It grabs your attention, and with good reason. Unlike some weather terms that are force-fed through the media because they sound Urban-Dictionary cool — snowpocalypse, anyone? — a bomb cyclone is a meteorological phenomenon. It applies to a dramatic drop in barometric pressure, 24 millibars in 24 hours, which has a profound effect on the weather. It signifies a rapidly intensifying storm that’s typically accompanied by heavy precipitation and fierce winds. It also goes by another apocalyptic-like name: bombogenesis.
Though generally uncommon, they mostly occur in the winter in the Northeast. That’s when conditions tend to be most favorable, requiring a clash of radical air masses and an energetic jet stream. Arctic air sweeps down from the north and collides with warm air coming up over the Atlantic Ocean. When that merger occurs just off the coast, low pressure rapidly intensifies and can lead to an old-fashioned New England blizzard. Old-time Yankees often called them winter hurricanes.
What’s unusual about Thursday’s bomb cycle is that it occurred in October. While nor’easters do smack us sometimes in the fall, this one broke records. According to the National Weather Service, the barometric pressure fell from 1,000 millibars to 975.3 millibars in less than 24 hours, a record for October. Those are hurricane-like numbers and thus no surprise that a 90-mph wind gust was recorded in Provincetown on Cape Cod Thursday night, with sustained winds near hurricane strength on the coast.
Meteorologist Benjamin Sipprell, in the weather center’s Boston office, said the drop in pressure was so rapid it can affect the human body, causing headaches, in particular.
And yet we were fortunate, especially in the Monadnock Region. Sometimes these storms stall right where they form and instead of a 12-hour rain and snow event, they linger 36 hours or so, exacerbating the damage. This one scooted up the coast fairly rapidly in the early morning hours Thursday, though the tail whipped around and gave us a good dose of rain much of the day. And if you really want to get technical, it was a sou’easter, since the fiercest winds blasted us from the southeast, rather than whipping around the storm’s center and coming down from the northeast.
Meteorologists have little time to rest. Nestor was upgraded to a Tropical Storm Friday at 1:35 p.m., and it’s on the move. Wind shear and dry air should prevent it from developing into a major hurricane, but the Florida Panhandle and much of the Southeast are in line for a good drenching, and possible flooding, this weekend. Nestor is expected to hug the coast up through North Carolina, then get booted into the Atlantic by a cold front approaching from the west.
That means we should be spared its wrath and can look forward to a stretch of benign weather — good for raking — through the weekend and into this coming week, with seasonal high temperatures in the 50s and lows in the 30s.
Showers are possible mid-week, but nothing like the bomb cyclone of this past week.