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Tropical Depression Ten Forecast Discussion Number 1

2020-07-31 22:48:40| National Hurricane Center (Atlantic)

Issued at 800 PM CVT Fri Jul 31 2020 000 WTNT45 KNHC 312048 TCDAT5 Tropical Depression Ten Discussion Number 1 NWS National Hurricane Center Miami FL AL102020 800 PM CVT Fri Jul 31 2020 Deep convection has recently developed closer to the center of a small area of low pressure located a couple of hundred miles east of the Cabo Verde Islands. Earlier ASCAT data indicated peak winds of around 25 kt, but given the small size of the system and the likely undersampling of the instrument, the initial intensity is set at 30 kt. This is in line with a Dvorak classification of T2.0 from SAB. With the recent increase in convective organization, there is a potential for the depression to become a short-lived tropical storm tonight. After that time, the environment is forecast to become increasingly hostile as cooler waters and less favorable thermodynamic conditions lie ahead. As a result, weakening should begin on Saturday and the system is forecast to become a remnant low Saturday night and dissipate shortly thereafter. The system is moving slightly west of due north, or 355/8 kt. The depression is forecast to turn northwestward tonight, then west-northwestward to westward on Saturday and Saturday night as it moves around the eastern side of a large cyclonic gyre over the far eastern Atlantic. The track guidance is in relatively good agreement, and the NHC forecast is near the TVCA multi-model consensus. FORECAST POSITIONS AND MAX WINDS INIT 31/2100Z 15.9N 20.0W 30 KT 35 MPH 12H 01/0600Z 17.3N 21.0W 35 KT 40 MPH 24H 01/1800Z 18.5N 23.2W 30 KT 35 MPH 36H 02/0600Z 18.8N 25.8W 20 KT 25 MPH...POST-TROP/REMNT LOW 48H 02/1800Z...DISSIPATED $$ Forecaster Brown

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Hurricane Isaias Forecast Discussion Number 14

2020-07-31 17:01:08| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT Fri Jul 31 2020 000 WTNT44 KNHC 311501 TCDAT4 Hurricane Isaias Discussion Number 14 NWS National Hurricane Center Miami FL AL092020 1100 AM EDT Fri Jul 31 2020 After a brief convective hiatus earlier this morning, a small burst of deep convection with cloud tops as cold as -80C to -85C have developed over the previously exposed low-level center. Dry downslope air coming off of Hispaniola wrapped into the center, eroding the inner-core convection somewhat. However, a surge of moist southwesterly inflow moving through the Windward Passage now appears to be fueling the recent increase in convection. The upper-level outflow has improved somewhat in the western semicircle but still remains restricted due to some modest southwesterly vertical wind shear. The initial intensity has been lowered to 65 kt based on 700-mb flight-level winds of near 70 kt and an increase in the central pressure of 992 mb, which is based on a dropsonde report of 995 mb with 26-kt surface winds. Isaias is continuing on a steady northwestward motion of 305/14 kt. This motion is expected to continue for the next 36 hours or so as the hurricane rounds the southwestern periphery of the Bermuda-Azores High. A gradual turn to the north-northwest and north is expected by 48-60 hours due to a break in the ridge developing in response to a central U.S. shortwave trough digging into the southeastern United States. The timing and strength of this trough will determine how far west Isaias moves before the hurricane turns northward. The 06Z UKMET and and GFS have shifted westward, closer to the Florida coast, which is similar to the westward shift seen in the 00Z ECMWF model fields. By 72 hours, the hurricane is forecast to begin accelerating northeastward, possibly passing over eastern North Carolina by day 4 and across eastern New England on day 5. As a result of the westward shift in the latest model guidance, the new NHC forecast track has also been shifted farther west closer to the southeastern U.S. coast, and lies a little to the west of the consensus models TVCA and NOAA-HCCA. The westward shift in the track forecast has required the issuance of a Hurricane Watch for portions of the Florida east coast. Although some slight weakening has occurred, radar data from the aircraft and the Bahamas radar indicate about a 60-percent eyewall has formed in the northeastern semicircle, which is an indication that the cyclone is trying to reorganize. As a result, strengthening is still expected during the next day or so, especially tonight and Saturday morning during the convective maximum period when the hurricane will be moving over the Gulf Stream where SSTs are 30C or warmer and while the vertical remains reasonably low. Increasing southwesterly shear could cause a gradual decrease in intensity over the weekend. The new official intensity forecast is similar to the previous advisory, and is a little above the available model guidance out of respect for continuity. Key Messages: 1. Isaias will produce heavy rains and potentially life-threatening flash flooding and mudslides across the Dominican Republic, northern Haiti, Turks and Caicos, and the Bahamas. Heavy rains associated with Isaias may begin to affect south and east-central Florida late Friday night, and the eastern Carolinas by early next week, potentially resulting in isolated flash and urban flooding, especially in low-lying and poorly drained areas. Isolated minor river flooding is possible in the Carolinas early next week. 2. Hurricane conditions and dangerous storm surge are expected in portions of the Bahamas today and Saturday, and Hurricane Warnings are in effect. Preparations to protect life and property should be rushed to completion. 3. Tropical storm conditions are expected with hurricane conditions possible along portions of the Florida east coast beginning Saturday, and a Tropical Storm Warning and Hurricane Watch are in effect. Storm surge watches or warnings could be needed for part of this area this afternoon. 4. There is a risk of impacts from winds, heavy rainfall, and storm surge beginning late this weekend along the northeastern Florida coast and spreading northward along the remainder of the U.S. east coast through early next week. Interests along the entire U.S. east coast should monitor the progress of Isaias and updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 31/1500Z 21.7N 74.5W 65 KT 75 MPH 12H 01/0000Z 23.0N 76.1W 70 KT 80 MPH 24H 01/1200Z 24.8N 78.0W 80 KT 90 MPH 36H 02/0000Z 26.3N 79.2W 80 KT 90 MPH 48H 02/1200Z 27.7N 79.8W 75 KT 85 MPH 60H 03/0000Z 29.2N 80.0W 70 KT 80 MPH 72H 03/1200Z 31.6N 79.5W 65 KT 75 MPH 96H 04/1200Z 36.4N 76.2W 55 KT 65 MPH 120H 05/1200Z 43.3N 69.0W 55 KT 65 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart

