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Any chance of frost?

March 27, 2012

  This March has been incredibly warm. Our monthly temperature is more than 16 degrees above average, and this will go down as the warmest March in Twin Cities history! We topped out the upper 60s today, and we have reached 70 degrees or warmer on eight days this month. The previous March record for days of 70 degrees or warmer was five, set back in 1910. We have set eight new record highs this month, and have witnessed record warm low temperatures on ten days. On St. Patrick’s Day, we set a new record for our earliest 80 degree reading in any year. You can see a nice summary of our March 2012 records, courtesy of the Minnesota State Climatology Office, at  http://climate.umn.edu/doc/journal/mild_march_2012.htm .

  Despite our recent warm weather, some people are thinking about frost. Could plants, like these emerging daylilies, still be nipped by frost?

  The Minnesota State Climatology Office looked back at the ten warmest Marches in the “modern” Twin Cities climate history, which begins in 1872.  They compiled this list of the final 32 degree or colder reading in each of those years:

Rank Year  Final Spring Frost Date
 1     1910    April 25
 2     1878    April 6
 3     1946    May 12
 4     2000    April 21
 5     2010    May 9
 6     1973    May 17
 7     1945    May 10
 8     1918    April 30
 9     1968    May 5
10    1987    April 23

  The list shows that a warm March doesn’t necessarily mean that we won’t see some frost in April or May. Our final 32 degree or colder reading happened as early as April 6, and as late as May 17, in the years with our warmest Marches. I should mention that light winds can allow frost to form at ground level when temperatures are above 32 degrees at instrument level, but historic frost data is not available. That is why the final occurrence of 32 degrees was used to estimate the dates of our last frost. Our next dip into the 30s could happen late Wednesday night/early Thursday morning in the Twin Cities. Check those forecasts Wednesday night if you have some tender plants to protect.

  I’ll leave you with a photo taken this afternoon at Lake Calhoun. Wind gusts of around 40 mph kicked up some whitecaps on the lake, and the water was splashing onto the walking path north of Tin Fish.  By the way, a sign at Tin Fish said that they expect to open after mid-April this year…another sign of our early warmup.

              (You can left-click on the photo for a detailed view)

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