What Do Warm Marches Tell Us About the Date of the Final Spring Frost?
One of the most extraordinary warm spells in Minnesota's modern climate record is slowly moderating. In terms of the magnitude of the temperature anomaly, the duration of the episode, and the geographic extent; this spell of weather has had few peers in the record books.
Over a 15-day period from March 10 through March 24, the Twin Cities set 18 new temperature records. Eight new daily maximum high temperature records were set and ten new daily maximum low temperature records were established. The mean temperature over the two-week period averaged an astonishing 27 degrees above normal. Additionally, eight new all-time daily high dew point temperature records were set. Details can be found through our article about this mild March .
While these records reference the Twin Cities, similar patterns emerge when reviewing data from nearly any Minnesota recording station. In fact, the same could be said for much of the eastern two-thirds of the United States.
The abnormally warm March temperatures have rapidly advanced spring phenology and hastened early-spring agricultural field work at a pace seldom seen at this time of year. Two questions arise ... 1) What will be the impact of spring frosts when they inevitably occur? and 2) Do exceptionally warm Marches foreshadow earlier-than-average last spring frost dates? The answer to question #1 is of course highly dependent on the nature of the impacted landscape. However, the answer to question #2 can be addressed using historical climate data.
Using the past as our window to the future, it is apparent that a warm March offers us no obvious indication about when the final spring frost will occur. 139 years of Twin Cities temperature data were reviewed. Keeping in mind that much of the latter portion of the record is influenced by the urban heat island affect, the long-term median (half of all years before - half of all years after) date of the final frost (minimum temperature less than or equal to 32 degrees) of the spring is April 28th. Here are the final spring frost dates following the ten warmest Marches in the Twin Cities record:
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 table indicates that the final spring frost date following warm Marches has been very early (April 6, 1878), very late (May 17, 1873), and everything in between. Although the table was created using data from the metropolitan area, it is reasonable to extrapolate the large degree of variation around the median, and thus uncertainty about the future, to other Minnesota reporting locations. Median frost dates for various locales for the period 1948 to 2005 can be found at: http://climate.umn.edu/text/historical/frost.txt .