Sunday, March 4, 2012

Lowland Snow Timing

Sunday morning's model runs are holding with a pulse of snow tomorrow morning and early afternoon with the front.  0 to 2 inch for most, but some could get more.  Here are the latest forecast output for the 3-hr ending 10 AM.

This run shows pretty substantial amounts 2-4 inches over north Seattle and the eastside.  The previous model run had much less, so we don't have stable solutions.  However, each run has tended toward more snow and the other model simulation done at the UW (MM5-NAM) shows a similar snow burst.   The model run for the next three hours (ending 1 PM) is show next.   The snow is moving southward with the front.

This would be wet snow.  The situation above is a very unusual one for lowland snow around here.  Temperatures are really marginal, but the models "think" that the large precipitation rates of the front, coupled with cooling air behind , will be sufficient to drive the snow level to the ground. I would expect quite a snow gradient, with far less near warm water and near sea level.  More on hills and away from the water.

Right now my colleagues at the National Weather Service are calling for rain with the front, which is what one usually expects with such situations.  There is considerable uncertainty with this forecast. We will have to examine tonight's model forecasts very carefully to see if they back off.

The implications for the commute are mitigated by the fact we are warm today and the ground temperatures are above freezing.  This wet snow will tend to melt and should melt further during the day on road surfaces.   If wet snow does fall, road salt will really help. (No...I am not on a retainer for the Salt Institute).

3 comments:

smokejumper said...

I wish you guys luck. Like you said, its too bad there's not the usual moisture pool behind this system. It looks chilly.

But what about a sneaky convergence zone? Strong NW flow could set something more diagonal further S. I'll live vicariously thru Seattles weather tomorrow. Dry, cold, and wind at my locale.

Scott said...

What happened to all the great weather we were supposed to get today?

Justin Wilkerson said...

I can say that the UW's GFS and NAM are both showing the possibility of ~1/2 inch of snow in the Portland area. And the NWS right now has a chance of rain/snow mix Monday evening and Tuesday morning with a chance of just snow showers late Monday/early Tuesday, which would correlate with what the models show.

I think it will all depend on exactly how the cold air and the brief showers behind the front will coincide. As you mentioned in your previous post with this setup there won't be very much moisture behind the front, but if the cold air (helped by NW or WNW flow) can move into the valleys and lowlands before all the moisture is gone then I think it is reasonable to expect maybe a little accumulation even at the lowest elevations.

It is definitely not ideal, but I think its possible. My concern would be that things end up slightly different. Since we are now into March the air could stay too warm, the trough may not push as deep (mainly a concern for the Portland forecast I would think), etc.