National Hurricane Center Weather Outlook

Tropical Storm Peter Forecast Discussion

WTNT41 KNHC 201444

Tropical Storm Peter Discussion Number   8
NWS National Hurricane Center Miami FL       AL162021
1100 AM AST Mon Sep 20 2021

West-southwesterly vertical wind shear on the order of 30 kt is 
pummeling Peter this morning. Just like yesterday morning, the 
low-level center of the storm is pulling away from the deep 
convection and is now displaced greater than 80 n mi. An Air Force 
Reserve Hurricane Hunter aircraft has been investigating Peter this 
morning and has measured peak 925-mb flight-level winds of 54 kt 
and several SFMR values of 40-45 kt, supporting keeping the initial 
intensity at 45 kt for this advisory.  

Peter refuses to slow down, and for the past several hours has been 
moving 290/14 kt. This west-northwestward motion is forecast to 
continue for the next two days as the system remains to the 
southwest of a subtropical ridge. By Wednesday, Peter should slow 
down and turn northwest then north as it reaches a developing 
weakness in the ridge carved out by a mid-latitude trough 
moving across the northeastern United States later this week. The 
model track guidance has shifted slightly left for the 24-72 h time 
frames, and the NHC forecast track was nudged in that direction as 
well. Otherwise, the latest NHC forecast track is little changed 
from the previous one. 

An upper-level trough to the west of Peter is forecast to remain 
near the cyclone for the next few days, keeping the storm in a 
high-shear environment. Therefore, despite being over warm waters 
the storm is forecast to slowly weaken. If Peter survives the next 
72 h, there could be a window of opportunity late in the forecast 
period for some modest strengthening as the cyclone would have 
lifted to the north of the upper trough. It should be noted that a 
vast majority of the GFS ensembles show Peter dissipating later this 
week as the system opens into a wave. This scenario is plausible, 
especially if deep convection fails to persist near the center of 
the cyclone. The latest NHC intensity forecast assumes Peter 
survives, and closely follows the various intensity consensus 

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


INIT  20/1500Z 19.5N  60.9W   45 KT  50 MPH
 12H  21/0000Z 20.1N  62.7W   40 KT  45 MPH
 24H  21/1200Z 20.8N  64.8W   40 KT  45 MPH
 36H  22/0000Z 21.8N  66.6W   35 KT  40 MPH
 48H  22/1200Z 22.7N  67.9W   35 KT  40 MPH
 60H  23/0000Z 23.5N  68.6W   30 KT  35 MPH
 72H  23/1200Z 24.5N  68.7W   30 KT  35 MPH
 96H  24/1200Z 26.5N  67.6W   30 KT  35 MPH
120H  25/1200Z 28.6N  66.4W   35 KT  40 MPH

Forecaster Latto

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