000 WTNT41 KNHC 210250 TCDAT1 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. FORECAST POSITIONS AND MAX WINDS 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