Henri headed toward the Hudson Valley

Update #460b

Hi there! There’s a lot to cover today.

This post will outline Henri’s most likely impact on the Hudson Valley from Sunday through Sunday night.

In short, prepare for the worst and hope for the best!

For more frequent updates on the storm, you can follow me on Twitter. For the official forecast, head to the National Hurricane Center.

Henri is currently a strong tropical storm with wind gusts up to 70 mph. It’s expected to strengthen into a hurricane as it moves northward today.

The animation below illustrates the immense amount of moisture that Henri will transport northward from the tropics.

Once it arrives in the Northeast, the combination of lots of moisture, already soaked soils, and a slow-moving storm will lead to big problems when it comes to flooding.

Numerous watches and warnings have been issued for the Hudson Valley, including a Tropical Storm Warning for Westchester County and a flood watch for much of the rest of the region.

For us, this one will be much more about the rain than the wind.

Key information:

• 🌀 Landfall — at this time, the most likely outcome is for an initial landfall over eastern Long Island early on Sunday afternoon before a 2nd landfall in Connecticut.

• 🌧️ Rain — is expected to develop over the Hudson Valley on Sunday morning, turning steady to heavy during the afternoon and lasting into the night. Torrential downpours are likely at times. Amounts of 2-5 inches will be possible by Monday morning with scattered amounts in excess of 6 inches, especially over elevated terrain. If you live in a flood prone area, take steps to protect your property. Check that your drains are clear. Sodden soils from frequent rain this summer means that it will take less rain than normal to cause a flood. Remember to never drive through floodwaters.

• 🌬️ Wind — will be strongest along and east of the storm’s track. This means Long Island, Connecticut, Rhode Island, and parts of Massachusetts. For the Hudson Valley, the current indication is that the strongest winds will be to the east of the river and closer to the Connecticut border, especially across higher elevations. With saturated grounds, trees will come down more easily.

• 💥 Impact — expect delays or cancellations across most regional airports on Sunday, building as the day goes along. Flooding may result in road closures. Scattered, but probably not widespread, power outages are possible in the Hudson Valley. Widespread power outages are likely across Long Island, Connecticut, and Rhode Island.

• 🤔 Confidence — the storm will be pretty compact, so even a *small* deviation in track can result in a shifting of the swath of strongest winds and area of heaviest rain. As of now, confidence of these impacts over the Hudson Valley is moderate.

Additional heavy rain is likely on Monday as the storm very slowly pulls away to the north and east — this will exacerbate issues that develop on Sunday. The weather looks dry by Tuesday.

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The science behind the storm

One way to assess forecast confidence is to use an ensemble — a collection of model “members” whose initial conditions are tweaked very slightly to give a range of possible forecast outcomes.

The more the members agree, the higher forecast confidence is.

In the plot below, 50 possible tracks for Henri are shown, color-coded by storm strength. The black line represents the average of the 50 possible tracks and is usually considered the most likely track.

Colloquially, this is known as a spaghetti plot 🍝

Compared to the plot I showed in my post yesterday, there are more members indicating a “left turn” toward Long Island, Connecticut, and the Hudson Valley.

Even so, there are is still a chance for the track to deviate to the right, so we must keep watching! Since this is a particularly fluid situation, the forecast will change over the next 24 hours. I’ll have another email update for you tomorrow morning.

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