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Tropical Depression Eleven Forecast Discussion Number 3
2020-08-12 11:00:28| National Hurricane Center (Atlantic)
Issued at 500 AM AST Wed Aug 12 2020 000 WTNT41 KNHC 120900 TCDAT1 Tropical Depression Eleven Discussion Number 3 NWS National Hurricane Center Miami FL AL112020 500 AM AST Wed Aug 12 2020 After an earlier burst of deep convection near the center and in the western semicircle of the circulation, overall thunderstorm activity has decreased somewhat. This is likely due to the entrainment of mid-/upper-level dry air as noted in GOES-16 high-resolution mid- and upper-level water vapor imagery, in conjunction with some modest southeasterly vertical wind shear. The latest subjective and objective Dvorak satellite classifications from TAFB/SAB and UW-CIMSS ADT of 30 kt and 33 kt, respectively, support maintaining an initial intensity of 30 kt. The depression continues moving a little north of due west, or 280/13 kt. This general motion is expected to continue through tonight due to easterly flow on the south side of a strong subtropical ridge situated to the north of the cyclone. By 24 h, all of the global models are in decent agreement that a slight weakness will develop within the ridge, allowing the cyclone to turn more toward the west-northwest and then continue that motion through the end of the forecast period. The latest NHC model guidance is tightly packed about the previous forecast track, so no significant track changes were made. The aforementioned unfavorable conditions of dry air entrainment and southeasterly shear are expected to give way to more conducive environmental conditions by 24 h when the cyclone will begin to move underneath the center of a synoptic-scale upper-level anticyclone. This will result in the shear decreasing to near zero, along with a pronounced improvement in the upper-level outflow pattern. The much lower shear conditions should also reduce the amount of dry air entrainment, while allowing for some moistening of the surrounding environment to occur. These more favorable conditions are expected to persist through at least the 60-h period, and thus slow but steady strengthening is forecast during that time. By 72 h and beyond, the global models and regional models show the system moving out from underneath the positive influence of the upper-level anticyclone, encountering moderate southerly to southwesterly vertical wind shear, which is expected to induce gradual weakening. It should be noted that during the 48-60 h period when the shear will be the lowest and sea-surface temperatures will be near 28.5C, there is a narrow window of opportunity where the intensity could peak higher than what is currently indicated. The new NHC intensity forecast is very similar to the previous advisory, and is slightly higher than the consensus intensity consensus models IVCN and HCCA due to anticipation of the very favorable low-shear conditions. FORECAST POSITIONS AND MAX WINDS INIT 12/0900Z 12.2N 42.9W 30 KT 35 MPH 12H 12/1800Z 12.6N 44.8W 35 KT 40 MPH 24H 13/0600Z 13.2N 47.3W 40 KT 45 MPH 36H 13/1800Z 14.0N 49.6W 45 KT 50 MPH 48H 14/0600Z 15.1N 52.1W 50 KT 60 MPH 60H 14/1800Z 16.3N 54.6W 50 KT 60 MPH 72H 15/0600Z 17.6N 57.0W 45 KT 50 MPH 96H 16/0600Z 20.1N 61.8W 40 KT 45 MPH 120H 17/0600Z 22.9N 65.6W 35 KT 40 MPH $$ Forecaster Stewart
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