Snow Update will be posted at 4 PM Thursday, Snow has spread over Portland into SW WA at 1:30 PM.
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It is going to snow Thursday evening at most locations in western Washington--even near sea level.
And if you want an impressive accumulation of snow you will be disappointed, unless you are in a few favored locations. Many will only see 1-2 inches. Some will only have a dusting.
And there will be lots of wind for those near the western Cascade foothills.
And yes, the forecast timing has shifted a bit: a few hours later.
The origin of the snow is clear: a warm front is approaching from the south and we have cold, dry air over us--air of continental origin. Dry is important, because that allows evaporative cooling when precipitation from the warm front falls into it..
The forecast map for 1 AM Thursday shows the front and associated trough off northern CA (this map shows sea level pressure in solid lines and lower atmosphere temperatures--actually at 925 hPa-- in color). Note the large change in pressure over the Washington Cascades: that will produce strong easterly winds.
By 10 PM Thursday, the front is off the SW WA coast, with cold air still over western WA.
Run after model run has progressively weakened the warm front, with less precipitation. That reduces the potential snowfall. Furthermore, it has slowed down as it weakened. The latest models hold off the snow until right after the evening commute--so less worry about driving tomorrow. If the current models are right, snow will begin in Olympia around 7 PM and Seattle around 9 PM.
But a weaker front will be slower to scour out the cold air and will force less snow-eating easterly flow.
OK...you want to look at the snow forecasts....so let me not delay you anymore. Here is the 24-hour total snow ending 4 AM Friday. A big east-west snow gradient across the Sound. Virtually nothing over the western Cascade foothills where easterly, downslope flow will be strong, and 3-4 inches over the western Kitsap Peninsula. Seattle gets 1-1.5 inches, with more on the northern side of the city. Half a foot in the mountains. This is not a very wet system.
By 4 AM, the precipitation will turn to rain over the lower elevations. So the early Friday commute could be a bit slushy, but by later in the morning driving should be ok.
Is there some uncertainty in this forecast? Of course. The latest National Weather Service short-range ensemble forecast (SREF) snow prediction for Seattle Tac shows an ensemble mean (the average of all the forecasts) of 1.5 inches at the airport, with quite a range of forecasts (from nearly nothing to 6 inches).
The European Center ensemble system has 1-2 inches at Sea Tac for most members for the late Thursday event, with more over the weekend. But perhaps 30% of the members have nothing.
Based on these and other ensemble systems, there is a good 30-35% chance that the city will only get a dusting.
It will be nice to see snow tomorrow evening and equally nice to know it won't stick around long in the urban areas.
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It is going to snow Thursday evening at most locations in western Washington--even near sea level.
And if you want an impressive accumulation of snow you will be disappointed, unless you are in a few favored locations. Many will only see 1-2 inches. Some will only have a dusting.
And there will be lots of wind for those near the western Cascade foothills.
And yes, the forecast timing has shifted a bit: a few hours later.
The origin of the snow is clear: a warm front is approaching from the south and we have cold, dry air over us--air of continental origin. Dry is important, because that allows evaporative cooling when precipitation from the warm front falls into it..
The forecast map for 1 AM Thursday shows the front and associated trough off northern CA (this map shows sea level pressure in solid lines and lower atmosphere temperatures--actually at 925 hPa-- in color). Note the large change in pressure over the Washington Cascades: that will produce strong easterly winds.
Run after model run has progressively weakened the warm front, with less precipitation. That reduces the potential snowfall. Furthermore, it has slowed down as it weakened. The latest models hold off the snow until right after the evening commute--so less worry about driving tomorrow. If the current models are right, snow will begin in Olympia around 7 PM and Seattle around 9 PM.
But a weaker front will be slower to scour out the cold air and will force less snow-eating easterly flow.
OK...you want to look at the snow forecasts....so let me not delay you anymore. Here is the 24-hour total snow ending 4 AM Friday. A big east-west snow gradient across the Sound. Virtually nothing over the western Cascade foothills where easterly, downslope flow will be strong, and 3-4 inches over the western Kitsap Peninsula. Seattle gets 1-1.5 inches, with more on the northern side of the city. Half a foot in the mountains. This is not a very wet system.
By 4 AM, the precipitation will turn to rain over the lower elevations. So the early Friday commute could be a bit slushy, but by later in the morning driving should be ok.
Is there some uncertainty in this forecast? Of course. The latest National Weather Service short-range ensemble forecast (SREF) snow prediction for Seattle Tac shows an ensemble mean (the average of all the forecasts) of 1.5 inches at the airport, with quite a range of forecasts (from nearly nothing to 6 inches).
The European Center ensemble system has 1-2 inches at Sea Tac for most members for the late Thursday event, with more over the weekend. But perhaps 30% of the members have nothing.
Based on these and other ensemble systems, there is a good 30-35% chance that the city will only get a dusting.
And winds? The easterly winds have already started to rev up...here are the max gusts for the past 24 hr around the region (mph, locations with 31 mph and more). Mountain stations are already getting above 50 mph and foothills reports have jumped to 40-45 mph. They will get stronger during the next few hours.
It will be nice to see snow tomorrow evening and equally nice to know it won't stick around long in the urban areas.
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Help Weather Prediction Research and Enjoy a Great Free Weather App for Your Smartphone: uWx
At the UW, we have developed a wonderful FREE weather app for Android smartphones that also collects pressure for use in weather forecasting. If you want to try it, please go to the Google PlayStore and download it.
At the UW, we have developed a wonderful FREE weather app for Android smartphones that also collects pressure for use in weather forecasting. If you want to try it, please go to the Google PlayStore and download it.
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