May 2016: Hot End Moderates an Overall Cool Month

Posted: May 19, 2016, 4:56 pm by bmarmo

After months of cold conditions, May often features some of the most pleasant weather of the year. With temperatures typically in the 60s and 70s, the fifth month is usually a great time to go outside and enjoy some sunshine. Unfortunately, this was not the case for a majority of last month.

May 2016 did begin quite active in the mid-Atlantic, with severe thunderstorms on the 2nd. The storms were triggered by unstable air pushing into a lingering warm front over Maryland. With the help of an upper level trough pushing in, favorable winds, and cold air aloft, multiple t-storms fired with even well-developed supercells. While some trees and power lines were downed by 50 - 60 mph winds, large hail was the biggest story with up to baseball-sized hail reported around Rockville and St. Charles, MD.

May 2nd archived radar of the severe thunderstorms courtesy of the College of Dupage

After the early excitement, overall, cloudy and cool conditions dominated the Northeast and Mid-Atlantic during the first two-thirds of May. The biggest anomalies were across southeastern New York, New Jersey, Pennsylvania and Maryland, where temperatures were around 5 degrees below normal through the third week of the month. In addition, persistent areas of rain also impacted parts of the East Coast. While New England received closer to normal precipitation amounts (some spots have actually finished below average), locations around and south of the Mason-Dixon Line were stuck in an endless barrage of rain during much of the month. As result, portions of southern New Jersey, Delaware, Maryland and Virginia finished 1.5 to 3.0 inches (some spots up to 6.0 inches!) above what you would typically expect for May.

Temperature anomalies across the Mid-Atlantic and Northeast from 5/1 thru 5/23
 

Another interesting occurrence during May was the return of wintry-like conditions over the second weekend of the month. With a strong upper level trough settling over the eastern half of the country, locations from western Pennsylvania through New England received some wintry precipitation. Most snowfall totals were under an inch, though Caribou, Maine did measure around 5.0” on May 16th! While New Jersey and eastern Pennsylvania did not see any accumulating snowfall, graupel was not all too uncommon across the area (graupel is a type of wintry precipitation that occurs when water droplets coat up on a snowflake in the upper atmosphere).

 

So, why were temperatures so unseasonably cool during the first three weeks of May? The predominate pattern for much of the month featured an upper level ridge of high pressure across the northwestern US and western Canada, while the eastern third of the US was trapped under a trough of low pressure. As a result of persistent northerly winds, parts of the Northeast and Mid-Atlantic experienced temperatures well below what they should be. In addition, any warmth was relatively short-lived, as it was quickly followed by cool and cloudy weather. Another potential cause of the unusual temperatures can actually be traced back to the late winter. The weakening of polar vortex in the upper reaches of the atmosphere allowed chilly air to filter all the way down from the arctic into the eastern half of the US.

 

Figure shows the “average” pattern during the first two-thirds of May 2016. The blues basically represent upper level low pressure (and cooler temps) while the oranges and reds show upper level high pressure (and warmer temps)

 

While the first three weeks of May were dominated by the cool conditions, some heat did build in for the end of the month. Memorial Day certainly was the “unofficial start to summer” this year, as temperatures across some spots along the eastern seaboard topped the 90 degree mark multiple times over the holiday weekend. Not only did this heat wave moderate monthly temperature anomalies across the region, but much of New England actually finished May warmer than average.

With a hot summer expected across the East Coast, the cool weather during May could become a distant memory by the middle of this month.

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