First rule of weather – holiday weekends are guaranteed to include some forecast intrigue. Especially in New England. This Labor Day Weekend it’s the tropics that have our attention. Is it time to cancel all your plans and bring in the boats? No. But the tropical system in the Gulf of Mexico, likely to become Hermine, commands our attention for the next few days. It’s path through Friday morning is fairly high confidence. After that is where it starts to get tricky.
To start off with, we have a storm that hasn’t been in any hurry to organize. That’s not to say it can’t come together quickly while sitting over the bathtub waters of the Gulf, but we’ve been watching this system for over a week now and it hasn’t done much to impress. The best window for intensification is now through Wednesday, with wind shear at its lowest point. It’s likely that it reaches Tropical Storm status, and perhaps even hurricane status before landfall between the Big Bend and Panhandle of Florida. It just barely gets nabbed by an approaching trough, lifting the storm and accelerating it northeastward. After some brief weakening over land, we expect it to pop back out over the warm waters of the southeast coast and regain tropical storm status (if it loses it).
Tuesday guidance coming closer to the New England coast than it was on Monday. Source: Weatherbell
Then attention turns to us here to the north. The overall trend in guidance has definitely been farther west in the past 24 hours, and those chances go higher the slower the storm moves. If it really takes its sweet time to get to Florida (a Friday landfall), then the odds of it coming up the eastern seaboard or hugging closer to the coast will increase. The faster it moves, the higher the odds it escapes out to sea and stays there.
Capture or block? That’s the biggest forecast question right now.
The main player when it comes to steering is a large ridge building in from the west. This ridge is going to do one of two things – block it from moving our way or capture it. The best way to capture the storm would be via the slower storm movement I mentioned above. This ridge would build over the north side of the storm, blocking its escape hatch to the open Atlantic. Then you end up with a tropical system moving slowly, perhaps even stalling, near our coastline. And as depicted by some models, retrograding (drifting west) back toward land with time. If the ‘left hook’ reminds you of Sandy, it’s a different situation. It’s true Sandy was blocked by high pressure, but the biggest factor was a strong approaching trough that grabbed the storm and yanked it west. That trough also included some potent jet stream energy, which allowed Sandy to blow up in size and produce its incredible storm surge. That’s not in play here, meaning any storm that gets near us would not likely be anything similar in terms of strength. Even more so if the storm passes us and then moves back west with time, since it will spend more time over the cooler waters of the North Atlantic instead of the warm waters south of Long Island.
Here are the current sea surface temperatures along the East Coast. You can see that a storm would not be happy if it got into the water due east of Nantucket or farther northeast from there. Not good to sustain a strong tropical entity. Meanwhile, waters are very supportive of a tropical storm south of Long Island. Source: Weatherbell
When it comes to timeline, we’re looking at Sunday and Monday (and if it really moves out to sea before swinging back in, possibly lingering into Tuesday). At the very least, I’d expect some rough surf and big waves at area beaches this holiday weekend. That’s even if the storm completely misses us and stays out to sea. When it comes to plans – stay the course for now. Five days is an eternity when it comes to tropical systems, especially ones that aren’t very organized. They’re tougher to forecast and more erratic than stronger/well established storms. We have a couple days to nail down the likelihood of this turn west.
Best case scenario? It would be fantastic if this got blocked by the ridge and slowly backed toward us late Monday into Tuesday. Some soaking rainfall without winds that would be overly damaging. But that’s a small needle to thread. We can only hope that we’ll manage to squeeze out some sorely needed rain without the destruction of a tropical system. I currently don’t believe a storm would be in very strong condition if it makes it here, but we’ll have updates on the future of T.D. 9 all week!