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Tropical Storm Henri Forecast Discussion Number 10
2021-08-18 10:54:14| National Hurricane Center (Atlantic)
Issued at 500 AM AST Wed Aug 18 2021 000 WTNT43 KNHC 180854 TCDAT3 Tropical Storm Henri Discussion Number 10 NWS National Hurricane Center Miami FL AL082021 500 AM AST Wed Aug 18 2021 There's been little change in Henri's satellite appearance this morning. The surface center is still located just to the northwest of a gradually expanding central dense overcast, and there still appears to be an eye-like feature in the Bermuda Doppler radar presentation. However, there are no indications in the GOES-16 BD-curve enhanced infrared images of a developing warm spot. Subjective and objective satellite intensity estimates remain steady, and the initial intensity is held at 55 kt for this advisory. Although the surrounding thermodynamic environment is ripe for further significant development, moderate northwesterly shear continues to impinge on the cyclone causing difficulty in inner-core convective development. Consequently, little change in strength is expected during the next couple days, and in fact the statistical SHIPS model indicates an increase in a more northerly shear component on Thursday. By Saturday afternoon, however, the shear should decrease as the cyclone slides beneath an upper ridge axis off of the southeast U.S. coast. Henri is expected to further intensity through the remaining period as the upper-wind flow becomes much more diffluent. The official intensity forecast is similar to the previous advisory, and lies between the SHIPS model and the HFIP Corrected Consensus Approach aid after the 60-hour period. The initial motion is estimated to be westward, or 260/7 kt. A high amplitude mid- to upper-level anticyclone situated north-northeast of Henri should steer the cyclone toward the west during the next 36 hours, or so. Thereafter, the aforementioned ridge is forecast to weaken and shift eastward in response to a mid-latitude shortwave trough moving over the northeastern U.S. As a result of the change in the synoptic steering pattern, Henri should turn toward the northwest, north, and then northeast through the remaining portion of the forecast. With so much uncertainty or spread in the global and regional models beyond day 3, the best approach at this point is to base the NHC forecast on the better performing multi-model consensus guidance, which has once again shifted a bit to the left of the previous track forecast. I think it's worth noting that the GFSv16 model is now showing a shortwave ridge building over eastern Canada on Sunday, which causes Henri to move more northward toward the New England coast. Therefore, additional changes or shifts of the track beyond the 60-hour period may be required on subsequent advisories. Due to the increased uncertainty in the track forecast, interests along the New England coast should monitor the progress of Henri. FORECAST POSITIONS AND MAX WINDS INIT 18/0900Z 30.1N 65.7W 55 KT 65 MPH 12H 18/1800Z 29.9N 66.9W 60 KT 70 MPH 24H 19/0600Z 29.8N 68.7W 60 KT 70 MPH 36H 19/1800Z 29.8N 70.6W 60 KT 70 MPH 48H 20/0600Z 30.4N 71.8W 60 KT 70 MPH 60H 20/1800Z 31.4N 72.3W 65 KT 75 MPH 72H 21/0600Z 33.0N 71.9W 70 KT 80 MPH 96H 22/0600Z 37.6N 69.0W 75 KT 85 MPH 120H 23/0600Z 40.5N 66.1W 65 KT 75 MPH $$ Forecaster Roberts
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