December has been very mild and a bit drier than average. The mild weather is here to stay, likely through Christmas. Sorry kids, hard to find cold & snow in this weather pattern! But I think we'll be making up quite a bit of ground with respect to rain. A strong westerly jet and atmospheric river will arrive in the Pacific Northwest Saturday night. Expect about 48 hours of moderate to heavy rain (depending on location) across NW Oregon and SW Washington from Saturday night through Monday evening. Notice the west/southwest flow at 500mb midday Sunday
Then by Tuesday a cold trough is pushing the whole system well east and south of us
Look at the precipitable water loop from Saturday through Monday; you can see the moisture plume moving into the region
Integrated Vapor Transport (IVT) shows the situation very well. Strong wind overhead and copious moisture supply combines to create an "atmospheric river" aimed at the mouth of the Columbia River Sunday morning
How much rain? Each model is slightly different of course, but you get the idea...somewhere between 2-4" in the western valleys and 5-8" in the mountains!
This might be enough to produce minor river flooding on the usual suspect north coastal rivers: Wilson, Nehalem, Trask, & Willapa. Since we've been relatively dry with no recent flooding, my gut feeling is we won't see anything widespread in the western valleys. But we'll keep a close eye on it of course.
Beyond Monday, we're headed into a least a brief period of drier weather, chilly east wind, and even some sunshine Tuesday-Christmas Eve. You can see a bit of that precipitation gap in the ECMWF ensemble forecast
Notice temperatures cool off a bit on the ensembles too
But if you want snow? Very unlikely west of the Cascades. Possibly some sort of ice/snow transition around Christmas or beyond IN THE GORGE if that cold east wind is still blowing...maybe.
That 15 day ECMWF ensemble snow forecast now takes us to New Year's Eve. Wow, just no sign of snow/cold on any of the 51 members.
That's it for now, I'm out of time this evening. We've been busy the past few days (and this evening) implementing a graphics change. Nothing too dramatic, but changing all those fonts, banners, backgrounds, colors takes awhile...
Chief Meteorologist Mark Nelsen