After a yet another bitterly cold and snowy winter, it's time to start gearing up for a transition to severe weather season. Every spring, we trade in our shovels and jackets for t-shirts and umbrellas as the cold and relatively dry conditions are replaced by heat and humidity. Coincidentally, it is the engagement of these two air masses that leads to the formation of severe weather across the Northeast. However, the Northeast (especially along the coast) has the added threat of sea breezes causing storm development. This occurs when the land becomes warmer than the ocean, causing breezes to develop off of the sea, which then allows for additional uplift and the threat for thunderstorm development inland.
Above: How a thunderstorm develops from a sea breeze (Image courtesy of Wikipedia)
There is clearly a peak season for thunderstorms and severe weather in the Northeast, and in general, this peak occurs from June through August. On average, the region sees anywhere from around 20 to 40 thunderstorm days per year (could see multiple storms for each of those days), with the lower end of that range toward coastal New England and the higher end into the Mid-Atlantic.
The reason for this peak is due to cooler air retreating north, with warmer air and moisture following it from the south, creating an unstable atmosphere that is conducive for thunderstorm development. This warm and moist air rises and condenses in the colder air aloft, helping to create a towering cloud which has the capability of producing heavy rain, hail, wind, and even tornadoes under the right conditions. Early in the season, the process is assisted by stronger dynamics within the atmosphere, allowing for stronger cold fronts and upper-level winds, which lead to more intense lift for thunderstorm development.
Speaking of tornadoes, there is also a peak season for that phenomenon as well (though not nearly as major as areas in the Midwest). Twisters are not completely uncommon for the area and are formed by the turning of winds with height and lift. They are most common in June and July and are usually relatively weak, with higher amounts inland (10-15 per year on average in VA, PA, and NY) and lower amounts along the Mid-Atlantic coastline and into New England (0-2 per year for states along the Northeast coast).
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Above: Severe weather days per year from 2003-2012 (Image courtesy of the SPC)
Data from the Storm Prediction Center between 2003 and 2012 actually further breaks down each criteria for severe weather by days of occurence within a 25-mile radius of a certain point on the map. On average, the Northeast and Mid-Atlantic will generally see 3-7 severe hail days and upward of 10-20 severe wind days per year. When all is said and done, the region usually experiences around 15-30 severe weather days in total. The image above gives a display of severe weather days per year across much of the United States from the Storm Prediction Center.
