25 February 2011

 

This snowstorm was an exceptional forecast challenge, as the precipitation transition zone was difficult to forecast.  Forecast guidance depicted a relatively narrow precipitation transition zone just north and west of the Capital District until 12 hours prior to the onset of precipitation when one set of forecast guidance shifted the precipitation transitions zone just south and east of the Capital District.  It was not known if this one set of guidance was accurate or not, so it was difficult to determine whether a major change in the forecast was needed in the near term.  The subtle shift in guidance was so subtle that it was difficult to determine of the shift was enough to warrant major changes to the forecast precipitation type forecasts across the Capital District.  The clearest indicator of major changes to expected precipitation types across the Capital District was in the plume diagram from the GEFS initialized at 00Z 25 February, which changed from a mix to all snow, which was different than the SREF which kept indicating a mix.  However, this was only one set of guidance, and not enough of a consensus, compared to the SREF and the deterministic NAM and GFS.  The precipitation transition zone did shift south and east of the Capital District, and areas from the Capital District north and west received 10 or more inches of snow, including rapid accumulation during the morning commute.  Frequent forecast updates were required through the event as the reality of the shifting precipitation transition zone became clear through monitoring of observations and trends.  The precipitation transition zone was quite narrow, with freezing drizzle in the eastern Berkshires, becoming a mix of sleet and snow along the NY/MA border, to all sleet between the NY/MA border and the Taconics, then all snow from the Hudson River and points west.

 

  

 

Above:  Loops of MSLP from (left to right) the GEFS, GFS, SREF and NAM.

 

 

Above:  Loops of 850 hPa winds from (left to right) the GEFS, GFS, SREF and NAM.

 

   

 

Above:  Loops of 850 hPa temperatures from (left to right) the GEFS, GFS, SREF and NAM.  Note the very tight thermal gradients and slight shift eastward of the 0C line in the later guidance.

 

 

 

 

Above:  Loops of 925 hPa temperatures from the GEFS and GFS.  Note the very tight thermal gradients and slight shift eastward of the 0C line in the later guidance.

 

 

 

 

Above:  Loops of probabilities for 1 inch in 12 hours from the GEFS and SREF.

 

 

 

Above:  Plume diagrams for Albany, NY from the GEFS.  Note all members were indicating snow, and clustering around 1 inch liquid equivalent precipitation on the 00Z 25 February GEFS.

 

 

   

 

Above:  Plume diagrams for Albany, NY from the SREF.  Note the SREF was indicating a mix even as late as the 03Z 25 February initialization.

 

 

Above:  Loop of surface maps showing the track of the surface low.

 

  

 

  

 

Above:  Successive soundings from Albany, NY (top row), and Upton, NY (bottom row) from 00Z 25 February through 00Z 26 February.

 

   

 

Above:  Loops of observed 300 hPa, 500 hPa, 700 hPa and 850 hPa heights, temperatures and winds.

 

 

Above:  Radar loop of reflectivity through the event.

 

Link to the public information statement with snowfall amounts across the region.