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Tropical Storm Henri Forecast Discussion Number 17
2021-08-20 05:00:01| National Hurricane Center (Atlantic)
Issued at 1100 PM EDT Thu Aug 19 2021 000 WTNT43 KNHC 200259 TCDAT3 Tropical Storm Henri Discussion Number 17 NWS National Hurricane Center Miami FL AL082021 1100 PM EDT Thu Aug 19 2021 The satellite presentation on Henri this evening continues to exhibit a persistent bursting pattern, with the center estimated to be just to the north and west of the coldest cloud tops which occasionally have been below -80 C in the overshooting tops. This current satellite presentation is primarily due to continued moderate to strong northerly vertical wind shear, which is forcing the convection underneath the cirrus canopy down-shear of the low-level center, as seen on a 2230 UTC SSMIS microwave pass. While the mid-level vortex with the convection also remains tilted down-shear of the low-level center, it has not completely separated due to the persistent convection, preventing the low-level center from escaping poleward in more shallow low-level steering. Tonight's subjective Dvorak estimates from SAB/TAFB are in agreement with T3.5/55 kt and given that this value is near what the earlier Air Force Reconnaissance mission found, the latest intensity is being maintained at 55 kt for this advisory. There is a bit of uncertainty determining if Henri has begun a more poleward motion since the center remains under the convective cirrus plume, but my best guess is now 285/9 kt. Over the next 12-24 hours, the mid- to upper-level ridging that has dominated the synoptic steering pattern for Henri the last few days will quickly break down, as a shortwave trough drops in from the Great Lakes into the Mid-Atlantic and cuts off. This feature is now forecast to continue digging in to the west of Henri. To the east, a new mid-level ridge is also forecast to build in to the right of Henri. This synoptic pattern should draw the cyclone poleward with an acceleration to the north-northeast in the 24-48 h period. Afterwards, the aforementioned trough takes on a negative tilt to the southwest of Henri, helping to reorient the mid- to upper-level flow out of the south-southeast, and this flow could result in a slight leftward bend in the track between 48-72 h. The majority of guidance this cycle now is forecasting the mid-level ridge east of Henri to build poleward with the storm, blocking an easy path for the storm to stay on a more northeast heading out to sea. Consequently, the latest NHC forecast track now explicitly shows landfall in southeast Massachusetts at 72 h. The track guidance this cycle has come into better agreement, though there remain some leftward (UKMET) and rightward (ECMWF) outliers. The latest forecast track lies very close to the HFIP corrected consensus approach (HCCA) guidance, which is also very near the latest GFS forecast track. Data from the NOAA G-IV synoptic mission around Henri shows that just north of the tropical cyclone there remains some very dry mid-latitude air, which is being advected into the storm by 20-25 kt of northerly vertical wind shear. Over the next 24-36 hours, this shear is forecast to gradual subside, as Henri moves near the center of an upper-level ridge axis. By 36-48 hours, the vertical wind shear is forecast to be under 10-kt by both the GFS- and ECWMF-based SHIPS guidance, while the storm is also traversing 28-29 C sea-surface temperatures (SSTs). Thus, the latest NHC intensity forecast still calls for strengthening beginning after 12 hours, and the rate of strengthening could be a bit quicker as the storm moves over the warm gulf stream waters between 36-48 hours. Thereafter, Henri will cross a very sharp SST gradient with sea-surface temperatures down below 23 C near the New England coast to the east of Long Island. Henri is forecast to begin weakening after 48 hours, but the storm could still be near hurricane intensity by the time Henri is forecast to be near the Northeast coastline. Transition to a post-tropical storm is expected to begin shortly thereafter which should be sometime in the 96-h to 120-h points as deep convection ceases over the storm over cold SSTs As noted previously, the wind field of Henri is expected to expand, especially as it interacts with a mid-latitude trough located to its west. Therefore, users are reminded to not focus on the exact forecast points as impacts will extend far from the center. Key Messages: 1. Henri is forecast to be near the northeast coast of the U.S. on Sunday and Monday, and the risks of storm surge, wind, and rain impacts in portions of southern New England and eastern Long Island are increasing. Watches will likely be required for portions of this area early Friday. 2. Swells from Henri will begin to reach much of the east coast of the U.S. and Atlantic Canada by the end of the week and continue through the weekend. These swells could cause life-threatening surf and rip currents. 3. Heavy rainfall may lead to flash, urban, and small stream flooding over portions of southeastern New England Sunday into Monday. FORECAST POSITIONS AND MAX WINDS INIT 20/0300Z 29.8N 72.3W 55 KT 65 MPH 12H 20/1200Z 30.3N 73.2W 55 KT 65 MPH 24H 21/0000Z 31.8N 73.3W 60 KT 70 MPH 36H 21/1200Z 34.4N 72.2W 65 KT 75 MPH 48H 22/0000Z 37.6N 71.0W 80 KT 90 MPH 60H 22/1200Z 40.1N 70.6W 75 KT 85 MPH 72H 23/0000Z 41.7N 70.8W 60 KT 70 MPH...INLAND 96H 24/0000Z 42.5N 70.1W 40 KT 45 MPH...OVER WATER 120H 25/0000Z 43.7N 65.4W 35 KT 40 MPH...POST-TROPICAL $$ Forecaster Papin/Brown
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