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Tropical Storm Isaias Forecast Discussion Number 23
2020-08-02 22:57:46| National Hurricane Center (Atlantic)
Issued at 500 PM EDT Sun Aug 02 2020 000 WTNT44 KNHC 022057 TCDAT4 Tropical Storm Isaias Discussion Number 23 NWS National Hurricane Center Miami FL AL092020 500 PM EDT Sun Aug 02 2020 The earlier intense recent burst has waned since this morning, but the large convective cell has persisted. At the peak of the activity around 1500Z, several small patches of Doppler velocities of 90-96 kt were co-located with the pronounced mid-level circulation that was evident in the Melbourne radar reflectivity data. However, these intense wind speed values were short-lived for only about 20 minutes and, thus, were not considered to representative of Isaias' tangential wind field. Since that time, the cyclone has become more steady state with Doppler radar and Air Force Reserve aircraft data indicating surface winds in the 56-63 kt range. Therefore, the initial intensity of 60 kt is an average of these values. Radar and aircraft fixes indicate that Isaias is still moving toward the north-northwest or 345/08 kt. The latest model guidance remains in excellent agreement on Isaias moving north-northwestward through a break in the subtropical ridge tonight and turning northward by Monday morning, all the while remaining offshore of the coast from east-central Florida to Georgia. By Monday night, Isaias is forecast to turn northeastward and accelerate toward the Carolinas, reaching the mid-Atlantic states on Tuesday and New England by early Wednesday. The new NHC forecast track during the first 24 hours lies a little east of the previous one, but is essentially just an extension of the previous advisory track thereafter, and lies close to the various consensus models, which are lightly packed around the previous NHC foreast. Isaias will continue to move slowly over the warm Gulfstream waters for the next 36 h or so. Despite unfavorable vertical shear conditions of around 25 kt, Isaias is expected to maintain its current intensity until landfall, and could restrengthen to hurricane status in 24-36 h when the vertical shear vector is forecast to switch from westerly to southwesterly which would align the shear along the direction of storm motion. Some baroclinic interaction is expected on days 2-3 when Isaias will move into the right-rear quadrant of an anticyclonically curved jet streak, which is expected to hold the intensity a little above what would normally be expected for a post-landfall tropical cyclone. The NHC intensity forecast is similar to the HFIP corrected consensus model and the IVCN intensity consensus model, which agree well with the GFS and ECMWF model intensity forecasts. Key Messages: 1. There is the danger of life-threatening storm surge inundation of 2 to 4 feet above ground level from Edisto Beach South Carolina to Cape Fear North Carolina along the immediate coastline and adjacent waterways. Life-threatening storm surge is possible along the North Carolina coast from Cape Fear to Duck. Residents in these areas should follow advice given by local emergency officials. 2. Tropical storm conditions will spread northward within the Tropical Storm Warning area from Florida to North Carolina through Monday night. Isaias is expected to be near hurricane strength when it reaches the coast of northern South Carolina and southern North Carolina Monday night, and strong tropical storm force winds are likely with hurricane conditions possible in the Hurricane Watch area. 3. Heavy rainfall from Isaias will continue to result in potentially life-threatening flash flooding in the Northwest Bahamas through tonight. Flash and urban flooding, some of which may be significant in the eastern Carolinas and the mid-Atlantic, are expected through midweek along and near the path of Isaias across the U.S. East Coast. Widespread minor to isolated moderate river flooding is possible across portions of the Carolinas and the mid-Atlantic. 4. A tropical storm watch is in effect for the U.S. mid-Atlantic coast, including the Chesapeake Bay, Delaware Bay, and Long Island Sound, as tropical storm force winds are possible in these areas on Tuesday and Tuesday night. Additional watches and warnings will likely be issued tonight and Monday as Isaias is expected to move northward near or over the mid-Atlantic and New England states Tuesday and Wednesday. FORECAST POSITIONS AND MAX WINDS INIT 02/2100Z 27.8N 79.8W 60 KT 70 MPH 12H 03/0600Z 29.0N 80.1W 60 KT 70 MPH 24H 03/1800Z 30.9N 79.9W 60 KT 70 MPH 36H 04/0600Z 33.7N 78.8W 60 KT 70 MPH 48H 04/1800Z 38.1N 75.8W 50 KT 60 MPH...INLAND 60H 05/0600Z 42.5N 72.2W 45 KT 50 MPH...INLAND 72H 05/1800Z 46.3N 68.4W 40 KT 45 MPH...INLAND 96H 06/1800Z 54.2N 58.4W 30 KT 35 MPH...POST-TROP/EXTRATROP 120H 07/1800Z...DISSIPATED $$ Forecaster Stewart
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