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Tropical Storm Peter Forecast Discussion


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000
WTNT41 KNHC 191458
TCDAT1

Tropical Storm Peter Discussion Number   4
NWS National Hurricane Center Miami FL       AL162021
1100 AM AST Sun Sep 19 2021

Earlier this morning, Peter began to encounter increasing 
southwesterly shear emanating from flow around an upper-level 
trough to its northwest. This shear caused the low-level center of 
the storm to separate from the deep convection and as of now is 
located over 100 n mi from the edge of that band of convection. An 
Air Force Reserve Hurricane Hunter aircraft has been investigating 
the cyclone for the past few hours and has provided helpful data on 
the structure and intensity of Peter. Based on the aircraft data, 
tropical-storm-force winds extend at least 100 n mi to the northeast 
of the center, while there are no tropical-storm-force winds in the 
southern semicircle. The initial intensity of 40 kt is based on 
aircraft passes through the northeastern quadrant that measured 
peak 925 mb flight-level winds of 56 and 54 kt. 

The initial motion is 290/15 kt. Peter is forecast to continue to 
move in this west-northwestward direction for the next couple of 
days as it is steered to the south of a subtropical ridge. This 
ridge is expected to weaken in a few days which should cause the 
cyclone to turn northwestward. Late in the forecast period a turn to 
the north and possibly northeast is expected to occur as Peter gets 
caught in the flow around a large trough to its north. As mentioned 
in the special advisory discussion a couple hours ago, a shift to 
the west-southwest of the track was required to accommodate a 
initial position adjustment. Some additional southward adjustments 
were made to the official NHC track for this advisory to come into 
better agreement with a blend of the GFS/ECMWF solutions that also 
indicate a shallower system. 

The UW-CIMSS shear analysis suggests about 20 kt of southwesterly 
shear is impacting Peter. Just to its west and northwest, where the 
cyclone is heading, the shear is analyzed as 30 kt or greater. 
Given that Peter is already exhibiting the structure of a highly 
sheared tropical cyclone, some slight weakening is now forecast 
tonight into tomorrow. There is quite a bit of spread in the 
environmental forecast between the GFS and ECMWF beyond day 3, as 
the GFS shows shear increasing to 40 kt, while the ECMWF indicates 
a less hostile environment with 20 kt of shear. Assuming Peter 
survives its interaction with the upper trough to its northwest 
over the next few days, additional weakening is indicated 
due to the ongoing shear. It should be noted that quite a few GFS 
ensemble members open Peter into a trough by the end of the 
forecast period. The latest NHC intensity forecast is near the 
various consensus solutions. However, due to the possibility the 
cyclone may not survive the next few days, the confidence in this 
forecast is lower than normal. 

Based on the track, intensity, and wind radii forecast, no tropical
storm watches or warnings are required for the northern Leeward
Islands, Virgin Islands or Puerto Rico.  However, interests there
should monitor the progress of the system as locally heavy rain is
possible on Monday and Tuesday when it is expected to pass to the
north of the area.


Key Messages:

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


FORECAST POSITIONS AND MAX WINDS

INIT  19/1500Z 17.6N  56.5W   40 KT  45 MPH
 12H  20/0000Z 18.3N  58.6W   40 KT  45 MPH
 24H  20/1200Z 19.2N  60.8W   35 KT  40 MPH
 36H  21/0000Z 20.2N  63.0W   35 KT  40 MPH
 48H  21/1200Z 21.3N  64.9W   35 KT  40 MPH
 60H  22/0000Z 22.5N  66.3W   35 KT  40 MPH
 72H  22/1200Z 23.8N  67.0W   35 KT  40 MPH
 96H  23/1200Z 25.5N  67.4W   30 KT  35 MPH
120H  24/1200Z 27.2N  66.7W   30 KT  35 MPH

$$
Forecaster Latto