We have a very cool air mass sitting over us. Even under sunny skies we just barely made it to 40 at the lowest elevations today. The top of the West Hills just scraped above freezing, and at the top of our TV tower (1800′), it stayed below freezing.
Drier air has been gradually working its way in from the north and also mixing down from above. Dewpoints are in the 20s all across the metro area and we’ll probably see them even lower tomorrow, down around 20 or even upper teens. Wind will be a non-factor tonight as well with only 1 millibar northerly gradient from Olympia to Eugene and a very light westerly gradient through the Gorge.
As a result, temps will be in a free fall the next few hours. All of us should be below freezing by 9pm, even in the city. Coldest spots may drop into the upper teens, with the main city areas into the mid 20s. So far this season PDX has dropped to 23; I think we’ll just barely stay above that tonight, but tomorrow night could be a 20-23 degree night out at the airport with an even drier air mass. Here are last night’s temps:
Watch out for freezing fog later tonight. Tonight we’ll see more FROST on the roads (although some black ice is still possible) since roads will freeze first, then the moisture in the air will go straight to the solid state. With some fog, clear ice is possible too as the water droplets freeze to those cold roads.
Not much happening otherwise through the foreseeable future. The only forecast issue next week is whether we get stuck in fog at the lower elevations. Common sense tells me we will, even though mesoscale models don’t show it. I see one forecast sounding for 7am Wednesday shows 20s in the valley, but around 55 degrees at that time at 3,000′. THAT is what we call an inversion. With no significant pressure difference across the Cascades, or north to south for that matter, it’s extremely difficult to believe we won’t get locked into low-level moisture and muck. MAYBE initially starting with this drier air will help…maybe.
Other than that, big ridge of high pressure remains over us through the next 10 days while very cold weather plunges out of Canada and into the eastern USA.
Here are the 12z ECMWF and 18z GFS ensemble charts for 850mb temps since Eric A. asked for them:
Have a nice weekend, it looks perfect for outdoor work with dry vegetation, partially frozen ground, and no cold east wind! That’s a rare combination. Can’t wait to get those chainsaw chaps on and cut something down…
Chief Meteorologist Mark Nelsen
Posted by Mark Nelsen 
FOX 12 Weather Blog 