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Tropical Storm Sam Forecast Discussion Number 5
2021-09-23 22:59:56| National Hurricane Center (Atlantic)
Issued at 500 PM AST Thu Sep 23 2021 000 WTNT43 KNHC 232059 TCDAT3 Tropical Storm Sam Discussion Number 5 NWS National Hurricane Center Miami FL AL182021 500 PM AST Thu Sep 23 2021 Sam could be in the beginning stages of developing a small inner-core this afternoon as a central dense overcast is beginning to take shape. On geostationary visible satellite imagery, the storm has occasionally exhibited a clear area, though this feature has been tilted a bit southeast of the estimated low-level center position. In addition, overshooting convective cloud tops have been recirculating cyclonically around this feature, suggesting convective symmetrization that may help shield the low-level vortex from dry-air intrusion as seen earlier on the northwest side of Sam. Unfortunately, I have not received a high resolution microwave pass over Sam in 9-12 hours, so there remains some uncertainty on the structure underneath the cirrus canopy. The latest subjective Dvorak estimates from SAB and TAFB were 55-kt and 45-kt respectively. Taking a blend of these intensity estimates yields 50-kt for this advisory. Sam's estimated motion is nearly identical to this morning, moving north of due west at 280/14 kt. A large and anomalously strong mid-level ridge is located poleward of Sam and should maintain the current west to west-northwest heading over the next 2-3 days. However, this ridge-axis is forecast to build in to the northwest ahead of Sam, which is likely to gradually slow down this forward motion over the next few days. In fact, Sam is forecast to move unusually slow for a tropical cyclone in the deep tropics east of the Lesser Antilles. Afterwards, a weakness in the ridge to the northwest is forecast to gradually develop as a broad deep-layer trough parks itself just offshore of the eastern United States by early next week. This could potentially allow Sam to gain a bit more latitude at a somewhat faster forward motion as the ridge axis shifts back east. The track guidance this cycle has slowed down a bit compared to this morning and is also a bit further south in the early portion of the forecast. The NHC official track will follow suit this advisory, though is not quite as slow as the GFS model. It is interesting to note that both the GFS and ECMWF ensemble guidance this afternoon shows a significant spread in track solutions by day 5 in the cross-track direction, so this track forecast is a bit more uncertain than usual. With the apparent improvement in inner-core structure inferred on satellite today, Sam appears poised to rapidly intensify in the short-term. In fact, the latest GFS-SHIPS guidance is now explicitly forecasting rapid intensification over the next 24-48 hours. This seems feasible given that the guidance is forecasting 5 kt or less vertical wind shear over the next 60 hours as the cyclone traverses over warm 28-29 C sea-surface temperatures. Thus, the intensity guidance in the short-term has been raised once again, now showing a 50-kt increase in intensity over the next 48 hours taking Sam to major hurricane intensity by Saturday afternoon. Remarkably, this rapid intensification is still below the latest HCCA and LGEM guidance over this time span. After this period, Sam is likely to start undergoing inner core fluctuations, related to eyewall replacement cycles that are challenging to forecast far in advance. A modest increase in southwesterly vertical wind shear could also occur in the day 3-5 period. Thus, the NHC intensity forecast still tops off Sam as 110-kt hurricane by the end of the forecast period, which is on the high end of the intensity guidance envelope. FORECAST POSITIONS AND MAX WINDS INIT 23/2100Z 11.1N 39.7W 50 KT 60 MPH 12H 24/0600Z 11.5N 41.9W 60 KT 70 MPH 24H 24/1800Z 11.9N 44.2W 75 KT 85 MPH 36H 25/0600Z 12.3N 46.0W 90 KT 105 MPH 48H 25/1800Z 12.6N 47.4W 100 KT 115 MPH 60H 26/0600Z 13.1N 48.7W 105 KT 120 MPH 72H 26/1800Z 13.7N 50.0W 110 KT 125 MPH 96H 27/1800Z 15.2N 52.5W 110 KT 125 MPH 120H 28/1800Z 17.5N 55.5W 110 KT 125 MPH $$ Forecaster Papin
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