A look at weather extremes and the big-picture climate connections.

Historic March Heat Wave Sets New Milestones

The March heat wave continues to shatter longstanding records from the Upper Midwest to the Northeast, with more than 2,200 warm temperature records set during the month so far. It’s quite possible that this March heat wave will be considered an unprecedented event in the U.S. historical record, which extends back to the late 19th century, based on the margin by which records are being exceeded, the wide geographic scope of the heat wave, the duration of the event and the time of year when it is occurring. 

“This will be a March event that we’ll look back on as one of the big March events of modern history,” said Deke Arndt, who leads the climate monitoring branch of the National Climatic Data Center in Asheville, N.C.. “If it’s not unprecedented, it’s definitely very impressive.”

According to the HAMweather website, 1,192 record daytime highs were set in the U.S. from March 12-18, along with 708 high minimum temperature records. This compares to just 66 coldest maximum temperature records, and only eight records for the coldest overnight low temperature. More records are likely to be set today through the end of this week, when a cooler airmass finally moves eastward (as it does so, it may spark rounds of severe weather). This data may be missing some records set after March 15, since there have been some problems obtaining data from the National Climatic Data Center's website.

According to the CapitalClimate blog, so far this month warm weather records have been outpacing cold records by a lopsided ratio of 19-to-1. Since January 1, the ratio has been closer to 14-to-1.

Temperature departures from average on during the period of March 1-18. Credit: NOAA/Southern Regional Climate Center.

In a long-term trend that has been found to be inconsistent with natural variability alone, daily record-high temperatures have recently been outpacing daily record-lows by an average of 2-to-1, and this imbalance is expected to grow as the climate continues to warm. According to a 2009 study, if the climate were not warming, this ratio would be expected to be even. Other studies have shown that climate change increases the odds of extreme heat events.

The exceptional heat is especially evident when viewed location by location.

Take, for example, the case of International Falls, Minn. Known as the “icebox” of the nation, the city is famous for its frigid temperatures and late onset of spring. Yet on March 18, the temperature at International Falls reached 79°F, setting a record for the month of March — beating the record of 77 degrees that was set just the day before! Before this heat wave, the warmest temperature during the month of March was 73°F, set in 1960.

In Chicago, the National Weather Service described the heat wave as “historic and unprecedented.” Chicago has recorded five days in a row in which temperatures reached or exceeded 80°F, shattering the previous record of two consecutive days. When Chicago hit 82°F on March 16, it was the earliest it had ever been so warm there — breaking the old record by more than a week. 

U.S. temperature records set between March 12-19. Credit: HAMweather.com.

Prior to this year, there had been just 10 March 80-degree days in Chicago, which means on average in March there’s an 80-degree high once every 14 years or so. Average temperatures for the month are on track to exceed the record for the warmest such month on record in the windy city. “It is likely that Chicago and Rockford will not only break… but shatter their current record warmest Marches,” the Weather Service stated.

Milwaukee, Wisc. is also on track to set a record for the warmest March, beating the old record by 3.5°F. Even with cooler temperatures in the forecast for late this week, it’s likely that Milwaukee and Madison will both set records for the warmest three months of the year.

Just as it was a feature of the 2011 summer heat wave, this event has also seen many overnight low-temperature records being set. In fact, in parts of the Upper Midwest, some of the overnight lows threatened daytime-high temperature records.

In Madison, the temperature only got down to 60°F on Saturday — an all-time March record-high minimum temperature. The average temperature in Madison that day was 70°F, which is the warmest average temperature of any March day on record there.

Average temperature in Chicago during the warmest Marches on record, showing how much warmer this month has been so far. Credit: National Weather Service.

In Minneapolis, Sunday’s record high of 79°F was 37 degrees above average for the date. The Twin Cities recorded seven record highs and five record warm nighttime lows since March 10, according to meteorologist Paul Douglas.

Minneapolis set records for the earliest 80-degree temperature on record (March 17), and the maximum low temperature for the month (59 degrees on March 17).

“Hyperbole aside, I honestly can't remember anything like this, even back in 1988, when Minneapolis-St. Paul experienced 44 days above 90. I'm afraid we've run out of red dots,”  Douglas wrote on his Minneapolis Star-Tribune weather blog, referring to the red dots signifying warm temperature records on weather maps.

Daily high temperature records have also been set in the Northeast, where Boston reached 74°F on March 18, for example. The warm weather is expected to continue through most of the week in this region.

The weather pattern resulting in this heat wave is remarkably similar to ones that lead to prolonged periods of extreme heat during the summertime, with a huge dome of high pressure blocking the eastward movement of storms and cooler air off the Pacific. Some meteorologists are studying whether global warming is leading to more such “blocking patterns,” and there are some indications that this may be the case, although there is considerable uncertainty surrounding this question.

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