First, don’t get too excited about snow anywhere close to the metro area tomorrow. Sticking snow level drops to around 1,500′ in the morning and stays there all day. We’ll just see scattered showers the first half of the day so it’s possible anyone could see a snowflake mixed in with the showers but that’s it. Those of you above 1,000′ in the Cascade foothills have a better chance of a dusting early in the day since it’ll be wetter up against the mountains. Same thing in the Coast Range. More obvious will be the change from highs in the upper 50s to afternoon temps just above 40…that winter feeling in the air is back!
Tomorrow night and Sunday we have some modified arctic air working down out of Canada and into the Pacific Northwest. It’s not as cold as the last event two weeks ago and sure isn’t an “arctic blast” with 850mb temps only around -7/-8 and high temps in sunshine around 40 Sunday. But quite a chill again and this time it’s straight from the north instead of coming at us from the northeast. That said, the effect is the same. Easterly gradient through the Gorge builds quickly and by Sunday morning it’s in the 8-10 millibar range from The Dalles to Portland. Sunday will be a sunny, but very cold and windy day across the entire metro area. Windchill temps will likely only be in the 20s and lower 30s all day!
Things get REALLY INTERESTING late Sunday night and Monday…Moisture moves north from the upper-level low off the California coast and forms a sort of deformation band directly over northern Oregon and the extreme southern part of Washington. This sits overhead for more than 24 hours. You can see the convergence of the northern part of the jet and the flow from the south midday Monday…
As of this evening, all models are on board with this general idea, which brings up three questions:
1. How cold will the airmass be? Cold enough for anything in the Gorge to be frozen through all of Monday. PROBABLY not cold enough in the metro area for frozen, although the timing of precip arrival (at sunrise or soon after) would increase the chance for something frozen.
2. How much moisture? This is a tough one because the placement of the precipitation is farther north on some than others. Some models are far wetter than others too. The ECMWF is a real soaker, but keeps the heaviest stuff just south of us, like what happened a couple of weeks ago. It shows 1/2″ liquid over PDX, but 2″ around Albany by early Tuesday! The GFS is a little wetter and farther north showing around 1″ over PDX.
3. What precipitation type? This is the easier one! Hopefully all the forecasters can agree this time that whatever falls west of the Gorge will be in liquid form. The atmosphere 2-4,000′ up is forecast to be even warmer than two weeks ago with forecast soundings showing temperatures as high as 40 degrees 2,000′ or so overhead. It’ll be either rain or freezing rain Monday morning in our area. In the Gorge, freezing rain at the west end and a wintry mix central/east end. Could start as snow and then change to freezing rain.
If heavier precipitation forecasts pan out…the potential exists for a very disruptive snow/ice storm in the Gorge again…That’ll be twice so far this season by December 1st!
I work Saturday and Sunday…actually every day through next Friday…so I’ll update again as we get closer.
Chief Meteorologist Mark Nelsen