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Hurricane Isaias Forecast Discussion Number 13

2020-07-31 10:56:02| National Hurricane Center (Atlantic)

Issued at 500 AM EDT Fri Jul 31 2020 000 WTNT44 KNHC 310855 TCDAT4 Hurricane Isaias Discussion Number 13 NWS National Hurricane Center Miami FL AL092020 500 AM EDT Fri Jul 31 2020 Satellite images indicate that Isaias is maintaining a small central dense overcast, with a new burst of convection recently forming near the center. Earlier aircraft data indicated that the pressure had fallen to 990 mb, although there was little from the aircraft that supported more than the previous advisory's intensity of 70 kt. The hurricane has been moving between west-northwest and northwest during the last several hours, estimated at 305/15 now. Isaias should gradually turn to the north-northwest and north over the weekend as it moves around the western periphery of a weakening western Atlantic subtropical ridge. Guidance is in fairly good agreement over the first couple of days of the forecast, so little change is made at that time, other than a small westward adjustment near the Bahamas. After that time, there is a considerable spread, growing to over 700 miles, in the reliable models on how quickly Isaias moves northeastward near the east coast of the United States. The large spread appears to be related to both how strong the cyclone is by early next week and how much mid-level ridging remains near the Carolinas. There are no obvious reasons to choose either the fast GFS or slow ECMWF right now, so the official forecast will remain close to the previous one and the model consensus. It should be noted that given this large spread, the extended forecast could be subject to large speed/timing changes if either of those solutions become more likely. Strengthening is expected during the next day or so while the hurricane remains over the very warm waters near the Bahamas with reasonably low vertical shear. The forecast for the first 24 hours has been raised from earlier, at the top of the guidance spread. Increasingly southwesterly flow aloft should cause an increase in shear over the weekend, which is forecast by most of the guidance to lead to a gradual decrease in intensity. The details are not very clear at all, especially considering the track uncertainty, but there is a notable chance of a hurricane moving close to the U.S. East coast, so the forecast continues to show that scenario. Key Messages: 1. Isaias will produce heavy rains and potentially life-threatening flash flooding and mudslides across the Dominican Republic, northern Haiti, Turks and Caicos, and the Bahamas. Heavy rains associated with Isaias may begin to affect South and east-Central Florida beginning late Friday night, and the eastern Carolinas by early next week, potentially resulting in isolated flash and urban flooding, especially in low-lying and poorly drained areas. Isolated minor river flooding is possible in the Carolinas early next week. 2. Hurricane conditions and dangerous storm surge are expected in portions of the Bahamas today and Saturday, and Hurricane Warnings are in effect for these areas. Preparations to protect life and property should be rushed to completion. 3. Tropical storm conditions are possible along portions of the Florida east coast beginning Saturday, and a Tropical Storm Watch remains in effect. While storm surge watches are not currently needed for this area, they may be required later today if the forecast track shifts closer to the coast. Heavy rains associated with Isaias may begin to affect South Florida and east-Central Florida beginning late tonight, potentially resulting in isolated flash and urban flooding, especially in low-lying and poorly drained areas. 4. There is a risk of impacts from winds, heavy rainfall, and storm surge late this weekend from the northeastern Florida coast and spreading northward along the remainder of the U.S. east coast through early next week. The details of the track and intensity forecast remain uncertain, and it is too soon to determine the magnitude and location of these potential impacts, but interests along the entire U.S. east coast should monitor the progress of Isaias and updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 31/0900Z 20.9N 73.3W 70 KT 80 MPH 12H 31/1800Z 22.3N 75.0W 80 KT 90 MPH 24H 01/0600Z 24.0N 77.0W 85 KT 100 MPH 36H 01/1800Z 25.7N 78.5W 80 KT 90 MPH 48H 02/0600Z 27.3N 79.4W 75 KT 85 MPH 60H 02/1800Z 28.8N 79.6W 75 KT 85 MPH 72H 03/0600Z 30.6N 79.3W 70 KT 80 MPH 96H 04/0600Z 36.5N 75.5W 65 KT 75 MPH 120H 05/0600Z 42.5N 69.0W 55 KT 65 MPH $$ Forecaster Blake

