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Tropical Depression Eighteen Forecast Discussion Number 1
2021-09-22 22:54:37| National Hurricane Center (Atlantic)
Issued at 500 PM AST Wed Sep 22 2021 000 WTNT43 KNHC 222054 TCDAT3 Tropical Depression Eighteen Discussion Number 1 NWS National Hurricane Center Miami FL AL182021 500 PM AST Wed Sep 22 2021 The tropical wave that NHC has been monitoring over the last several days has gradually become better organized. The satellite structure in particular is quite impressive for a tropical depression, with both the 1800 UTC subjective Dvorak estimates from TAFB and SAB at T2.5/35 kt. This bigger question, however, was if the system possessed a well-defined closed earth-relative circulation. An ASCAT-C pass from earlier this morning hinted that the circulation was becoming better defined, with the development of westerly low-level winds to the south of the convective shield. This westerly low-level flow is also confirmed by atmospheric motion vectors (AMVs) available from the GOES-16 meso domain over the system. While the low-level circulation may still be somewhat broad, it now appears to be well-defined enough to mark the formation of a tropical cyclone. The initial intensity is set at 30-kt, in agreement with peak wind retrievals of 28-30 kt by the earlier scatterometer data. The initial motion is estimated to be at 270/13 kt, though this is somewhat uncertain given that the center has only recently formed. An expansive mid-level ridge is located to the north and west of the cyclone, which should maintain its heading toward the west, though with a gradual gain in latitude as the system approaches the western extent of the ridge by day 5. The track guidance is in excellent agreement on this track evolution for the first three days, with just a bit more spread in the guidance thereafter. The ECMWF and its ensemble mean towards the end of the forecast is on the left side of the guidance envelope, while the GFS and HWRF models are currently on the right side. For the first NHC track forecast, I have elected to stay close to the track consensus aids, taking a blend of the HCCA and TCVN aids which are near the middle of the guidance envelope. The environment ahead of the tropical depression appears quite favorable for intensification. Vertical wind shear is forecast by both the ECWMF- and GFS-based SHIPS guidance to stay at or under 10 kt for the next 3-5 days as the storm traverses warm 28-29 C sea surface temperatures. Most of the guidance responds to this environment by indicating strengthening, and the NHC intensity guidance follows suit, showing a steady increase in intensity throughout the forecast period. While it might take a bit of time for the formative low-level circulation to become vertically aligned with the mid-level center, once that occurs, it is possible a period of rapid intensification could occur during the five day forecast. The forecast intensity by 120 hours (100 kt) is on the higher end of the guidance envelope, but not as high as the latest HWRF or HAFS-B forecasts. FORECAST POSITIONS AND MAX WINDS INIT 22/2100Z 10.1N 33.9W 30 KT 35 MPH 12H 23/0600Z 10.1N 36.0W 35 KT 40 MPH 24H 23/1800Z 10.5N 38.6W 40 KT 45 MPH 36H 24/0600Z 11.0N 41.1W 50 KT 60 MPH 48H 24/1800Z 11.7N 43.4W 60 KT 70 MPH 60H 25/0600Z 12.4N 45.3W 70 KT 80 MPH 72H 25/1800Z 13.0N 46.8W 80 KT 90 MPH 96H 26/1800Z 14.2N 49.5W 90 KT 105 MPH 120H 27/1800Z 16.0N 52.5W 100 KT 115 MPH $$ Forecaster Papin
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