As I mentioned earlier, National Weather Service has a Winter Storm Warning out for the metro area for late tonight through tomorrow morning. My thoughts haven’t really changed much since last night’s post.
1. Nice cold air over us, but not as dry as with our last few ice storms, so evaporational cooling won’t have as strong an effect (temps won’t dip much with start of precip).
2. That dip in temp with precip may easily be countered by increasing southerly wind by midnight. This is very different than when a normal ice storm starts and 70 mph wind are gusting out of the Gorge. For example, we usually get gusty east wind here at the station at the beginning of ice storms. That’s due to the easterlies coming down off the West Hills. It’s calm here right now. So I wouldn’t be surprised if at 6pm we are at our lows for the night.
3. If so, then only spots close to the Gorge or in the hills will get freezing rain at the beginning. Forget west of the West Hills, north of Hazel Dell, or south of a Downtown PDX-Gresham line.
4. Cold front passes through around 4am. A sharp switch to SW wind with this EVEN IN THE GORGE means a jump to a well-mixed atmosphere with a 4,000′ snow level. That pushes metro temps into the low-mid 40’s by daybreak! Hard to believe with this cold of air over us now, but I see no good reason to doubt it.
5. As for snow…maybe briefly at the beginning, but seems unlikely with such strong warm advection above.
I don’t think this warrants a Winter Storm Warning in the metro area, but maybe in the Gorge. I’m not sure why the NWS doesn’t just issue a Freezing Rain Advisory instead. Maybe because snow or sleet could be mixed in?
Very simple forecast beyond tonight’s…ridging or split-flow through the foreseeable future. What a dramatic change that follows the calendar….wet November to dry December? Mark
UPDATE 11PM: Moving in quickly now…but the Portland Radar died at 9pm! Nice timing. Looks like temps will stay about where they are. If you are 32 or below, you’ve got ice in the next hour. Drive carefully! By the way…excellent timing by models…10pm to 1am arrival time right on.