Through the remainder of Thursday, lowland rain and mountain snow will continue. A Winter Storm Warning remains in effect for the mountains, and the Cascades could pick up another 6-10” of snow through tonight.
Attention will also turn to some lowland locations early Friday morning, where the combination of heavier precipitation during the chilly morning hours could bring snow levels down to near sea level in several locations, with minor accumulations of snow possibly bringing slippery driving conditions.
Lower snow levels Friday morning bring lowland snow threat
KIRO 7 meteorologists have been examining the forecast data and have called for a Pinpoint Alert Day on Friday for the possibility of snowy travel in some lowland spots during the Friday morning commute.
Temperatures will cool later tonight and be in the low to mid 30s by daybreak. Snow levels will generally be around 500 feet, and some minor accumulation of snow (mainly on grassy surfaces) can be expected in locations in the north Sound and north interior, mainly near and above 500 feet.
However, the forecast data we’re watching is pointing to a band of heavier precipitation associated with converging winds just off the surface in the vicinity of the central and south Puget Sound. Heavier precipitation in these colder overnight and morning temperatures could be enough to “drag” the snow level down to nearer sea level by the pre-dawn hours Friday morning.
Focusing on central and south Puget Sound and foothills Friday morning
The spots we’ll be closely watching for a slushy inch of snow accumulation (isolated spots could get somewhat more) will be from the eastern side of the Seattle metro, including the Eastside from Bothell to Issaquah, then south through the South Sound, including Auburn, Tacoma, Maple Valley, and farther south into Thurston County. The best chances of accumulation that sticks around for a bit will still be above 200 feet in elevation, and especially above 500 feet in elevation, but where precipitation is heaviest, snow could briefly accumulate to sea level.
While roadways (especially treated and well-traveled roads) would tend to have faster melting of snowfall, grassy and elevated surfaces like untreated bridges and overpasses could hold onto snow longer before melting.
For places like downtown Seattle, the chances of one inch of snow accumulation are about 30%, but it’s worth noting that with this setup — where we have plenty of moisture but temperatures are right on the edge of rain and snow — many more folks could “see” snow falling, but not accumulating. It could vary widely from place to place, and with elevation differences.
A note about temperature: While we will have plenty of available moisture this time (it has been lacking on some of our recent flirtations with lowland snow), if temperatures near the surface or aloft are even a couple degrees warmer than expected, far more of the area would just get cold rain instead of wintry precipitation. So the chance of disappointment is still high!
Next, a note about snow accumulation vs. lingering snow depth: If an inch of snow falls at a lowland location, once it stops, melting will begin within minutes. The likelihood of hanging onto snow accumulation on roadways, especially, is not high. However, if precipitation is heavy enough, there could be some travel issues for a time. We’ll be watching for any problem spots starting at 4:30 a.m. on KIRO 7 on Friday morning.
Farther north from Everett to the Canadian border, moisture will be less abundant Friday morning, though temperatures would better support some spotty areas of wintry mix or snow early Friday. Travel issues in these areas could occur, but would likely be rather isolated.
Heavy mountain snow continues into Friday
Another foot or more of snow will fall in the Cascades through Friday morning, with further totals of six inches or more through the day. Pass travel will remain very difficult.
Stay with KIRO 7 on air and kiro7.com as we update the potential for brief lowland snow accumulations overnight into Friday morning.