Wednesday, 4 January 2012

Top 10 Weather Events of 2011 - #5

#5 July Heat Wave
     The week of July 15 to 20 featured the worst heat wave since the infamous double heat-wave of July 2007. Temperatures reached 30 C for 6 consecutive days, 5 of which saw highs of at least 32 C. Even at night, temperatures stayed quite warm, offering little relief to the heat. For instance, The Forks saw 5 consecutive nights where the temperature stayed above 20 C. At the airport, no record highs were broken, although a high of 34.4 C on the 19th tied the record set in 1967. Generally, humidity levels during the heat wave stayed fairly reasonable, especially the further north you went. It was near the end of the heat wave, on the 19th and 20th, that humidity levels became particularly dangerous.
     July 19 featured the peak of the heat. By mid morning, temperatures were already close to 30 C, and with the humidity it felt closer to forty. By afternoon, temperatures peaked in the mid thirties. That, along with dewpoint temperatures in the mid twenties gave humidex values in the mid to high forties. In Winnipeg, the peak humidex of 47.2 was just 0.1 shy of the all-time record of 47.3 on July 25, 2007. Here are some selected highs reached July 19. Records are in bold:

High temp.
High dewpoint
High humidex
All-time humidex record
52.6 (July 25, 2007)
49.7 (July 24, 1960)
47.0 (July 24, 2007)
47.3 (July 25, 2007)
47.3 (July 30, 1975)

     Both Brandon and Dauphin broke their all-time humidex record. Amazingly, Brandon broke it’s humidex record at 7 pm, until temperatures finally began to drop off slowly. Slowly is the key word, considering the dewpoint in Brandon was still 27.4 C at 9 pm, giving a humidex of 45. An incredibly uncomfortable night for sleeping. Overnight lows over southern Manitoba that night were in the low to mid twenties, and humidex values remained in the high thirties all night.
     There was little relief the next morning. At 8 am, relative humidity in Melita sat at 100%, while the dewpoint temperature was 26.3 C. That gave a humidex of 40.
      A cold front sweeping through the south in the afternoon offered relief to the heat, and ended the heat wave.
     On the evening of July 19, history was made in Minnesota when the dewpoint rose to 31 C in Moorhead. It was the highest dewpoint temperature ever recorded in the state, and just shy of the all-time highest dewpoint of 32 C for the entire United States.

     As the heat wave moved east, southern Ontario experienced temperatures in the high thirties and humidex values near 50 on July 21. As well, in Windsor, July 2011 ended up being its warmest month in history, with a mean monthly temperature average of 25.9 C. It was also the warmest month in history for any large Canadian city.

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