Monday, December 17, 2012

Now Lowland Snow Showers

Cold air has moved in behind the low pressure center;  in fact, the atmosphere is now cold enough that snow can reach the surface in heavy convective showers.

The latest visible satellite picture shows lots of convection in the cold unstable air approaching the region.
 We now have strong onshore flow of  cool, shower-filled air with marginal temperatures for snow.   The snow level is now around 1000 ft, but heavy showers can lower the snow level by melting and evaporation.  Snow showers have already begun around the region

The flow is generally westerly, which means heavy snow in the Cascades.  But there where will be some locations like Seattle, that will be in the rainshadow (snow shadow!) of the Olympics.

Here is the UW WRF model snow forecast for the 24 hr ending 5 AM Tuesday.  Some snow reaching the surface over the lowlands, but not much.  Lots in the Cascades.

 What if the model is wrong about how much snow reaches the surface?  Lets look at how much total precipitation the model is forecasting over that 24-h period.  You can see the rainshadow around Seattle.

So bottom line:  don't be surprised if there are few snow showers....even some light accumulations away from the water and on hills....particularly away from the lee of the Olympics (e.g., Seattle).

The threat of lowland snow doesn't end tomorrow AM, here is the predictions for the next 24h:

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