Nowcast: Light Snow Reaches Seattle

Snow has reached southern/central Seattle and will push northward during the next few hours.

The system is pushing somewhat northward of what the models suggested this morning--yes, this reflects some of the problems of predicting weak Pacific disturbances that I talked about in my previous blog.

Here is the 6 PM radar image..you can see the precipitation shield reaching Seattle.  But remember the radar beams are above the surface and there is a lot of dry air below, which causes serious evaporation.

The other thing to keep in mind is that the snow is light and temperatures have warmed at low levels--well into the mid to upper 30s today.  And with a cloud cover there will not be good nighttime cooling.  Furthermore, the road temperatures are all above freezing, so light snow will melt initially.   Here are the latest surface air temperatures and road surface temperatures from Seattle SnowWatch--you can see what I mean.

Evaporation and melting will bring the temperatures down over the next few hours, but the main roads should be fine for quite a while.  And I know that SDOT is now revving up a response team.  This is NOT going to be another Atlanta.

My favorite short-term forecasting model is NOAA's HRRR...let me show you the accumulating snow it predicts.  Here are totals for the period from 4 PM through 9 PM, 2 AM Sunday, and 7 AM Sunday.  You can click on the images to expand.  You see the drying along the eastern slopes of the Cascades?  That is due to easterly, downslope flow...or "snoweaters" as some call them.  Quite light near the water and in Seattle..perhaps a half-inch by daybreak over north Seattle....a bit more to the south.  More in the Cascades and over southwest Washington.

This minor snow situation will end by 9 AM on Sunday morning.  And then the atmosphere will start warming.  I bet tonight's snow is the last you will see for a long time...