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Tropical Storm Peter Forecast Discussion


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WTNT41 KNHC 200251
TCDAT1

Tropical Storm Peter Discussion Number   6
NWS National Hurricane Center Miami FL       AL162021
1100 PM AST Sun Sep 19 2021

Peter is a sheared tropical cyclone. Satellite imagery and microwave
data indicate that an earlier convective burst near the center of
Peter has collapsed tonight. Although the low-level center is now
displaced at least 60 n mi west of the edge of the convective cloud
mass, an Air Force Reserve Hurricane Hunter aircraft investigating
Peter has found the cyclone is a bit stronger tonight. The aircraft
measured flight-level (925 mb) winds of 58 kt and SFMR winds of 45
kt or so. These data support raising the initial intensity to 45 kt
for this advisory.

Peter's wind field is very asymmetric, with tropical-storm-force
winds extending outward up to 120 n mi from the center only in its
northeast quadrant. The moderate to strong southwesterly vertical
wind shear that is plaguing the system is forecast to persist during
the next several days. Thus, despite sufficient oceanic heat content
along its forecast track, the official NHC intensity forecast does
not show any further intensification. In fact, some gradual 
weakening is forecast since the system appears likely to struggle 
sustaining organized convection near its center, as suggested by 
GFS and ECMWF model simulated satellite imagery. The official NHC 
intensity forecast has been adjusted upward in the near-term to 
account for the stronger initial intensity, but otherwise closely 
follows the HCCA and IVCN aids and shows gradual weakening this 
week. While the official NHC forecast shows Peter remaining a 
tropical cyclone through the forecast period, the GFS suggests 
Peter could struggle to even maintain its closed low-level 
circulation in the coming days. Therefore, it is plausible that the 
cyclone could degenerate into an open wave and weaken somewhat 
quicker than forecast.

Peter is moving west-northwestward, or 295/12 kt, along the 
southwestern periphery of a subtropical ridge over the central 
Atlantic. This general motion should continue for the next couple of 
days. Thereafter, the track forecast becomes more challenging. A 
low- to mid-level ridge is expected to build over the western 
Atlantic by midweek, which would keep the weakening cyclone on a 
more northwestward trajectory. But, a mid-level shortwave is 
forecast to drop southward and erode the southern extent of the 
ridge, which should eventually draw Peter more northward during the 
middle and latter parts of the week. There is more spread noted in 
the track guidance at days 3-5, as the timing of this northward turn 
is uncertain. The official NHC track forecast has been adjusted 
slightly to the left of the previous one, and it lies near the 
center of the guidance envelope and closer to the TVCA and HCCA 
aids.

Based on the latest track, intensity, and wind radii forecasts, no
tropical storm watches or warnings are required for the northern
Leeward Islands, Virgin Islands, or Puerto Rico at this time.
However, locally heavy rain is possible on Monday and Tuesday when
Peter is expected to pass to the north of these locations.


Key Messages:

1. Rainfall around the southern periphery of Tropical Storm Peter
may lead to areas of urban and small stream flooding through Tuesday
across Puerto Rico, the Virgin Islands, and the Northern Leeward
Islands.


FORECAST POSITIONS AND MAX WINDS

INIT  20/0300Z 18.6N  58.5W   45 KT  50 MPH
 12H  20/1200Z 19.3N  60.6W   40 KT  45 MPH
 24H  21/0000Z 20.1N  62.9W   40 KT  45 MPH
 36H  21/1200Z 21.0N  65.0W   35 KT  40 MPH
 48H  22/0000Z 22.0N  66.7W   35 KT  40 MPH
 60H  22/1200Z 23.0N  67.9W   30 KT  35 MPH
 72H  23/0000Z 23.9N  68.6W   30 KT  35 MPH
 96H  24/0000Z 25.7N  68.5W   25 KT  30 MPH
120H  25/0000Z 28.0N  67.5W   25 KT  30 MPH

$$
Forecaster Reinhart