For the first time in about 20 years I’m sitting here on Christmas Eve figuring out how a “classic” snow/ice event will proceed for the Portland metro area.
Of course in 2008 we had all the snow on the ground and expected a little more the next day, that was easy. You have to go all the way back to 1998 to find a couple of inches of morning snow Christmas Eve; then it melted by evening as the cold east wind died. This time it appears the cold air is here to stay THROUGH TONIGHT. That’s a problem for many of us in the metro area and Gorge. It’s also a change from my post 24 hours ago.
A band of light precipitation is moving through the North Willamette Valley and into the south metro area. It appears to be mainly rain/freezing rain with some sleet (ice pellets) mixed in.
Temperatures are near/below freezing throughout the metro, Willamette Valley, and of course in the Gorge. A layer of cold/dry air pouring out of the Gorge is about 2,000′ thick in the metro and thinner south of us in the valley. It’s a few degrees colder than I was expecting. Where it is above freezing (by 1-4 degrees), the air is dry enough that when moisture falls into it, temperatures fall (evaporative cooling). This means ANYONE could get frozen precipitation of some sort the next few hours from Longview to Salem/Albany. This part of the forecast hasn’t changed. We thought there would be scattered icy areas if/when that first band of precip arrives.
THE MAIN MESSAGE
Models are bringing in this afternoon/evening’s system a bit farther south, prolonging the period that at least 1/2 of the metro area will get freezing rain (and icy roads) the next 24 hours. It appears more of the metro area will see freezing rain than I was thinking 24 hours ago.
WESTERN VALLEYS FORECAST HIGHLIGHTS
- Oregon Coast will be too warm for frozen precipitation, all clear out there
- After patchy areas of freezing rain this morning, some weak daytime “heating” in much of the Willamette Valley south of a Newberg-Wilsonville-Sandy line will be enough to keep roads mainly clear through tonight; temps will hover just above freezing. AFTER late morning, most of the Willamette Valley should be done with icy roads.
- IN the metro area the cold temps mean precipitation with this first band could ice up roads anywhere before noon. As we go through midday and early afternoon, main roads should be fine away from the Gorge as pavement temperatures stay above freezing. Near the Gorge and in the West Hills icy areas will stick around all day.
- After sunset and into this evening, more solid precipitation arrives. It should be mainly freezing rain across the entire eastern half of the metro area north of Oregon City and in the West Hills. Expect widespread icy roads for all these areas through the night and into Christmas Morning. A Silver Christmas for many of us tomorrow morning.
- West of the West Hills and Clark County there may be more snow mixed in with slightly deeper cold air. Temperatures may go a few notches above freezing. Ice on roads should more patchy in those areas.
- Christmas Day temperatures will gradually go above freezing and ice even near the Gorge will melt off. High temps reach into the upper 30s.
IN THE GORGE
- Light snow arrives midday, then picks up a bit this evening and overnight.
- Expect 2-4″ fresh snow central/east Gorge
- West of Multnomah Falls, it’ll be more freezing rain & sleet than snow. Expect glaze ice on roads around Cape Horn and Corbett/Bridal Veil areas as we go into the overnight hours
- Not much melting Christmas Day since cold easterly wind will just weaken a bit.
There are 2 reasons the forecast has changed back to more ice the past 24 hours. First is that the cold air pouring through the Gorge is a few degrees colder than expected. That gives more of us an “opportunity” to enjoy frozen precipitation. And second, sure enough the ECMWF model is looking like the smarter one, bringing in low pressure farther south. Check out morning WRF-GFS low position at 10pm this evening:
There it is…right around Tillamook. This is far enough south to keep the cold easterly wind going in the metro area through the night. But it’s also just far enough north to push mild air over the top of the cold air, that mostly eliminates the snow chances keeping our precipitation in mainly a liquid form. Of course that liquid can refreeze into sleet (ice pellets) or freeze on contact (freezing rain). Notice there is no southerly wind in the metro area even by 7am Christmas morning; temps will rise a couple of degrees in the valley overnight, but we’ll stay frozen through the entire night in that easterly flow out of the Gorge in the entire eastern half metro area.
Be careful on the roads today and if you live on the east side of the metro area I’d plan to stay at home this evening through Christmas morning.
Chief Meteorologist Mark Nelsen