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