Atlantic Hurricane Feed

Tropical Storm Sam Forecast Discussion

WTNT43 KNHC 240240

Tropical Storm Sam Discussion Number   6
NWS National Hurricane Center Miami FL       AL182021
1100 PM AST Thu Sep 23 2021

Microwave data from around the time of the previous advisory 
revealed that Sam has developed a well-defined inner core, including 
a nearly closed eyewall feature in the 91-GHz channel.  Dvorak 
estimates from TAFB and SAB have increased to T3.5 and T4.0, 
respectively, and the initial intensity is therefore estimated to be 
60 kt.  Sam's 24-hour intensity change qualifies as rapid 
intensification, since the cyclone was only a 30-kt tropical 
depression at this time yesterday.  Sam is small, however.  Recent 
scatterometer data showed that tropical-storm-force winds only 
extend a maximum of 40 n mi from the center, and the radius of 
maximum winds is estimated to be about 15 n mi.  This small size is 
likely to have implications on Sam's future intensity.

Sam is moving a little slower toward the west (280/13 kt).  A 
mid-tropospheric ridge is expected to build across the Caribbean 
Sea and extend northeastward over the central Atlantic during the 
next few days.  This blocking high is expected to cause Sam to slow 
down to 6 to 7 kt by late Saturday and Sunday while it maintains a 
westward to west-northwestward heading.  Global models suggest the 
ridge may weaken and shift eastward a bit by days 4 and 5, allowing 
Sam to turn to the northwest and speed up a little by the end of 
the forecast period.  The ECMWF, HWRF, and HCCA models lie along 
the southern edge of the main pack of models, while the ECENS 
ensemble mean is even farther south.  The GFS and the HMON are on 
the northern side.  Partially due to an adjustment of the initial 
position, the new NHC track forecast has been shifted southward a 
bit from the previous forecast, and it lies just south of the TVCA 
consensus aid--but not as far to the left as the previously 
mentioned southern models.

Environmental conditions and Sam's structure and size are ideal for 
continued rapid intensification (RI).  The SHIPS RI index for a 
30-kt increase in 24 hours is nearly 50 percent.  The DTOPS 
methodology, which uses inputs from the statistical-dynamical and 
deterministic models, shows RI indices for various forecast periods 
exceeding 90 percent.  Therefore, rapid strengthening is being 
forecast at least for the next 36 hours, with Sam becoming a 
hurricane very soon and then a major hurricane by Friday night or 
Saturday morning.  One caveat to this forecast is that although 
Sam's small size more readily allows for RI to occur, it can also 
make the cyclone more prone to weakening if, for example, vertical 
shear increases.  By 48 hours, the intensity models show the 
strengthening trend leveling off, and the NHC intensity forecast 
follows suit.  Fluctuations in intensity from days 3 through 5 are 
likely, both due to normal internal dynamics within the storm and 
the cyclone's response to the surrounding environment in relation 
to its small size.


INIT  24/0300Z 11.2N  40.9W   60 KT  70 MPH
 12H  24/1200Z 11.4N  42.9W   75 KT  85 MPH
 24H  25/0000Z 11.8N  45.0W   90 KT 105 MPH
 36H  25/1200Z 12.1N  46.6W  105 KT 120 MPH
 48H  26/0000Z 12.5N  47.8W  110 KT 125 MPH
 60H  26/1200Z 13.0N  49.1W  110 KT 125 MPH
 72H  27/0000Z 13.5N  50.4W  110 KT 125 MPH
 96H  28/0000Z 15.3N  53.2W  110 KT 125 MPH
120H  29/0000Z 17.3N  56.1W  110 KT 125 MPH

Forecaster Berg

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