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Tropical Depression Thirteen Forecast Discussion Number 3

2020-08-20 16:52:11| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Thu Aug 20 2020 000 WTNT43 KNHC 201452 TCDAT3 Tropical Depression Thirteen Discussion Number 3 NWS National Hurricane Center Miami FL AL132020 1100 AM AST Thu Aug 20 2020 The organization of the depression has not changed much overnight or this morning. An area of convection has persisted near the estimated center, with some banding noted over the northwestern portion of the circulation. An earlier SSMIS overpass was very helpful in locating the center of what appears to be a small circulation. A very recent ASCAT overpass has also revealed a small circulation that is weak on the southeastern side, but with winds near tropical storm strength to the north of the center. The ASCAT data along with subjective Dvorak classification from TAFB and SAB support maintaining the 30-kt initial intensity. The depression continues to move briskly west-northwestward or 290/18 kt. The track forecast philosophy has not changed from before. A subtropical ridge over the central Atlantic is forecast to build westward and strengthen over the next several days. This pattern is expected to keep the cyclone on a west-northwestward heading throughout the forecast period. The dynamical models continue to agree on this overall scenario, but there some differences in both forward speed and how close it gets to the Greater Antilles. In general, the models that indicate a stronger cyclone favor a more northern track, while those which depicted a weaker system are along the southern and faster side of the envelope. The latest consensus aids are little north of the previous track, and the new NHC forecast lies between the HFIP corrected consensus and the TVCA multi-model consensus. This is slightly north of the previous advisory, and not far from the GFS ensemble mean. The environment consisting of light to moderate vertical wind shear is expected to allow for gradual strengthening over the next few days, and the NHC forecast calls for the system to become a tropical storm later today or tonight. The upper-level wind pattern is expected to remain favorable in the latter portion of the forecast period, and if there is minimal land interaction, a faster rate of strengthening is possible at that time. The NHC intensity foreast now shows the system becoming a hurricane by 96 hours, but it is a little lower than the consensus aids at days 4 and 5 due to uncertainty in how much the system will interact with the Greater Antilles. Key Messages: 1. Tropical storm conditions are possible across portions of the northern Leeward Islands by Friday night, and Tropical Storm Watches have been issued for some of these islands. Heavy rainfall is likely across this area beginning late Friday. 2. There is a risk of tropical storm conditions in the Virgin Islands and Puerto Rico Friday night and Saturday and Tropical Storm Watches could be required for these islands later today. Interests there should closely monitor the progress of this system. 3. The details of the long-range track and intensity forecasts are more uncertain than usual since the system could move over portions of the Greater Antilles this weekend. However, this system could bring some storm surge, rainfall and wind impacts to portions of Hispaniola, Cuba, the Bahamas, and Florida this weekend and early next week. Interests there should monitor this system's progress and updates to the forecast over the next few days. FORECAST POSITIONS AND MAX WINDS INIT 20/1500Z 16.0N 52.0W 30 KT 35 MPH 12H 21/0000Z 17.0N 54.7W 35 KT 40 MPH 24H 21/1200Z 18.0N 58.3W 40 KT 45 MPH 36H 22/0000Z 18.9N 61.6W 45 KT 50 MPH 48H 22/1200Z 19.6N 64.7W 50 KT 60 MPH 60H 23/0000Z 20.4N 68.1W 55 KT 65 MPH 72H 23/1200Z 21.5N 71.8W 60 KT 70 MPH 96H 24/1200Z 24.0N 78.6W 65 KT 75 MPH 120H 25/1200Z 27.0N 83.4W 65 KT 75 MPH $$ Forecaster Brown

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Hurricane Genevieve Forecast Discussion Number 17

2020-08-20 16:41:00| Tropical Depression LIDIA

Issued at 900 AM MDT Thu Aug 20 2020

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Tropical Depression Thirteen Forecast Discussion Number 2

2020-08-20 11:01:35| National Hurricane Center (Atlantic)

