Friday, January 13, 2012

Quick verification of yesterday's snow and turning to the Pac NW

Yesterday's snowstorm across the upper midwest is well on its way to moving out of the area.  I thought I'd take a quick minute to look at what the final snowfall pattern looked like and compare it to the model outputs I was showing yesterday.

First, final totals from around the Chicago area are now in.Most areas got at least three inches of snow, with totals up to 8 inches being reported in some areas of the Chicago suburbs.  Here's an analysis map from the WFO Chicago webpage of 24-hour snowfall amounts as of 7 AM today.

Let's compare this to the forecast map I showed yesterday.  The time periods don't match up, so the amounts won't be the same.  However, I'm more interested in the geographical distribution of the snow.
There's actually remarkably good agreement in the extent of the areas being forecast to have snow.  The model is correctly leaving snow out of the Detroit area, and the western edge of the 0.1 inch or greater snow line matches up well with the 1-2 inch line on the analysis.  Also,the areas of the most snow--from northern Michigan and eastern Wisconsin back down through northern Illinois--also agree fairly well.  I'm rather impressed with this model forecast.

However, I want to turn my attention now to the potential for snow where it's more unusual--in the western Washington lowlands.  A shortwave trough aloft is moving down the west coast of Canada, bringing with it cooler air and a setup that's very favorable for snow in the lowlands.  Here's the 500mb setup now:
A large, deep, cold trough is sitting in the Gulf of Alaska.  This trough has brought some of the very heavy snows that Anchorage and southern Alaska have been getting over the past few days.  The same trough is not done yet--by Sunday morning it's forecast to have moved south over the Pacific northwest.  Here's a 48 hour forecast:
The cold air accompanying this trough will linger for several days--through at least the middle of next week.  With westerly flow both aloft and near the surface, the air moving in will contain some moisture as it moves in off the Pacific.  However, the models are all forecasting low-level temperatures near or below freezing to persist.  With "moist" air moving in over sub-freezing temperatures, there definitely is a potential for snow.

And the models are really starting to go a little crazy with this.  Here's our UW WRF 12-km forecast of 24-hour snowfall accumulations ending Sunday night:
Definitely some snow creeping down into the lowlands.  That scale shows around an inch in the Seattle area on Sunday.  There also looks like there may be some convergence zone enhancement to the north of the city.    The mountains are definitely getting hammered.  However, if you look at the breakdown of the precipitation, most of the lowland snow is coming from several scattered showers.  Here's the next 24-hours of accumulation ending on Monday night:
Interesting pattern here that puts a lot of snow to the north and to the south of the Olympics.  This snows 2-3 inches falling over Tacoma, and another inch over Seattle.   Once again, though, this isn't an organized, massive snow event--it's more just snow showers. Here's the next map ending on Tuesday night, though we're starting to get beyond the model's predictive confidence...:
More convergence zone enhancement to the north of the city, and lots of snow in many places of the lowlands.  Another 1-2 inches forecast for the Seattle area.  And finally the map ending Wednesday night:
Wow--4 inches forecast for the Seattle area.  If you sum that all up, that's a forecast potential for up to 8 inches if you believe the longer range forecasts.  But Sunday is just two days away, and the last several model runs have consistently pointed to at least a small amount of accumulating snow in the Seattle area.

There's a lot of factors that could complicate this.  Water temperatures are still above freezing in the sound, so low-level temperatures will have that to compete with to maintain snow all the way to the ground.  There's also concern that the westerly flow that's bringing moisture in from over the Pacific may spend too long over the water, warming the air enough that we don't get below freezing. Also, with this concern about the temperatures, any light accumulations will probably melt off during the day, meaning that we won't see too much on the ground at once.  My main concern is just how many days we're facing the potential for snow.  Even with small amounts, the constant threat of snow could cause some problems, particularly for commuters.

We'll have to watch this.  The coastal radar and the dual-pol capabilities of the radars will be put to good use next week.

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