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Tropical Storm Henri Forecast Discussion Number 13
2021-08-19 05:00:12| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Wed Aug 18 2021 000 WTNT43 KNHC 190300 TCDAT3 Tropical Storm Henri Discussion Number 13 NWS National Hurricane Center Miami FL AL082021 1100 PM AST Wed Aug 18 2021 Henris appearance this evening is one of a tropical cyclone that is battling vertical wind shear. The satellite structure has evolved into a large convective plume with overshooting cold -70 to -75 C cloud top temperatures near the center of the convective mass. Underneath the cirrus, Henris structure has remained steady-state, with a SSMIS pass at 2243Z showing a ragged mid-level eye on the 91 GHz channel that remains displaced a bit southeast of the low-level signature on 37 GHz. The latest satellite intensity estimates have a large spread in values ranging from 55-kt up to 80-kt. I have elected to maintain Henris intensity at 60 kt for this advisory, but there is a larger than normal uncertainty in this estimate. The initial motion continues to be just south of due west at 265/8 kt. The short-term track guidance is in general agreement that Henri will continue a general westward motion for the next day or so, as the cyclone is caught to the south of a pronounced mid- to upper-level ridge. However, the models disagree on how far westward Henri is able to advance in the short-term, and these track differences appear to be related to Henri's intensity and vertical depth of the circulation. This track dependency is nicely illustrated by the most recent ECMWF ensemble guidance, where stronger members move further south and west in the short-term, ending up on the left-side of the ensemble mean, while weaker members move slower and more poleward. The deterministic guidance is similar, with the weaker ECMWF and Canadian runs off to the east/right while the stronger GFS and UKMET runs are further left/west. These short-term track differences appear pivotal, because the ridge over Henri is then expected to quickly erode as a mid-latitude trough digs in over the Ohio Valley and becomes negatively tilted over the Mid-Atlantic. The leftward track guidance has Henri interacting with this trough leading to a track forecast much closer to the northeastern US, while the rightward track guidance is not captured by this feature, and stay further offshore escaping out to sea. In general, the mean of the track guidance is very similar, or just a shade to the right of the previous cycle. I have elected to maintain a very similar track to the previous advisory, splitting the difference between the previous forecast just left, and the reliable HCCA consensus aid just right. As mentioned previously, a NOAA Gulfstream IV aircraft has been scheduled to investigate the synoptic environment of Henri tomorrow and will hopefully provide more data ingested into the model guidance to reduce the large track spread seen in the current cycle. Henri is now under moderate northerly shear that is forecast to increase to 25-30 knots over the next 24 hours. While this would ordinarily weaken the tropical cyclone, the system also remains under very warm 29-30 C sea-surface temperatures that are somewhat above normal for this time of year. Due to these offsetting factors, the latest NHC intensity forecast maintains Henri at 60-kt for the next 24 hours. Afterwards, the shear is forecast to decrease as the storm turns more poleward directly under the upper-level ridge axis. This favorable synoptic pattern should allow Henri to intensify and gradual intensification is shown up to a 80-kt peak in 72-h. The NHC intensity forecast is more or less in the mean of the large guidance spread, though it should be noted that the regional hurricane models (HWRF, HMON, COAMPS-TC) have somewhat stronger peak intensities later in the forecast period. Key Messages: 1. The forecast track of Henri remains near the northeast coast of the U.S. this weekend and early next week, and the risks of storm surge, wind, and rain impacts in portions of the northeastern U.S. and Atlantic Canada remains a distinct possibility. Interests in these areas should closely follow the progress of Henri and check for updates to the forecast. 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. FORECAST POSITIONS AND MAX WINDS INIT 19/0300Z 29.8N 68.5W 60 KT 70 MPH 12H 19/1200Z 29.8N 69.9W 60 KT 70 MPH 24H 20/0000Z 30.0N 71.8W 60 KT 70 MPH 36H 20/1200Z 30.7N 72.7W 65 KT 75 MPH 48H 21/0000Z 32.3N 72.8W 70 KT 80 MPH 60H 21/1200Z 34.4N 71.8W 75 KT 85 MPH 72H 22/0000Z 36.9N 70.7W 80 KT 90 MPH 96H 23/0000Z 40.4N 69.4W 70 KT 80 MPH 120H 24/0000Z 41.8N 68.3W 60 KT 70 MPH $$ Forecaster Papin/Beven
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