To Snow or Not To Snow

January 19,2009, 7 p.m.

William & Mary may start the spring semester under a blanket of snow.  That’s a big deal around here.  Snow is not common in Williamsburg, and depending on your perspective, it is an unusual winter treat or unexpected headache.  I am quite pleased because the arrival of snow would be well timed for the first day of my Weather, Climate, & Change course that kicks off on Wednesday.

The National Weather Service has issued a Winter Weather Advisory for southeastern Virginia- it is good reading.  “Upper level disturbance will approach from west” and “strong surface low will develop…. tracking northeast and rapidly intensifying through midday”.  The Weather Channel is predicting 2 to 5 inches of snow.  Wow, that’s enough to make college administrators tremble.

The weather map and satellite imagery paint the picture: a series of surface low pressures (L) defining a frontal boundary, cold air to the north and moist air to the south.  Atmospheric circulation, around regions of low pressure, is counterclockwise (in the northern hemisphere) so that ahead (to the southeast) of the low, moist air blows in from the south, which, along the front, overrides cold air to the north.  In January that is a recipe for snow.

From: http://www.intellicast.com/National/Surface/Current.aspx

But will W&M be merely teased by a dusting or walloped under 8 inches?  Whether snow falls on the Sunken Garden will be largely controlled by the position of the frontal boundary as it passes.  It looks good for a white first-day of class, as the northerly flow of air behind the front should keep Williamsburg frozen.

But what if synoptic weather maps, satellite imagery, and computer models were not readily available, would you predict that snow is on the way?

Today’s temperatures were cool, the relative humidity high, a weak wind from the southwest with a sky streaked by elongate clouds in the late afternoon.  And the barometer (measuring atmospheric pressure) has been slowly falling for two days (check out the current and past weather conditions at William & Mary on the Keck Lab’s website, http://web.wm.edu/environment/Watershed/Weather/KeckWeather.htm).  Based on these observations would you confidently predict a snow day?

January 20, 2009, 7 a.m.

So just where is the snow?  I awoke and was immediately disappointed- no white stuff, no snow day for the Presidential inauguration and its attendant celebration.  Twelve hours ago I wrote “It looks good for a white first-day of class”; should the College actually let me teach a course on weather?  I was, however, in good company, as the all predictions called for snow in Williamsburg.  Even with modern tools for meteorological prediction, the forecast, a mere 12 hours out, was in error.  What happened?

The frontal boundary moved on a west-to-east track, rather than a southwest-to-northeast track.  For Williamsburg to have gotten more than mere snow flurries, the frontal boundary needed to track over Williamsburg for an extended period, thereby bringing the moist air from the south to be lifted, condensed, and finally dropped out as snow.  The low pressure and frontal boundary has now pulled well offshore.  At William & Mary, it will be a cold day with a chilling north wind, but the prospects of snow are rapidly diminishing.  Bummer.

From: http://www.intellicast.com/National/Surface/Current.aspx

Weather forecasting is tricky business.  This recent flirtation with snow is grist for the meteorological mill and frames an excellent starting point for the Weather, Climate & Change course.  Let the semester begin.