Thursday, 5 January 2012

Top 10 Weather Events of 2011 - #4

#4 Wacky December
     This December was anything but normal. With an average monthly mean temperature of -8.1 C, it was the 9th mildest December on record in the city. That is an amazing 6.3 C above normal, and warmer than the normal high for the month! With a measly 5 cm of snow through the month, it was also the 9th least snowy December on record. There was very little snow cover throughout the month with a measly 1 to 3 cm. Outside of the city fields were quite bare, with just a dusting of snow on most days. 27 days had above normal highs, with cold snaps being very brief, and only bringing temperatures closer to normal before rebounding a day or two later. This prolonged spell of warm and dry weather will definitely make winter feel a lot shorter, as it was looking and feeling a lot more like November. 
     It was the same story in many parts of southern Canada. Temperatures were above normal, and snowfall way below normal in many places. As a result, it was a brown Christmas for many. Basically, the only areas which had a widespread white Christmas were the Maritimes and of course northern Canada. It was the same story in the United States, with the majority of people seeing a brown Christmas. And if you wondered where all the snow was, Texas and New Mexico had several snowstorms in December!
Emerson, Christmas morning
     Closer to home, it was a rare brown Christmas as well through much of south-western Manitoba, and through the western and southern RRV. A 2 cm snow depth in Winnipeg ensured that it was officially a white Christmas in the city, although many fields were quite bare. In addition, there were still many patches of open water on the Red River. This is the least snow on Christmas day in southern Manitoba since 1997, our last brown Christmas. The normal snow depth for Christmas day is 16 cm, and the deepest snow depth was 80 cm in 1955. Since snow depth keeping began in 1955, only 7 Christmases have had 3 cm or less of snow on the ground.
Dec 24 in St. Boniface, patch of water on the Red
     It was also a very mild Christmas, as a mild Pacific flow of air moved west. From the 24th to 26th, temperatures were well above zero, and about 13-18 C above the average of -11. Records were shattered on the 26th, thanks to a westerly downsloping wind and sunshine in the afternoon. The day started astonishingly mild in the morning, as overnight lows were above zero, some 20-25 C above normal. Winnipeg's high of 4.5 C broke a 55 year old record. Here are some highs and lows:
Dec 24 high
Dec 25 high
Dec 26 morning low
Dec 26 high

Windsor Park Nordic Centre
     The warm and snow-less month was a nightmare for snow-enthusiasts like snowmobilers and skiers. Snowmobilers were warned to be careful on trails as rivers, streams and swamps still had some open water, and thin ice. As for skiers, ski areas such as Springhill delayed their opening since it was not cold enough. Although snow can be made artificially, it was still too warm for it to stick around.
     Despite being in La Nina, which typically brings colder and snowier winters, we saw the complete inverse. The warm start to winter can be attributed to the Arctic Oscillation (AO). Briefly, the AO is the difference in pressure between the Polar Region and mid latitudes. This December, the AO was strongly positive, causing a strong westerly flow of air over North America. This prevents arctic air from plunging southwards, and keeps it bottled up way up north. The stronger flow also causes systems to move much faster, since the pattern is not blocked. As a result, clipper systems raced across our area rapidly, precipitation lasting a few hours only, limiting our monthly precipitation. At times, there would be half a dozen clipper systems in just over a week.

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