If April has earned its reputation for being the “cruelest” month, then May enjoys its title as the “Merry Month of May.”
Temperatures through the month climb about 10 degrees, though the typical spring chaos means a wide range in possibilities. For example, May starts with an average high temperature near 65, but May 2 of last year saw the mercury soar to 85 degrees. That’s quite a contrast to the chilly 48 degrees from only three years ago. Once you reach the end of May, average temperatures warm into the 70s, though temperatures over the last five years show an interesting variety, topping out at 69, 86, 83, 59, and 80 from 2014 through 2018. Nights start the month fairly cool, averaging 38 degrees at daybreak, with one third of those mornings at 32 degrees or below, keeping gardeners on their toes for the possibility of frost. Reaching the end of the month, nights hold in the upper 40s, only dipping into the 30s five times in the past 30 years, which included a touch of frost in 2004.
April showers have found their way into verse and song, but May features more rain than April in most years, averaging 3.25 inches, while April squeezes out 2.75 inches. In recent decades, May has become even wetter, with only 10 of the last 30 years ending up on the dry side of normal. Wettest of all, the disastrous May of 2011, which featured several moderate weather events through the month. With saturated ground, the conditions were primed for some serious flooding. In the late afternoon hours of May 26, a steamy, summer-like airmass enveloped the Northeast Kingdom, with temperatures in the 80s, and dewpoints in the 60s, which makes for our classic hot, humid weather in July. A cold front stretched through eastern Quebec, southwest to the Great Lakes, edging southeast toward us. This forced the steamy air to rise rapidly, however, the weather front was moving in very slowly. This created a situation where thunderstorms developed over the same locations, and traveled northeast in the moist southwest airflow preceding the front, to be followed by another thunderstorm moving along the same track, and yet another, and another.
This is referred to as “training,” as the storms form and move along the same “track,” like a train on a railroad track. The thunderstorms persisted for six to eight hours, each producing heavy rain, so that four to six inches of rain drenched the saturated ground, and soon overwhelmed the gullies, culverts, streams, and rivers. The damage was considerable, washing out hundreds of back roads and culverts. But the worst damage locally occurred in Passumpsic, where logs from Robinson’s lumber yard were swept into the Water Andric stream, feeding into the Passumpsic River. The logs jammed against the culvert that ran underneath Route 5, causing the untamed water to force its way around the culvert. This quickly eroded the roadbed, causing the road to collapse, followed in short order by the adjacent railroad track. Quick repairs took a few days, but it was several weeks before they were completed. The rainfall of 4.75 inches at the Fairbanks Museum was the greatest one-day total for the month of May, and brought the month’s accumulated rainfall to 11.12 inches, the greatest monthly total in the entirety of the Museum’s 125 years of weather records.
While such events are rare, thankfully, each May features a phenomenon born over the retreating snowfields of eastern Canada. Most weather systems – storms, fronts, and fair-weather high pressure – travel from west to east here in northern New England. However, the strengthening sun warms the land much more quickly than the bone-chilling ocean. Interior portions of Labrador and eastern Quebec are still losing their generous accumulations of snow. This creates a breeding ground for cold high-pressure areas to our northeast. If the rising temperatures of the spring lighten the air over New England and the mid-Atlantic states, this cold high pressure can build southwest behind a feature called a “backdoor cold front.” Because our weather systems normally travel west to east, this front’s westward movement can be characterized as moving backwards, and hence the name. In addition, the chill seems to set Spring back a few to several days, adding to the backwards nature of these fronts. The cold coastal waters off eastern Canada and New England provide a refrigerated surface, free from obstacles, for the cold air to ooze south, and then spread into eastern New England. The White Mountains can sometimes offer a barrier to the cold, though the backwards weather can even go south of the White Mountains, and turn back north toward us. The Green Mountains seem to hem in this late-season chill a bit more effectively, so that the Champlain Valley can be enjoying a partly sunny May afternoon in the 70s, while the Northeast Kingdom languishes under a low, maritime overcast, and thermometers stuck in the 40s.
Such weather disappointments rarely last too long, as the May sun regains the upper hand and the warmth of spring returns. Just in time to welcome the Ontario “bird of prey,” the blackfly.