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Hurricane Franklin Discussion Number  35
NWS National Hurricane Center Miami FL       AL082023
500 AM EDT Tue Aug 29 2023
 
Near the time of the prior advisory, we received a GPM microwave 
pass at 0231 UTC, strongly suggesting the initial stages of an 
eyewall replacement cycle (ERC) were underway. This observation is 
also supported by the recent trends on infrared satellite imagery, 
which show the coldest cloud tops relative to Franklin's warm eye 
becoming more focused towards outer concentric bands seen earlier on 
microwave imagery. Finally, a recently received 0635 UTC AMSR2 
microwave pass confirms the ERC is ongoing, with the outer eyewall 
already starting to become dominant on the 89-GHz channel. All this 
data means is that the hurricane is likely past its peak intensity, 
but may be broadening its inner-core wind field as it undergoes this 
structural change. Subjective final T-number intensity estimates 
were 6.0/115 kt from both TAFB and SAB, with UW-CIMSS ADT also at 
T6.0/115 kt. However since the earlier aircraft data showed that 
Franklin was stronger than the satellite signature would suggest, 
the initial intensity is only being lowered to 120 kt for this 
advisory.
 
Franklin is now beginning to lose longitude again, with the latest 
estimated motion at 015/8 kt. A further turn to the northeast is 
expected later today as the hurricane becomes caught in the flow 
between a subtropical ridge positioned to its southeast and an 
approaching mid-latitude trough ejecting out of Canada from the 
northwest. Ultimately, this trough is still forecast to capture 
Franklin, though the along-track spread remains high beyond the next 
48 h. Once again the GFS and ECMWF are book-ending the track 
guidance this cycle, with the GFS the fastest and furthest north, 
and the ECMWF the slowest and furthest south, missing the trough 
connection. The ensemble guidance also continues to exhibit large 
spread between these scnerios after the next 48 h. The NHC track 
forecast continues to favor a solution closest to the the consensus 
aids TVCA and HCCA, which trended a bit slower this cycle. On the 
forecast track, Franklin should bypass Bermuda well to the northwest 
on Wednesday, but tropical-storm-force winds associated with 
Franklin will come much closer to the island due to the hurricane's 
large and expanding wind field.
 
With the ongoing eyewall replacement cycle, Franklin is likely past
its peak intensity. It is forecast to continue weakening, especially
as the hurricane also begins to move over cooler waters. In
addition, shear out of the northwest increases markedly in both the
GFS and ECMWF forecasts after 48 hours, which should hasten the
weakening process. The latest NHC intensity forecast is again a bit
lower than the previous cycle, but remains somewhat higher than the
consensus aids which show a slightly faster rate of weakening. There
are still timing differences in the guidance regarding exactly when
Franklin will become extratropical. The day 4 extratropical
transition time frame in the NHC official forecast is a compromise
between the faster GFS and slower ECMWF solutions.
 
KEY MESSAGES:
 
1. Tropical storm conditions are possible on Bermuda beginning
Wednesday morning, when Franklin is forecast to make its closest
approach to the island.
 
2. Life-threatening surf and rip currents are occurring along the
coast of the southeast United States. These conditions are
expected to spread northward along the remainder of the U.S.
east coast, Atlantic Canada, and Bermuda later today into Wednesday.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  29/0900Z 30.2N  70.8W  120 KT 140 MPH
 12H  29/1800Z 31.3N  70.3W  110 KT 125 MPH
 24H  30/0600Z 33.0N  68.8W  100 KT 115 MPH
 36H  30/1800Z 34.8N  66.3W   95 KT 110 MPH
 48H  31/0600Z 36.7N  62.8W   90 KT 105 MPH
 60H  31/1800Z 38.6N  58.5W   80 KT  90 MPH
 72H  01/0600Z 41.4N  53.1W   75 KT  85 MPH
 96H  02/0600Z 48.2N  41.1W   65 KT  75 MPH...POST-TROP/EXTRATROP
120H  03/0600Z 59.1N  27.5W   50 KT  60 MPH...POST-TROP/EXTRATROP
 
$$
Forecaster Papin
 
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