Tropical Depression Fifteen Forecast Discussion Number 4

By | September 16, 2023

WTNT45 KNHC 160857

Tropical Depression Fifteen Discussion Number   4
NWS National Hurricane Center Miami FL       AL152023
500 AM AST Sat Sep 16 2023

Finding the exact center of the tropical depression this morning has 
been a challenge. While there is a large region of mid-level 
cyclonic rotation in the shortwave IR imagery, there isn't much 
evidence that the low-level circulation has tightened up any from 
the earlier ASCAT passes. This was confirmed by a 0501 UTC AMSR2 
microwave pass which only had a subtle hint of banding with the 
ongoing convection in the depression's northern semicircle. 
Subjective Dvorak estimates were 25 kt and 35 kt from SAB and TAFB, 
respectively. Objective intensity estimates were also in the 30-35 
kt range. Given the continued broad structure, the intensity is held 
at 30 kt for this advisory.

Given the current structure of the depression, its initial motion 
has higher-than-normal uncertainty but is currently estimated at 
330/12 kt. A general northwest to north-northwest motion is expected 
to continue for the next day or two, as the system is steered 
between a mid-level ridge centered to its northeast, and a 
deep-layer trough, centered to its west. The track guidance in the 
short-term has actually shifted a bit more poleward early on, and 
the NHC track was nudged in that direction over the first 48 hours 
or so. One interesting caveat to this track forecast is Margot, 
which has weakened the overall magnitude of the ridge steering 
TD15, and may explain the more poleward shift in the guidance 
the last few cycles. Thereafter, as Margot moves out of the 
picture, the mid-level ridge should briefly build back in, and 
the guidance this cycle shows a subtle bend more northwestward 
between 48-72 hours. However, another mid-latitude trough is 
expected to move off the eastern U.S. coastline in about 4-5 days, 
eroding this ridge enough to begin recurvature by the end of the 
forecast period. The NHC track forecast is in best agreement with 
the TVCN and HCCA consensus aids. 

The intensity forecast early on is largely dependent on how TD15's 
broad structure evolves, since environmentally vertical wind shear 
is low, sea-surface temperatures are anomalously warm, and the 
system is in a pocket of enhanced mid-level moisture. The 
uncertainty of structural evolution is highlighted by the regional 
hurricane models, which over the past few cycles have oscillated 
between the system developing a small inner core versus a broader 
structure that intensifies more slowly despite the favorable 
environment. The latest intensity forecast assumes it will take a 
day or so for the core to consolidate, only showing a slow rate of 
intensification for the next 24 hours, which is on the low side of 
the intensity guidance. However, assuming an inner core does form in 
the 36-48 hour time frame, a period of rapid intensification is 
possible, which is best highlighted by the ECMWF-SHIPS guidance 
which shows a 45 percent chance of a 65-kt increase in intensity in 
72 hours, more than eight times the climatological mean. This 
possibility is shown in the NHC intensity forecast, which shows a 
peak intensity of 95 kt in 72 hours. While not explicitly shown, it 
still remains possible TD15 could become a major hurricane sometime 
between the 72-96 hour forecast period, though some increase in 
southwesterly shear and inner core fluctuations will likely result 
in gradual weakening by the end of the forecast. The NHC intensity 
forecast is somewhat higher than the consensus aids at peak 


INIT  16/0900Z 17.9N  45.3W   30 KT  35 MPH
 12H  16/1800Z 19.6N  46.4W   35 KT  40 MPH
 24H  17/0600Z 21.5N  48.3W   40 KT  45 MPH
 36H  17/1800Z 23.1N  49.8W   50 KT  60 MPH
 48H  18/0600Z 24.5N  51.3W   60 KT  70 MPH
 60H  18/1800Z 25.9N  53.0W   75 KT  85 MPH
 72H  19/0600Z 27.2N  55.1W   95 KT 110 MPH
 96H  20/0600Z 31.0N  58.4W   90 KT 105 MPH
120H  21/0600Z 35.5N  56.5W   80 KT  90 MPH

Forecaster Papin

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