[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT43 KNHC 270858
TCDAT3

Hurricane Franklin Discussion Number  27
NWS National Hurricane Center Miami FL       AL082023
500 AM AST Sun Aug 27 2023

After becoming obscured for a time earlier tonight, the eye of
Franklin is beginning to re-emerge on infrared satellite images
early this morning. The last helpful microwave pass at 0246 UTC
showed some disruption of Franklin's inner core that could have
been due to southerly shear causing the eyewall to be open to the
east. This structure was also observed by both NOAA and Air Force
Reconnaissance missions earlier tonight. Intensity estimates from
SAB and TAFB were both T4.5/77 kt this morning, but with the recent
re-emergence of the eye, the advisory intensity is bumped up to 80
kt for this advisory.

Franklin is moving to the northwest this morning with an estimated
motion of 320/7 kt. The track forecast over the next 48 h is
relatively straightforward as the cyclone is expected to round the
western side of a mid-level subtropical ridge to its east. The
track guidance over this time span is in good agreement, and the
latest NHC track forecast is very close to or just a bit west of
the prior one through 48 h. Afterwards, uncertainty quickly
increases, primarily related to whether Franklin will be captured
by a mid-latitude trough emerging off the eastern US coastline.
While the GFS continues to show the hurricane being captured by
this trough, the latest runs of the ECMWF, CMC, and UKMET now leave
the cyclone behind, resulting in it turning more eastward and
slowing down as the trough bypasses it to its north. Both the
ECMWF and GFS ensembles show a huge amount of spread, spanning more
than 1000 mi at the end of the forecast period. While the latest
NHC track forecast will not shift all the way to Franklin being
left behind by the trough, it is noticeably slower and eastward
compared to the prior track at days 4 and 5. Future adjustments to
the southeast may become necessary in subsequent cycles. On the
forecast track, Franklin will be making its closest approach to
Bermuda in 3-4 days, and is a bit closer to Bermuda than the prior
advisory.

As long as the small eyewall observed by earlier recon and microwave
imagery is closing back off, Franklin should continue to intensify,
likely rapidly due to favorable low shear and ample warm waters
under the hurricane. DTOPS continues to suggest between a 59-68
percent chance of a 30 kt intensity increase in the next 24 hours,
and the latest intensity forecast is close to this scenario, taking
the storm to a category 3 hurricane in 24 hours and peaking it as a
115-kt category 4 hurricane in 36 hours. This is near the higher
end of the intensity guidance, but is quite close to the latest
HFIP corrected consensus forecast. Afterwards, there are a couple
of factors that could cause Franklin to start weakening. First, the
storm will likely undergo fluctuations related to eyewall
replacement cycles, which would also broaden its overall wind
field. In addition, the motion of Franklin is relatively slow, and
the warm 29-30 C waters it will be traversing are fairly shallow,
leaving the cyclone prone to cool upwelling along its track as the
wind field expands. Vertical wind shear out of the northwest also
increases after 36 hours. All of these factors are why the latest
NHC intensity forecast shows weakening beginning after this period,
which is also in good agreement with both the simple and corrected
consensus aids. Given the large spread of solutions at days 4 and 5,
it is unclear if Franklin will start to undergo extratropical
transition during that time, or will be left behind by the trough,
resulting in a slower motion that stays over warmer waters
south of the Gulf Stream.


FORECAST POSITIONS AND MAX WINDS

INIT  27/0900Z 24.7N  68.7W   80 KT  90 MPH
 12H  27/1800Z 25.6N  69.4W   90 KT 105 MPH
 24H  28/0600Z 26.9N  70.3W  105 KT 120 MPH
 36H  28/1800Z 28.4N  70.6W  115 KT 130 MPH
 48H  29/0600Z 29.9N  70.4W  110 KT 125 MPH
 60H  29/1800Z 31.3N  69.5W  100 KT 115 MPH
 72H  30/0600Z 33.0N  67.9W   90 KT 105 MPH
 96H  31/0600Z 36.5N  61.0W   80 KT  90 MPH
120H  01/0600Z 40.0N  52.0W   70 KT  80 MPH

$$
Forecaster Papin
This weather bulletin was brought to you by:
www.nashvilleweather.net