New Englanders have enjoyed delightful sunshine and pleasant air for our post-Labor Day return to work and school, but a quiet Tuesday belies what’s actually a very active weather pattern in the days ahead.
There are two big players in our weather story: a cold front diving southeast out of Canada and Hurricane Dorian. The latter has taken the weather spotlight the last few days, and understandably so – this historic hurricane has proven catastrophic in the Bahamas. While the storm has weakened Tuesday, its eventual path is still a critical detail up and down the Eastern Seaboard.
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It seems quite likely Dorian will parallel the Florida coastline, remaining over water as it drifts slowly north. If you’re a regular reader of our First Alert weather posts here at NBC10 Boston, NECN and Telemundo New England, you may remember we first explained this potential for a stall and northward drift last Thursday, citing the jet stream winds aloft. The jet stream is the fast corridor of air that flows like a river through the atmosphere, steering disturbances and separating cool air to the north from warmth to the south.
In the coming days, the jet stream will start to buckle over the Northeast United States, dipping south, and this is a key factor in both our local forecast and the predicted path of Hurricane Dorian.
The first response from the southward dip in the jet stream will be evident on Wednesday – the southward intrusion of cooler, Canadian air will be represented by a cold front at ground level, and this influx of cooler air will intersect a warm, southerly wind. In the warm wind, New England will find Wednesday high temperatures in the 80s across central and southern New England and southerly gusts to 30 mph or greater, but the approaching cold front will trigger scattered showers and thunder throughout the day in northern New England, then deliver scattered afternoon thunder to central and southern New England. Any cloud-to-ground lightning, of course, might cause delays or cancellations to after-school practices and games, at least causing a period of pause as the storms move through from northwest to southeast.
Behind the cold front, new, crisp air will pour into New England for Thursday.
The cold front crossing New England at midweek has a bigger impact than delivering new air, however – that same cold front is driven by a jet stream disturbance that will cause the fast, storm-steering winds aloft to dip southward, and this will eventually reach the northward drifting Hurricane Dorian.
As Dorian nears the Carolina coastline Thursday, the hurricane will begin to change direction toward the northeast as it starts interacting with the jet stream and our southward moving cold front. This is the change in atmospheric wind necessary to ensure Dorian does not "run the coast" and charge northward into New England, and instead the storm’s deteriorating eye – the center of the storm with the most potential for destruction – will be swept out to sea to our southeast.
While the storm center will move out to sea, and even the broad cone of probability from the National Hurricane Center no longer includes New England for a center-pass of Dorian, that doesn’t mean New England is guaranteed to escape impact – on the contrary, it’s quite likely we’ll see impact from Dorian.
The most assured impact from the large hurricane will be building swell – large waves emanating from the storm center and reaching New England’s south coastal waters as 12 to 18 foot seas, posing a risk to the marine community that will keep mariners out of the waters to our south, and causing intense rip currents due to the powerful waves at our south-facing beaches Friday into the weekend.
Our exclusive NBCUniversal Forecast System, built right here at NBC10 Boston and NECN, shows a 60 to 80 percent chance the northwest periphery of Dorian’s rain shield may also reach Cape Cod and perhaps southeast Massachusetts Friday evening and night, though would be unlikely to cause flooding this far from the storm.
Finally, when tropical systems begin the process of becoming non-tropical – a process Dorian is sure to begin as it starts interacting with the colder jet stream winds in our northern latitude – the wind field around the system morphs from a circular pattern near the storm center, to an elongated pattern that separates from the storm core. On its face, this raises the potential for strong wind gusts for a time Friday night in at least southeast Massachusetts and at least some gusty breezes along most of eastern Massachusetts.
Often, I’ve seen storms like this take time to wrap wind all the way around them as they become non-tropical, meaning I wouldn’t be at all surprised to see the strongest wind southeast of the storm center as Dorian makes its closest pass to New England Friday night... and that would be over the water. So, while we’re indicating the potential for 40 to 50 mph gusts on Cape Cod if the wind field expands northwest, if I were a betting man I wouldn’t be too quick to put money down on that potential coming to fruition.
Whatever rain falls from Dorian in New England should wrap up by early Saturday morning, leaving a dry and pleasant weekend and another shot of cool early fall air for early next week before some moderation toward the end of next week in our exclusive First Alert 10-day forecast.