Monday, 2 January 2012

Top 10 Weather Events of 2011 - #9

#9 November Extremes

     November began well-above normal with temperatures 5-10C above average. Highs hovered around the 10C mark throughout southern Manitoba. Things began to change by the second week. The rest of the month featured several bouts of snow. Typically, snow by mid November will stay for the rest of the winter. November 15 is the average date when we normally get our first snowfall that sticks until spring. This November was a little different. Each snowfall was soon followed by a warm-up that melted much of the snow on the ground. This constant seesaw between winter and fall ensured that by the arrival of December, there were still many patches of bare ground with visible grass. This was despite the fact that 20-25 cm of snow had fallen during the month.

     The second half of the month was definitely a highlight. A cold blast of arctic air made its appearance during the third week. Temperatures were 5-15C below the average. Winnipeg registered a low of -22.1C on the 19th, and the mercury only rose to -11.5C in the afternoon. It was even colder out west; -28.2C in Brandon with a high of only -16.2C. Even farther west, Edmonton dipped to a bone-chilling -36.1C on the 20th, where the heart of the frigid air laid.

     A warm downslope southwest flow of Pacific air finally cleared the frigid air by the 4th week. Perhaps it even got a little out of hand, as temperatures rose to 10-15C above average. November 24 featured the peak of the warmth. Here are some highs and lows observed (records in bold) in degrees celcius:

Nov 23 high
Nov 24 high
Nov 24 old record
Nov 24 morning low
7.2 (1907)
10.0 (1984)

Normal for the period:   high -4 C    low -12 C
     Just a few days later, temperatures rose once again. Highs from 4 to 7 C were felt on the 28th, some 9-13 C above the average, but not record breaking.

     Winnipeg ended up with an average mean temperature in November of -2.0C, which is over 3C above average, and tied with 1908 and 1980 for 29th warmest. 22 days were above zero, above the average of 13.

     This November follows a trend. 8 of the last 10 Novembers have been above normal. Some theories do lie on the cause of November’s warm-up, such as the possibility that the lack of arctic-sea ice early in the season means that the arctic is generally warmer than it would be with lots of ice.

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