Atlantic Basin Tropical Cyclones

Hurricane Sam Forecast Discussion


000
WTNT43 KNHC 272051
TCDAT3

Hurricane Sam Discussion Number  21
NWS National Hurricane Center Miami FL       AL182021
500 PM AST Mon Sep 27 2021

Sam's weakening trend ended earlier today, with the intensity likely 
bottoming out between the issuance of the previous advisory and now. 
Over the past few hours, microwave data and satellite imagery 
indicate that the inner core and eyewall of the hurricane are making 
a comeback. There is now a clear eye in visible imagery, convective 
cloud tops colder than -65 degrees C wrap completely around the 
center, and the convective mass is becoming more circular once 
again. An Air Force Reserve Hurricane Hunter aircraft currently 
investigating Sam indicates the central pressure has dropped to 957 
mb, with peak 700-mb flight-level winds of 106 kt, SFMR values of 
100 kt, and dropsonde surface winds of 102 kt. Given possible minor 
undersampling and the recent dropping of the hurricane's pressure, 
the initial advisory intensity is set at 105 kt.

Sam continues to move northwest, or 315/8 kt. The hurricane is
expected to continue this northwestward motion around a subtropical
ridge over the next few days. By late Thursday or early Friday a
turn toward the north is anticipated as Sam rounds the western
periphery of the ridge. The cyclone should begin to gradually
accelerate during that time in the increasing flow ahead of a mid-
to upper-level trough emerging off the U.S. east coast later this
week. Model guidance remains in very good agreement on this
scenario and the latest NHC forecast track is little changed from
the previous one, and lies near tightly clustered consensus track
guidance.

The southwesterly winds that brought in some dry air and disrupted
Sam's impressive satellite appearance last night appear to have
abated. As long as environmental winds surrounding the hurricane
remain light, then the dry air surrounding the cyclone should not 
have as much of an impact on the hurricane's strength over the next 
few days. Based on the improving inner-core structure appearance of 
Sam, re-strengthening is forecast over the next 12 h. Thereafter, 
mainly minor fluctuations in intensity are indicated through 72 h. 
Beyond that time, increasing vertical wind shear and decreasing 
sea-surface temperatures should cause Sam to weaken. The latest NHC 
intensity forecast was nudged slightly upward through 36 h and is 
on the high end of the guidance. Thereafter, no changes were made, 
and that portion of the forecast is close to the various multimodel 
consensus solutions.

Key Messages:

1. Large swells generated by Sam are affecting the Leeward Islands 
and will spread to portions of the Virgin Islands, Puerto Rico, 
Hispaniola, the Bahamas, and Bermuda by Thursday or Friday.  
Significant swells will likely reach the east coast of the United 
States and Atlantic Canada by the weekend.  These swells will likely 
cause life-threatening surf and rip current conditions, and 
beachgoers and other interests along these coasts are urged to 
follow the advice of lifeguards and local officials through the 
upcoming weekend.


FORECAST POSITIONS AND MAX WINDS

INIT  27/2100Z 16.3N  52.7W  105 KT 120 MPH
 12H  28/0600Z 17.0N  53.5W  115 KT 130 MPH
 24H  28/1800Z 17.8N  54.6W  115 KT 130 MPH
 36H  29/0600Z 18.8N  55.8W  115 KT 130 MPH
 48H  29/1800Z 19.8N  57.3W  110 KT 125 MPH
 60H  30/0600Z 21.2N  59.0W  110 KT 125 MPH
 72H  30/1800Z 22.9N  60.5W  110 KT 125 MPH
 96H  01/1800Z 27.8N  62.0W  105 KT 120 MPH
120H  02/1800Z 34.7N  59.6W  100 KT 115 MPH

$$
Forecaster Latto



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