Tracking Henri – Still Many Questions

Note: This isn’t to say the coast is clear!

Tropical Storm Henri is forecast to intensify into a hurricane and lift northward up (off) the Eastern Seaboard today and tomorrow, approaching New England on Sunday. Over the last 12 hours, it’s become increasingly clear that the system will not escape out to sea and will bring direct impacts to New England. That said, the extent of those impacts for us here on the Cape remain a big question-mark, as Henri has multiple aspects to it that are untraditional for hurricanes impacting the region.

Traditionally, hurricanes that impact New England do so because of a specific alignment and position of the jet stream and upper level steering currents surrounding the Northeastern part of the country. Historically, when hurricanes have approached New England they have interacted with that jet stream causing both a rapid acceleration of the storm but also an important phase transition. The storm weakens but not quickly enough to offset that rapid motion. And similarly, the system expands in size as it races up the coast, with its wind field expanding outward to the east considerably. So while New England hurricanes pass through the region in a matter of hours, they ultimately cover a large geographic region both S to N and W to E.

In this case, the jet stream alignment we typically look for isn’t there. Consequently, Henri’s forward speed will be quite slow as it moves northward – perhaps at a record-slow forward speed (courtesy Sam Lillo on Twitter). Multiple pieces of guidance and the official forecast track from the National Hurricane Center, suggest the storm will slow to a crawl as it nears and crosses into Southern New England Sunday night.

At first, that sounds troubling. But that slow motion *could* be a savior for Southeast Massachusetts, Cape Cod and the Islands – depending upon Henri’s ultimate exact track.

There has been a notable trend in guidance over the last day of Henri tracking more westward, with landfall closer to RI or SE CT. In fact, some guidance is even further west than that – with landfall over Long Island…even as far west as NYC! Should this trend hold, and guidance hone in on a landfall that far west, I suspect impacts here on the Cape would be quite minimal…for two big reasons:

Henri is never forecast to be a particularly large hurricane, size-wise to begin with. The National Hurricane Center forecast has tropical storm force winds extending outward 125 to 130 miles from the center, with hurricane force winds only extending outward about 20-25 miles from the eye. When we combine that initial small size with that lack of jet-stream-aided transition to a large extra-tropical cyclone (the traditional New England hurricane) it is likely that we won’t see the large expanding wind field we normally expect. (Hurricane Bob made landfall in Newport RI…some 70 miles from the Outer Cape, but winds gusted well over 100 mph from Chatham to Provincetown (125 mph in Truro.) Most of our guidance suggests the core of Henri will remain small and rather intact.

Secondly, Henri’s slow forward speed over progressively cooler water will cause the system to weaken and “fill in” as it wobbles toward the coast. While weakening is normal for storms approaching the area, in this case, the slow forward speed will result in a much weaker and more gradual change in surface pressure (and over a smaller area). The traditional rapid pressure fall/rise component of these systems is a big part of the wind transfer here in New England. And again, it’s not as geographically spread out. Furthermore, we’ll end up with a somewhat stable low level environment.

Guidance would suggest that the corridor for damaging wind may be a rather limited geographic region, very near where the center comes ashore. Consequently, if landfall is over CT or Long Island, we may be spared serious wind issues.

Also, it is quite likely that the heaviest rains will be along and to the LEFT/west of the track. As such, a track into central or western New England would yield very little in the way of rainfall locally. In fact, multiple pieces of guidance show almost NO precipitation beyond a few early showers here on the Cape.

The system’s smaller size and smaller corridor of strong wind would also limit the storm surge issues to some degree. Henri is not expected to be a massive system with a huge expansive fetch of gale, storm and hurricane force winds. Instead, it’s surge is likely to be more localized nearer its landfall point. There will certainly still be water rises away from the center but it would be less concerning vs a landfall nearby.

So we could – emphasis on COULD – escape the storm without many issues on the Cape if the westward trends continue.

If the recent trend westward on guidance ceases and models adjust back eastward (certainly possible), and we ultimately get a landfall at or east of Narragansett Bay, we would endure a more impactful event. Likewise, if Henri strengthens into a larger and more powerful storm than is currently anticipated, we’d feel more impact regardless of a further west track.

The next 24 hours will be very telling and we should have a pretty clear picture of the details for us here on the Cape by midday tomorrow.

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