Wednesday, December 29, 2010

Dirty high pressure

I should have known when I washed my car yesterday on the promise of three or four days of dry weather, that snow would soon follow. I awoke this morning to a flurries and some menacing freezing drizzle that has forced road crews to put sown a layer of salt to combat the black ice. As a result no more clean car. So why the precipitation? Well we are under what is known as a dirty high pressure ridge. Normally temperature cools with altitude. In this case we have a layer of slightly warmer air above the surface (an inversion), that is trapping pollutants and moisture at the surface. Add to that a light flow of moisture from a water source, Atlantic or Great Lakes, and presto cloud cover and very light precipitation. The visibility is down to 6km at Dorval this morning with light snow and freezing drizzle (-5C). It should break up by afternoon but could redevelop tonight. Temperatures will be mild for December right into New Years.

Meanwhile the storm in Atlantic Canada is finally loosening its grip on the region with seas and winds subsiding today. The area was hammered yesterday with heavy rain and snow and strong winds. Widespread power outages were reported and another storm surge caused considerable damage. In New Brunswick alone, the price tag from 4 storms this month is expected to exceed 50 million dollars.

Our attention will now turn to another storm taking shape in the southwest US. It is forecast pass well west of Montreal putting us in mild air with showers on the weekend. However if you are traveling north or east of the city, a period of freezing rain is possible. In northwest Ontario, Manitoba and Saskatchewan, it will be heavy snow, windy and cold. We will have more on that as the day and week move along.

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