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Tropical Depression Peter Forecast Discussion Number 13
2021-09-21 22:50:48| National Hurricane Center (Atlantic)
Issued at 500 PM AST Tue Sep 21 2021 000 WTNT41 KNHC 212050 TCDAT1 Tropical Depression Peter Discussion Number 13 NWS National Hurricane Center Miami FL AL162021 500 PM AST Tue Sep 21 2021 Like clockwork compared to prior days, deep convection has reignited closer to the increasingly elongated low-level circulation of Peter. Despite this increase in convective coverage, there does not appear to be much if any organization to this activity, with the upper-level cirrus taking on the appearance of a shapeless blob. In addition, an earlier 1157 UTC ASCAT-A pass received just after the prior advisory showed a peak wind retrieval of only 29 kt, well to the north of the low-level circulation. Furthermore, NOAA buoy 41043, located north of the center of Peter, has been observing peak 1-minute sustained winds between 20-25 kt over the last 6 hours. These lower winds, in combination with the lack of tropical storm force winds observed by the earlier Air Force Reserve Reconnaissance mission provide enough justification to downgrade Peter to a tropical depression this advisory with maximum sustained winds of 30 kt. The ongoing afternoon convection appears to have slowed down the forward motion of Peter a bit this afternoon, but the heading remains off to the west-northwest at 300/7 kt. Peter is still expected to gradually move to the west-northwest in the short term, followed by a somewhat sharp turn to the north and north-northeast as a weakness in the low-level flow develops from a deep-layer trough positioned near Bermuda. Similar to this morning, the guidance is in general agreement on this solution, with some cross- and along-track spread. The latest NHC track forecast is just a bit more right compared to the previous advisory, leaning a bit closer to the consensus aids which have also shifted a bit right this advisory. Peter's convective activity is unlikely to help improve the increasingly elongated vortex, mainly because the convection is likely to entrain dry mid-level air that often results in cool downdrafts disrupting the low-level cyclonic circulation more than helping. With deep-layer shear between 25-35 kt expected to persist for the next 36 hours in both the GFS- and ECMWF-SHIPS guidance, gradual weakening is likely to continue. By 36 hours, while intermittent bursts of deep convection will remain possible over warm sea-surface temperatures, both the GFS and ECMWF forecast simulated IR brightness temperatures show the convection becoming increasingly disorganized and unlikely to sustain Peter's status as a tropical cyclone. Thus, the cyclone is now forecast to become a remnant low in 36 hours. However, given that Peter's circulation is becoming increasingly elongated, it is also possible the system may open up into a trough even before this time period. Key Messages: 1. Rainfall around the southern periphery of Tropical Depression Peter could lead to areas of urban and small stream flooding through the evening across northern Hispaniola, Puerto Rico, portions of the Virgin Islands, and the Northern Leeward Islands. FORECAST POSITIONS AND MAX WINDS INIT 21/2100Z 20.5N 65.0W 30 KT 35 MPH 12H 22/0600Z 21.3N 66.1W 30 KT 35 MPH 24H 22/1800Z 22.3N 67.2W 25 KT 30 MPH 36H 23/0600Z 23.2N 67.4W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 23/1800Z 24.2N 66.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 24/0600Z 25.3N 66.1W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 24/1800Z 26.7N 65.1W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 25/1800Z 28.7N 63.7W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 26/1800Z...DISSIPATED $$ Forecaster Papin
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