Monday, 17 December 2012

Major LES Outbreak On The Way

Heres my preliminary forecast for this upcoming LES Outbreak

1) There are major model disagreements with regards to duration of this event, with the euro showing a longer lasting event extending into monday night as a secondary low forms across the northeast and moves northwestwards. This allows the flow to turn more northwesterly across ontario as the pressure gradient tightens. The GFS doesn't show a very strong secondary which allows the low to move NE faster therefore limiting the LES activity between friday night into saturday evening. These details will need to be ironed out as we get closer to the event and models come into better agreement. Short range modelling will also make it easier to pinpoint more exact location of said bands, duration as well as strength.

2) A major Lake effect snow outbreak is likely between friday evening and saturday evening across much of southern ontario downwind of lake huron and georgian bay. With a strong low pressure system developing in the u.s midwest, moving NE and stalling across quebec the first major LES outbreak of the season is likely. Behind the low, mid level moisture will be enhanced as a result of the upper low stalling out over central ontario and quebec. 850mb temperatures will crash into the -5 to -10C range causing delta-t values to be in the 13-20C range. Lake effect temperatures are running above normal which adds to the temperature gradient setting up. With very low wind shear values and good alignment of surface, 925, 850 and 700mb wind direction dentritic snow growth is likely. If 850mb temperatures live up to forecasted values snowfall ratios will fall into the 10:1-17:1 range which supports higher accumulations.

2) With a predominantly NW flow event expected off of lake Huron I expect an area from Kincardine south to goderich and east to south huron to endure the worst of this lake effect snow event. I expect a general 10-20cm band to set up with local amounts possibly greater then 30cm especially if the european solution is correct. London will be spared from the worst of the snows with the heaviest activity to the NW of the city. Nonetheless there is still significant agreement that strong Northwesterly winds will be gusting up to 80kph off of lake huron prompting these bands to shift Southeastwards into the london and even strathroy(briefly to start off the event) areas although these bands will stay weaker in nature and will last for a shorter duration of time, limiting snowfall amounts. In this case i believe that london has a far better shot at more significant accumulations as opposed to strathroy(a trace to 5cm) and areas further southwest. 5-10cm+ amounts are possible across the london area with the LES bands which should shift southwards as the event progresses. 

3) Georgian bay bands will be oriented in a NNW direction to start of the event before shifting to a NW direction by late friday night into early saturday morning as the storm system pulls of to the northeast. These bands will continue into the day on saturday extending into the late evening and overnight hours. Areas effected will range from meaford east to collingwood and wasaga beach and south to barrie. Bands will have the potential to extend further south into the innisfil and newmarket areas although i expect lighter accumulations with shorter lasting bands of snow. The orillia area will most likely get in on some actions but i expect most of the LES to be focused from barrie northwestwards. I expect a general 10-20cm event in this band with local amounts exceeding 25cm.

4) I will post an update later this week as the event draws closer and the short range models shed more light on potential accumulations. I will post my forecast map later tonight or tomorrow when i have time to draw up the map but you get the main ideas from my write up. If you have any question feel free to ask away! 

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