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Hurricane Isaias Forecast Discussion Number 12

2020-07-31 06:06:33| National Hurricane Center (Atlantic)

Issued at 1200 AM EDT Fri Jul 31 2020 000 WTNT44 KNHC 310406 TCDAT4 Hurricane Isaias Special Discussion Number 12 NWS National Hurricane Center Miami FL AL092020 1200 AM EDT Fri Jul 31 2020 An Air Force Reserve Hurricane Hunter found that Isaias has become a hurricane. Maximum flight-level winds so far were 87 kt at 850 mb, with believable SFMR values of at least 65 kt. A blend of these values gives an initial wind speed of 70 kt. Some further strengthening is likely over the next 24 hours before increasing southwesterly shear could weaken the system. The intensity forecast is modified upward from 5-10 kt through 48 hours and unchanged after that time. There are no changes to the previous track forecast. Key Messages: 1. Isaias will produce heavy rains and potentially life-threatening flash flooding and mudslides across the Dominican Republic, northern Haiti, Turks and Caicos, and the Bahamas. 2. Hurricane conditions and dangerous storm surge are expected in portions of the southeastern Bahamas overnight, central and northwestern Bahamas late Friday and Saturday, and Hurricane Warnings are in effect for these areas. Preparations to protect life and property should be rushed to completion. 3. Tropical storm conditions are possible along portions of the Florida east coast beginning Saturday, and a Tropical Storm Watch is in effect. While storm surge watches are not currently needed for this area, they may be required on Friday if the forecast track shifts closer to the coast. Heavy rains associated with Isaias may begin to affect South Florida and east-Central Florida beginning late Friday night, potentially resulting in isolated flash and urban flooding, especially in low-lying and poorly drained areas. 4. There is a risk of impacts from winds, heavy rainfall, and storm surge late this weekend from the northeastern Florida coast and spreading northward along the remainder of the U.S. east coast through early next week. The details of the track and intensity forecast remain uncertain, and it is too soon to determine the magnitude and location of these potential impacts, but interests along the entire U.S. east coast should monitor the progress of Isaias and updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 31/0400Z 20.4N 72.2W 70 KT 80 MPH 12H 31/1200Z 21.6N 73.9W 70 KT 80 MPH 24H 01/0000Z 23.5N 76.1W 75 KT 85 MPH 36H 01/1200Z 25.2N 77.8W 75 KT 85 MPH 48H 02/0000Z 26.7N 79.0W 70 KT 80 MPH 60H 02/1200Z 28.3N 79.6W 65 KT 75 MPH 72H 03/0000Z 30.0N 79.5W 65 KT 75 MPH 96H 04/0000Z 34.6N 76.8W 65 KT 75 MPH 120H 05/0000Z 42.0N 69.0W 60 KT 70 MPH $$ Forecaster Blake

