Atlantic Hurricane Feed

Tropical Storm Peter Forecast Discussion

WTNT41 KNHC 210250

Tropical Storm Peter Discussion Number  10
NWS National Hurricane Center Miami FL       AL162021
1100 PM AST Mon Sep 20 2021

Peter has proven to be a resilient tropical cyclone, despite strong 
upper-level wind shear that continues to displace its deep 
convection well east of its now exposed low-level center. Data from 
an Air Force Reserve Hurricane Hunter aircraft investigating Peter 
tonight, as well as recent scatterometer data, indicate that the 
cyclone has maintained its tropical storm intensity. An ASCAT-A pass 
shows several 40-kt wind vectors, with tropical-storm-force winds 
extending outward up to 150 n mi from the center in the northeastern 
quadrant. The aircraft has found peak 925-mb flight-level winds of 
47 kt and SFMR winds of around 35 kt, although it did not sample the 
area where ASCAT depicted the strongest winds. The initial intensity 
is held at a possibly generous 45 kt for this advisory based on the 
scatterometer data.

An upper-level trough to the northwest of Peter should maintain 20 
to 30 kt of vertical wind shear over the cyclone for the next 2-3 
days. Thus, intensification seems unlikely during this period, 
despite 29 deg C SSTs along Peter's forecast track. If the tropical 
cyclone can endure these hostile upper-level winds, it could survive 
through the entire forecast period, although the drier mid-level 
environment at higher latitudes will also work against Peter later 
this week. However, an alternative scenario that has been favored by 
the GFS is that Peter weakens sooner due to a lack of sustained 
convection and opens up into a trough late this week. The long-range 
forecast is further complicated by the potential development of 
another non-tropical low to the north of Peter later this week, 
which could interact with or absorb Peter. The official NHC 
intensity forecast is similar to the previous one and only shows 
gradual weakening over the next several days, which is in good 
agreement with the HCCA and IVDR consensus aids.

Data from the aircraft indicate that the center has moved westward 
and slowed down a bit over the past several hours, and Peter's 
initial motion is estimated to be 285/10 kt. Peter is expected 
to continue moving generally west-northwestward through Tuesday, as 
it is steered around the southern extent of a low- to mid-level 
ridge over the central and western Atlantic. By Wednesday, a 
mid-level cutoff low is forecast to develop to the north of Peter 
over the western Atlantic, which will induce a weakness in the 
steering ridge. Therefore, the cyclone is forecast to slow down and 
turn northward and then north-northeastward through the latter part 
of the week and into the weekend. There are some larger along-track 
differences noted in the guidance at days 4-5, with the ECMWF moving 
or re-forming the center much farther north than the rest of the 
track guidance. The official NHC track forecast is shifted slightly 
to the right at 48 h and beyond, based on the latest TVCA and HCCA 
consensus aids. At longer ranges, the forecast is of much lower 
confidence and trends a bit slower than the consensus aids, which 
are heavily influenced by the outlying ECMWF solution.

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 northern Hispaniola, Puerto Rico, the Virgin Islands, and the
Northern Leeward Islands.


INIT  21/0300Z 19.8N  62.8W   45 KT  50 MPH
 12H  21/1200Z 20.4N  64.2W   40 KT  45 MPH
 24H  22/0000Z 21.3N  66.0W   40 KT  45 MPH
 36H  22/1200Z 22.3N  67.2W   35 KT  40 MPH
 48H  23/0000Z 23.2N  67.7W   35 KT  40 MPH
 60H  23/1200Z 24.0N  67.7W   30 KT  35 MPH
 72H  24/0000Z 25.1N  67.2W   30 KT  35 MPH
 96H  25/0000Z 27.5N  66.0W   30 KT  35 MPH
120H  26/0000Z 30.5N  64.5W   30 KT  35 MPH

Forecaster Reinhart

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