Issued at 500 AM AST Thu Aug 20 2020 000 WTNT43 KNHC 200901 TCDAT3 Tropical Depression Thirteen Discussion Number 2 NWS National Hurricane Center Miami FL AL132020 500 AM AST Thu Aug 20 2020 There has been little change in the organization of Tropical Depression Thirteen during the past several hours. An area of ragged but persistent convection continues near the estimated center position, and there is some outer banding in the southern semicircle. Dvorak-based satellite intensity estimates have changed little since the last advisory, so the initial intensity remains 30 kt. The initial motion is a somewhat uncertain 295/18. The subtropical ridge to the north of the cyclone is expected to build westward to the north of the Greater Antilles during the next several days, and this should steer the cyclone generally west-northwestward through the forecast period. The track guidance remains in fairly good agreement with this scenario, but there remains a spread between the GFS/UKMET on the south side of the guidance and the Canadian/HMON on the north side. Overall, the envelope has shifted a little to the south since the previous advisory and the new forecast track, which lies just to the north of the consensus models, is also nudged southward. The new track calls for the cyclone to pass near the Leeward Islands in 36-48 h, near or north of Puerto Rico and the Virgin Islands in 48-60 h, and then near or over Hispaniola and the southeastern Bahamas by 72 h. Overall, the environment looks generally favorable for strengthening, with the cyclone expected to encounter light- to moderate shear during forecast period. However, the guidance responds to this environment with a wide range of solutions. The HWRF/HMON forecast the cyclone to intensify into a major hurricane by 120 h. On the other hand, the GFS and ECMWF show the system degenerating into an open wave by 120 h. The UKMET and Canadian models are between these extremes. The weak GFS solution appears to be due to forecast dry air entrainment, which is a possibility as satellite imagery suggests Saharan dust not far from the cyclone. Between these factors and the possibility of land interaction, the low-confidence intensity forecast is changed little from the previous advisory. Key Messages: 1. Tropical storm conditions are possible across portions of the northern Leeward Islands by Friday night, and Tropical Storm Watches have been issued for some of these islands. Heavy rainfall is likely across this area beginning late Friday. 2. There is a risk of tropical storm conditions in the Virgin Islands and Puerto Rico Friday night and Saturday and Tropical Storm Watches could be required for these islands later today. Interests there should closely monitor the progress of this system. 3. The details of the long-range track and intensity forecasts are more uncertain than usual since the system could move over portions of the Greater Antilles this weekend. However, this system could bring some rainfall and wind impacts to portions of Hispaniola, Cuba, the Bahamas, and Florida this weekend and early next week. Interests there should monitor this system's progress and updates to the forecast over the next few days. FORECAST POSITIONS AND MAX WINDS INIT 20/0900Z 15.2N 49.8W 30 KT 35 MPH 12H 20/1800Z 16.4N 52.7W 30 KT 35 MPH 24H 21/0600Z 17.5N 56.4W 35 KT 40 MPH 36H 21/1800Z 18.4N 60.2W 40 KT 45 MPH 48H 22/0600Z 19.1N 63.9W 45 KT 50 MPH 60H 22/1800Z 19.8N 67.1W 50 KT 60 MPH 72H 23/0600Z 20.8N 71.0W 55 KT 65 MPH 96H 24/0600Z 23.0N 78.0W 60 KT 70 MPH 120H 25/0600Z 26.0N 83.0W 60 KT 70 MPH $$ Forecaster Beven

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Hurricane Genevieve Forecast Discussion Number 16

2020-08-20 10:40:36| Tropical Depression LIDIA

Issued at 300 AM MDT Thu Aug 20 2020

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Tropical Depression Thirteen Forecast Discussion Number 1

2020-08-20 04:52:34| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Wed Aug 19 2020 000 WTNT43 KNHC 200252 TCDAT3 Tropical Depression Thirteen Discussion Number 1 NWS National Hurricane Center Miami FL AL132020 1100 PM AST Wed Aug 19 2020 The low pressure system that NHC has been monitoring over the central tropical Atlantic has now developed a well-defined center of circulation and maintained enough organized deep convection to be classified as a tropical depression, the thirteenth cyclone of the 2020 Atlantic hurricane season. The depression already has some banding features on its north and west sides as evident in geostationary satellite and microwave images. The initial intensity is set at 30 kt based on ASCAT data from around 0100 UTC and a T2.0/30 kt Dvorak classifications from TAFB. The depression has been moving fairly quickly to the west-northwest, with the latest initial motion estimated to be 295/17 kt. It should be noted that the initial motion is somewhat uncertain given that the system has only recently formed. A subtropical ridge currently over the central Atlantic is expected to build westward during the next several days and should be the primary steering feature for the depression through the forecast period. This pattern should keep the depression on a fairly quick west-northwest track during the next several days, taking the cyclone near the northern Leeward Islands by Friday night and near the Greater Antilles and southeastern Bahamas this weekend. The models are in fairly good agreement, but there is some north-south spread with the GFS being on the southern side of the guidance envelope and the ECMWF on the northern end. The NHC track forecast lies down the middle of the guidance suite. The environmental conditions appear generally favorable for the depression to strengthen, with the wind shear expected to remain relatively low while the system moves over warm SSTs and remains in a moist airmass. These conditions should promote gradual strengthening and it seems quite likely that the cyclone will be a tropical storm when it moves near or north of the northern Leeward Islands in a couple of days. The bigger question is how much interaction will there be with the Greater Antilles. If the depression moves on the south side of the guidance envelope, further strengthening would be limited due to land interaction. Conversely, if the system gains more latitude and moves north of these highly topographic islands, it could have the opportunity for more significant intensification. The NHC intensity forecast, which is of low confidence, is roughly near the middle of the guidance envelope. Key Messages: 1. Tropical storm conditions are possible across portions of the northern Leeward Islands by Friday night, and Tropical Storm Watches have been issued for some of these islands. Heavy rainfall is likely across this area beginning late Friday. 2. There is a risk of tropical storm conditions in the Virgin Islands and Puerto Rico Friday night and Saturday and Tropical Storm Watches could be required for these islands tomorrow. Interests there should closely monitor the progress of this system. 3. The details of the long-range track and intensity forecasts are more uncertain than usual since the system could move over portions of the Greater Antilles this weekend. However, this system could bring some rainfall and wind impacts to portions of Hispaniola, Cuba, the Bahamas, and Florida this weekend and early next week. Interests there should monitor this system's progress and updates to the forecast over the next few days. FORECAST POSITIONS AND MAX WINDS INIT 20/0300Z 14.6N 47.9W 30 KT 35 MPH 12H 20/1200Z 16.1N 50.8W 30 KT 35 MPH 24H 21/0000Z 17.4N 54.6W 35 KT 40 MPH 36H 21/1200Z 18.4N 58.4W 40 KT 45 MPH 48H 22/0000Z 19.1N 62.1W 45 KT 50 MPH 60H 22/1200Z 19.9N 65.9W 50 KT 60 MPH 72H 23/0000Z 20.8N 69.7W 55 KT 65 MPH 96H 24/0000Z 22.9N 76.7W 60 KT 70 MPH 120H 25/0000Z 26.2N 82.2W 60 KT 70 MPH $$ Forecaster Cangialosi

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