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Tropical Storm Isaias Forecast Discussion Number 10

2020-07-30 22:55:23| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Thu Jul 30 2020 000 WTNT44 KNHC 302055 TCDAT4 Tropical Storm Isaias Discussion Number 10 NWS National Hurricane Center Miami FL AL092020 500 PM EDT Thu Jul 30 2020 Surface observations in the Dominican Republic indicate that the poorly defined center of Isaias moved over the southeastern portion of Hispaniola around 1600 UTC. There is currently a significant concentration of convection near the mid-level center that is located along the northern coast of the island, and recent surface observations suggest that a new center may be forming near the area of mid-level rotation seen in satellite imagery. The advisory position has been placed between the previous estimated center location and the mid-level center until it becomes more clear that reformation has occurred. Earlier ASCAT data that arrived shortly after the previous advisory and reconnaissance aircraft data from this morning supported an intensity of 45-50 kt, and since there has no significant degradation the overall organization since that time, the initial intensity remains 50 kt. The central pressure of 999 mb is based on a surface observation of 1001 mb and 25 kt of wind at Puerto Plata in the Dominican Republic. The initial motion estimate is a somewhat uncertain 310/17 kt. The track forecast reasoning has not changed since the previous advisory. The tropical storm should continue on a general northwest heading during the next 24 to 36 hours along the southern side of a subtropical ridge. After that time, a trough moving into the east-central United States should cause Isaias to turn north-northwestward and northward as the western portion of the ridge erodes. By early next week, Isaias is expected to turn northeastward and accelerate ahead of the aforementioned trough. The 1200 UTC dynamical model guidance has shifted eastward and now that Isaias is expected to become a stronger and deeper cyclone, that also favors a more eastward track. The updated official forecast is a blend of the various consensus aids. Since the new center is still in its formative stage some additional shifts in the track may occur. In addition, strong winds and heavy rainfall will extend far from the center. Visible imagery has shown significant banding near the mid-level circulation. Assuming that the center reforms near that feature and moves away from Hispaniola tonight, strengthening is expected during the next 24-36 hours and Isaias is now forecast to become a hurricane. After that time, there are mixed signals regarding the amount of southwesterly vertical wind shear and the models generally do not show much additional strengthening. Therefore, the NHC intensity forecast shows a leveling off in intensity after 48 h. It still needs to be stressed that that there is a higher than usual amount of uncertainty in the intensity forecast. Key Messages: 1. Isaias will produce heavy rains and potentially life-threatening flash flooding and mudslides across Puerto Rico, the Dominican Republic, northern Haiti, Turks and Caicos and the Bahamas. 2. Hurricane conditions and dangerous storm surge are expected in portions of the central and northwest Bahamas late Friday and Saturday. Preparations to protect life and property should be rushed to completion. Tropical storm conditions will continue to spread across portions of the Dominican Republic, Haiti, the Turks and Caicos and the southeast Bahamas tonight through Friday, and Tropical Storm Warnings are in effect for these areas. 3. Tropical storm conditions are possible along portions of the Florida east coast beginning Saturday, and a Tropical Storm Watch has been issued. While storm surge watches are not currently needed for this area, they may be required tonight or early tomorrow if the forecast track shifts closer to the coast. Heavy rains associated with Isaias may begin to affect eastern Florida this weekend, potentially resulting in isolated flash and urban flooding. 4. There is a risk of impacts from winds, heavy rainfall, and storm surge this weekend along the Florida east coast and spreading northward along the remainder of the U.S. east coast through early next week. The details of the track and intensity forecast remain uncertain and it is too soon to determine the magnitude and location of these potential impacts, but interests along the entire U.S. east coast should monitor the progress of Isaias and updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 30/2100Z 19.5N 70.6W 50 KT 60 MPH...INLAND 12H 31/0600Z 20.8N 72.7W 55 KT 65 MPH...OVER WATER 24H 31/1800Z 22.7N 75.1W 60 KT 70 MPH 36H 01/0600Z 24.5N 77.1W 65 KT 75 MPH 48H 01/1800Z 26.1N 78.5W 65 KT 75 MPH 60H 02/0600Z 27.7N 79.5W 65 KT 75 MPH 72H 02/1800Z 29.2N 79.7W 65 KT 75 MPH 96H 03/1800Z 33.3N 78.0W 65 KT 75 MPH 120H 04/1800Z 40.0N 71.0W 60 KT 70 MPH $$ Forecaster Brown

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