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