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Hurricane Sam Forecast Discussion


481 
WTNT43 KNHC 300256
TCDAT3

Hurricane Sam Discussion Number  30
NWS National Hurricane Center Miami FL       AL182021
1100 PM AST Wed Sep 29 2021

Sam is intensifying tonight after completing an eyewall replacement 
cycle.  A recent NOAA Hurricane Hunter aircraft showed peak 700 mb 
flight-level winds of 138 kt, multiple SFMR surface winds of 120 kt 
or greater, plus falling central pressure to about 940 mb.  This 
deepening trend is also seen on recent satellite imagery, with 
significant warming within the eye noted.  These data support 
raising the initial intensity to 125 kt for this advisory.  It is 
of note that the NOAA mission has been quite helpful for both the 
size and intensity analysis, with dropsondes and SFMR data showing 
that the hurricane has grown quite a bit since earlier today, 
mostly on the eastern side.  Additionally, reflectivity data 
from the core confirmed the end of the eyewall replacement cycle.

The hurricane could strengthen a little more overnight while it 
moves over a warm ocean eddy in fairly light shear.  Afterward, 
gradually cooling SSTs, falling ocean heat content, and the 
inevitable future eyewall replacement cycles should cause a 
weakening trend.  However, this will likely not be as steady as 
shown below, and significant deviations (upward or downward) from 
the forecast can be expected.  Faster weakening is expected by the 
weekend due to cool SSTs and increasing shear.  The new forecast is 
close to the intensity consensus, and is a little higher than the 
previous NHC prediction early on, primarily due to the initial wind 
speed.

Sam has sped up tonight, now estimated at 315/10 kt.  It sounds like 
a broken record, but there are no changes to the forecast during the 
first few days as model guidance is in excellent agreement on a 
gradually rightward-curving track, passing a couple of hundred n mi 
or more east of Bermuda.  Afterward, guidance is actually in worse 
agreement over the weekend, with widely divergent solutions from an 
upper-level trough picking up Sam and turning it north toward 
Newfoundland or the trough leaving Sam behind, causing a slow 
eastward motion.  The trend in the guidance is for a slower motion 
at long range, so I've elected to keep the day 4 and 5 points almost 
the same as the previous advisory, just a hair faster than the 
latest consensus aids.


Key Messages:

1. Swells generated by Sam will impact the Lesser Antilles during
the next several days.  Swells are expected to reach Bermuda
and the Bahamas in a day or so, and then spread to the United
States east coast by this weekend.  These swells could cause
life-threatening surf and rip current conditions.  Please consult
products from your local weather office.

2. A Tropical Storm Watch could be required for Bermuda on 
Thursday, and interests there should monitor the progress of Sam.


FORECAST POSITIONS AND MAX WINDS

INIT  30/0300Z 20.6N  58.4W  125 KT 145 MPH
 12H  30/1200Z 21.9N  59.6W  130 KT 150 MPH
 24H  01/0000Z 24.1N  60.9W  125 KT 145 MPH
 36H  01/1200Z 26.8N  61.8W  120 KT 140 MPH
 48H  02/0000Z 29.6N  61.8W  115 KT 130 MPH
 60H  02/1200Z 32.2N  60.7W  110 KT 125 MPH
 72H  03/0000Z 34.3N  59.2W  100 KT 115 MPH
 96H  04/0000Z 37.5N  54.5W   85 KT 100 MPH
120H  05/0000Z 42.0N  48.5W   70 KT  80 MPH

$$
Forecaster Blake




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