It was a year ago this week that I decided to drive the 500 miles from Manhattan, Kansas to Minnesota to spend time with my family for spring break. On St. Patrick’s Day, we spent the afternoon on the patio at the Brookside Bar and Grill in Marine on the St. Croix, enjoying summer-like weather and delicious burgers. The temperatures were so warm that in the three hours it took us to drive to Marine, eat, and drive home, the ice on White Bear Lake went out! Not only was it the warmest March on record (since 1895 when record keeping began), but also there were over 15,000 records broken across the country. A blocking high-pressure ridge east of the Rockies, a jet stream straddling the U.S.-Canadian border, and warm air advection out of the Gulf of Mexico brought temperatures to 20 to 40 degrees above average.
In contrast to a year ago, our weather this year has been dominated by meridonal flow (north–south) that has brought warm air from the south, cold air from the north, and a range of precipitation from sleeze to snow, and all points in between. Couple the meridonal flow with a trough of low pressure over the eastern US, a jet stream parked over northern Iowa, and you’ve got the recipe for a good ol’ fashioned Minnesota March!
We can also add two teleconnection patterns (long time scale variability) into the mix. The North Atlantic Oscillation compares atmospheric pressure between the Icelandic Low and the Azore High, which ultimately controls the strength of the Westerlies. The positive phase of the NAO is associated with above average temperatures in the east, with the reverse occurring during the negative phase. Naturally, March 2012 saw abnormally positive NAO values, while this year they are highly negative, ushering in our cold weather. The Arctic Oscillation measures the amount of Arctic air that penetrates the mid-latitudes; with the positive phase associated with low pressure over the Arctic Circle, largely zonal flow (west to east) and warm weather. The negative phase is associated with high pressure over the Arctic, meridonal flow, and an influx of cold weather from the north. I’ll let you guess what phase of the AO we’re currently experiencing…
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