 ensemble average ECMWF temps Monday (no snow). High temperatures those days would be in the 38-45 degree range. Models are hinting an organized wet system could move onshore late Monday or Tuesday though. The Euro has a slightly deeper trough and keeps the solid precipitation south of us late Monday and Tuesday AM. That’s something to watch because a rapidly deepening low pressure center close to a very cold airmass can lead to issues forecast-wise.
What about Tuesday? You’ll notice the last day of our 7 day forecast looks similar; cool, but not really COLD. I’m ignoring this morning’s very cold ECMWF model run. It pushes cold a bit farther south than other models; PDX gets -11 or -12 850mb temps. Unfortunately (or fortunately for my forecasting sanity), the ensemble average is only -4! That means most of those ensemble members aren’t nearly as cold and that’s the better forecast option. Not as fun, but a good move.
Based on all this, our 7 Day forecast, both on TV and app, looks like this:
We could easily get through the next 7 days with no sticking snow. For that reason I haven’t called any of those days a “Possible First Alert Weather Day” because at this point I don’t see any weather that would disrupt your plans in the lowlands of NW Oregon and SW Washington.
That said, keep in mind that cold arctic air will be just to our north Sunday through Tuesday. A slight change could bring moisture onshore and pull at least a bit of that cold air southward or westward through the Gorge. I’ll be watching closely! I don’t have any time off planned until Christmas so I’ll be checking things closely every day.
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