Weather

H Henri: Most likely track a direct hit on RI

IMPORTANT: This post is out of date and has been superseded; see motifri.com/wx-henri-2021-08-22-0500edt

As of 11pm EDT Sat, Aug 21, 2021 in RI:
• A Storm Surge Warning and Hurricane Warning are in effect for Newport and Washington Counties including Block Island.
• A Storm Surge Warning and Tropical Storm Warning are in effect for eastern Kent County.
• A Tropical Storm Warning is in effect for Providence and western Kent Counties.

Hurricane Henri is on a forecast track for a direct hit on RI, as of the guidance at 11pm EDT, Sat, Aug 21, with the most likely path coming straight up Narragansett Bay, making landfall as a Category 1 hurricane on the Saffir-Simpson scale. Maximum sustained winds are 75MPH: hurricane force winds extend 35 miles and tropical storm force winds 150 miles from the center.

Rain is likely to begin by Sun, Aug 22, 5am and end by Mon 2am with peak sustained winds exceeding 30MPH and gusts exceeding 50MPH from 4pm to 11pm. A second period of rain is possible Mon 10am to Tue 2am, but only about 30% likely and winds less than 15MPH.

Because low pressure centers such as storms have counter-clockwise airflow (in the Northern Hemisphere), regions to the east of the track receive higher velocity wind with the forward motion of the storm added while regions to the west of the track receive higher rainfall.

Providence has 98% probability of experiencing tropical storm force winds (39MPH) and 19% probability of hurricane force winds (74MPH), capable of significant damage.

Tropical Storm Henri: Probability of tropical storm force winds for five-day period beginning Sat Aug 21, 2021, 8pm EDT.

Although “probability cone” charts are liable to be misunderstood because they focus on the center of the storm and thereby ignore the field of wind that can extend across hundreds of miles on either side of that track, in this case the direct aim toward RI is striking and illustrative.

Tropical Storm Henri: Probability “cone” of track of storm center for five-day period beginning Sun Aug 22, 2021, 2am EDT.

RI remains in the “slight” 10% risk band for flash flooding due to excessive rainfall. RI is forecast to receive 1-2 inches of rain, considerably less than the 6-10 inches in central Long Island and 4-6 inches along the CT coast, but this soon could be revised upward. Storm surge of 3-5 feet is possible on the RI coast and within Narragansett Bay.

Tropical Storm Henri: Probability of excessive rainfall and flash flooding for three-day period beginning Sun Aug 22, 2021, 8am EDT.
Tropical Storm Henri: Probability of total rainfall for three-day period beginning Sat Aug 21, 2021, 8pm EDT.

H Henri remains at hurricane strength with maximum sustained winds at 75MPH, but there is little difference in practical effect whether the storm is slightly above or below the arbitrary threshold of 74MPH that would classify it officially as a hurricane, and the potential for harm from heavy rain, high winds, and storm surge, as well as life-theatening surf and rip currents, should be taken seriously. All of southern New England can expect what amount to hurricane conditions, regardless of whether they meet the technical criteria.

All tropical weather systems should be regarded as volatile and unpredictable to some extent, so it is a mistake to focus on the exact forecast point of landfall as that may change significantly before it occurs. While this storm is extremely unlikely to “go out to sea” harmlessly, it would be no surprise to see the forecast track move back and forth closer to and farther away from RI, and it would be prudent to prepare for any possibility, charging cellular telephones and making sure flashlights and other equipment is powered and ready. Loss of electrical power mains for as long as several days should be part of planning.

[The RI Emergency Management Agency uses the CodeRED system to allow the public to sign up for direct community notifications: public.coderedweb.com/CNE/en-US/BF1E5F52D694 ]

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