[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

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