Snow Threat Evaporates

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I just took a look at the latest model runs---both the UW and NWS models--and I had quite a shock.   The lowland snows suggested in the previous model runs were GONE.  To illustrate, here is the 24-h snowfall for the period ending 4 PM Monday.  Lots of snow in the mountains, but virtually nothing in the lowlands.

 So what is going on?  This is really an interesting case and reveals some dilemmas in how we forecast and communicate our forecasts.

As I noted in my earlier blog, getting much snow during a frontal passage, as will happen tomorrow morning, is very, very unusual.  It is going against all my experience and other forecasters I talked to about.    But the models "thought" it was physically reasonable.   Things would have to be just right...cold air moving in aloft, enough precipitation intensity to drive the snow level to the surface, etc.   It appears that relatively subtle changes in frontal intensity, precipitation timing, and other features caused the model to back off on the snow.

There is a real lesson in this event, a lesson discussed at the NW weather workshop yesterday.  We really need to get away from providing single value deterministic forecasts that can change in time sharply like this, but rather move to probabilistic predictions based on ensembles, providing the odds of snow and other parameters.

By the way, much of the rest of the forecast has not shifted that much, but because the snow forecast was so excruciatingly sensitive, minor variations in the solutions made a huge difference.

The latest forecasts indicate that the precipitation should reach us around 4 AM and be over around 9 AM.  There is substantial uncertainty in the snow forecast...We will know soon....
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