[kingstonwx]

Despite a solid consensus among NWP models Sunday for a 10-15 cm snowfall along the north shore of Lake Ontario beginning Sunday evening and ending Monday morning, most areas between Toronto and Kingston, ON only received between 1 and 3 cm of snow.  It was a large forecast bust for the area but it probably could have been mitigated somewhat by a realization that the upstream low level atmospheric environment was quite hostile for permitting much precipitation to reach the ground.  

The 12Z Sunday radiosonde sounding from CWMW (Maniwaki, Quebec) held one of the keys to the disappointing snowfall forecast for Southeastern Ontario.  The station lies northeast of the north shore of Lake Ontario and with a well developed northeasterly flow in place over the region, it provided a powerful hint at what the low level environment would be like once the precipitation-bearing mid cloud layers arrived from the southwest.  The 12Z Sunday sounding showed very dry air below 6K feet associated with arctic air that was spreading across Northern Ontario and Western Quebec at the time.  RH values of about 10% (a 27 deg C temperature-dew point spread) prevailed in the layer between 3-6K feet.  This dry air was being advected over all of Southeastern Ontario on Sunday by a brisk northeast wind.  Precipitation from the mid cloud layers was rapidly sublimated within this dry layer and because the dry air continued to advect in from the northeast all day, the layer had virtually no opportunity to saturate in situ.  Radar returns showed a solid precipitation shield over the region by late Sunday afternoon.  But the classic "donut" radar signatures over Buffalo and Montague, NY and Frankford, ON revealed a high based virga only situation.  The dry air was eating up the moisture precipitating from the mid level clouds.  Overnight however, the dry layer was slowly replaced by a moist layer circulating over the region ahead of the 850 mb low tracking just south of the lower Great Lakes. And light snow finally began reaching the ground after midnight.  By the time the snow began, the strongest isentropic lift and other dynamical factors had shifted to the east coast and prospects for a significant snowfall were over.  

NWP models now have the capability to handle the issue of changes of state of water and the role of latent heat in these changes of state.  But as I understand it, these capabilities are still rather crude in some synoptic situations.  So it remains for the duty forecasters to remain cognizant of factors that can impact precipitation amounts.  In this case, a low level dry conveyor out of an expanding arctic ridge to the north was one factor that resulted in a shortfall in expected precipitation amounts.