TOWNSHEND — At the Skywarn spotter training, a citizens' severe-weather seminar held in April at Grace Cottage Hospital, National Weather Service (NWS) Meterologist-In-Charge Raymond O'Keefe offered some stories and statistics about local weather and told attendees which types of storms are most common during warm-weather months.
The NWS Weather Spotter's Field Guide says, “The U.S. is the most severe-weather-prone country in the world.” The guide says that each year, “people in this country cope with an average of 10,000 thunderstorms, 5,000 floods, 1,200 tornadoes, and two landfalling hurricanes.”
But how bad is it here in Windham County?
Turns out it's not so bad.
“In Vermont, our main concern comes in the 'marginal' and 'slight risk'” categories, O'Keefe said, noting that “it's rare to see 'moderate' and 'extreme-risk'” summer storms in the state.
O'Keefe pointed out the distinction between a weather watch, which he said the NWS issues “with 50-percent confidence,” and which indicates “you should go about your routine and pay attention,” and a weather warning, which “implies a threat to life or property."
Windham County sees eight or nine severe thunderstorm watches per year and one tornado watch per year on average, O'Keefe said.
Basic thunderstorms are more frequent, with 20-25 per year in the entire state, most between April and September.
June is hail time, but the pieces tend to be small, O'Keefe said. “Two-inch hail here in Vermont is big-time hail,” he said.
“July is the most likely month for wind events,” O'Keefe said, because the warm, humid air creates instability, which can cause windstorms. The most common time of day for high winds is late afternoon or evening, he said, with 80 percent of wind storms happening during this most-humid time of day.
August is peak season for thunderstorms and flooding.
Thunderstorms need moisture, and August is a humid month, O'Keefe said.
O'Keefe noted “the best source of moisture for thunderstorm development in Vermont is the Gulf of Mexico, and some comes from the Atlantic."
Our local terrain also contributes to thunderstorms in the Connecticut River Valley, O'Keefe said.
“There's lots of upslope flow here,” he explained. Cool fog in the valley rising into the warm air at higher elevations creates clouds over the mountains, he explained.
Flooding is of particular concern in Vermont, O'Keefe said, because our hilly landscape traps thunderstorms in a particular area.
“They ignite, move a little, reignite, move, and go over the same area again and again,” he said.
There isn't as much wind in other months, too, so the storms tend to move slowly, “and that rain's going to pile up really fast,” O'Keefe said.
“In Vermont, it may not rain where you are, but when rain comes down from the mountain it might flood you,” he added.