Atlantic Basin Tropical Cyclones

Hurricane Sam Forecast Discussion


000
WTNT43 KNHC 270848
TCDAT3

Hurricane Sam Discussion Number  19
NWS National Hurricane Center Miami FL       AL182021
500 AM AST Mon Sep 27 2021

The satellite presentation of Sam deteriorated overnight, as GOES-16 
infrared imagery showed periodic disruptions to Sam's inner core 
convection. The eye has not been readily apparent in conventional 
satellite imagery for much of the night, although very recent 
imagery suggests an eye could be re-emerging. Data from the 
reconnaissance aircraft last night indicated that an eyewall 
replacement cycle (ERC) was underway, and some dry mid-level air 
impinging on the western side of the circulation could also be 
negatively impacting Sam. No recent high-resolution microwave data 
is available, but an earlier scatterometer pass revealed that the 
tropical-storm-force wind radii had expanded just a bit in the 
eastern semicircle of the hurricane. The initial intensity is 
lowered to 115 kt for this advisory based on a blend of the 
objective and subjective Dvorak current intensity estimates. A NOAA 
hurricane hunter aircraft is scheduled to investigate Sam later this 
morning, which will provide crucial data to assess changes in Sam's 
structure and intensity.

The initial motion is estimated to be northwestward, or 315/7 kt. A 
subtropical ridge to the north and northeast of Sam will remain the 
primary steering mechanism over the next several days, and Sam is 
expected to maintain a northwestward heading through midweek. Then, 
a mid- to upper-level trough is forecast to dig southward over the 
western Atlantic late this week. The deep-layer southerly flow ahead 
of this feature should cause Sam to move faster toward the 
north-northwest by 96 h, followed by a northward acceleration 
thereafter. The NHC forecast track is shifted slightly to the right 
of the previous one through the first 48-60 h of the forecast 
period, which accounts for the more northwestward initial motion 
observed during the past 12 h. Otherwise, the NHC track is virtually 
unchanged as the guidance has remained fairly consistent.  

Sam's intensity has likely peaked now that an ERC has commenced, but 
environmental conditions should allow Sam to persist as a major 
hurricane for the next several days. Sea-surface temperatures are 
expected to be 28 deg C or greater along Sam's track through 120 h, 
and the vertical wind shear is forecast to remain weak (



Source link