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Hurricane Isaias Forecast Discussion Number 17

2020-08-01 10:55:42| National Hurricane Center (Atlantic)

Issued at 500 AM EDT Sat Aug 01 2020 000 WTNT44 KNHC 010855 TCDAT4 Hurricane Isaias Discussion Number 17 NWS National Hurricane Center Miami FL AL092020 500 AM EDT Sat Aug 01 2020 Isaias has a somewhat ragged appearance in satellite imagery this morning, likely due to the impact of westerly shear and dry air entrainment. The area of central convection has shrunk in size, although radar data from the Bahamas shows some banding near the center and occasional attempts to wrap up an eyewall. Also, a dry slot is now present in the eastern semicircle between the central convection and the outer banding. The last Hurricane Hunter mission indicated that the maximum winds had increased to near 75 kt, so that is maintained for this advisory. NOAA and Air Force Reserve Hurricane Hunter aircraft are scheduled to investigate the storm during the next several hours. The initial motion is now northwestward or 315/10 kt. The track forecast philosophy remains unchanged, as Isais should continue to move northwestward on the southwest side of a mid-level ridge today and begin to turn north-northwestward through a weakness in the ridge on Sunday. After that, the storm should recurve into the mid-latitude westerlies, with a turn toward the north-northeast and northeast with an increase in forward speed likely. The track guidance envelope has shifted a little to the east near the Florida coast and calls for a slower forward motion that the previous guidance. Thus, the new forecast track has been adjusted a little east of, and slower than, the previous track. The hurricane is currently undergoing about 25 kt of westerly vertical shear, and some mid-level dry air is present west of the center. This combination should prevent any more intensification, and, while Isaias is expected to remain a hurricane as it passes near the Florida coast, at least slight weakening should occur during this time. Current indications from the global models are that the storm will continue to experience strong shear as it recurves, and thus the intensity forecast follows the previous forecast in calling for weakening during this time. Key Messages: 1. Hurricane conditions and dangerous storm surge are expected in portions of the Bahamas through Saturday, and Hurricane Warnings are in effect. 2. Hurricane conditions are expected along portions of the Florida east coast late Saturday and Saturday night, and a Hurricane Warning is in effect. Preparations to protect life and property should be rushed to completion. 3. Dangerous storm surge is possible along the Florida east coast from Jupiter Inlet to Ponte Vedra Beach where water rises of 2 to 4 feet above ground level are possible along the immediate coastline and adjacent waterways. Residents in these areas should follow advice given by local emergency officials. 4. Isaias will produce heavy rains and potentially life-threatening flash and urban flooding, especially in low-lying and poorly drained areas, across South to east-Central Florida, and across the Carolinas to the mid Atlantic. Minor river flooding is possible across portions of the Carolinas and into Virginia early next week. 5. There is a risk of impacts from winds, heavy rainfall, and storm surge spreading along much of the the U.S. east coast through early next week, and interests there should monitor the progress of Isaias and updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 01/0900Z 23.9N 77.1W 75 KT 85 MPH 12H 01/1800Z 25.0N 78.2W 75 KT 85 MPH 24H 02/0600Z 26.3N 79.2W 70 KT 80 MPH 36H 02/1800Z 27.6N 79.9W 70 KT 80 MPH 48H 03/0600Z 29.1N 80.3W 65 KT 75 MPH 60H 03/1800Z 31.2N 79.8W 60 KT 70 MPH 72H 04/0600Z 34.0N 77.8W 60 KT 70 MPH 96H 05/0600Z 41.5N 71.4W 50 KT 60 MPH...INLAND 120H 06/0600Z 47.5N 61.5W 40 KT 45 MPH...POST-TROP/EXTRATROP $$ Forecaster Beven

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Tropical Depression Ten Forecast Discussion Number 3

2020-08-01 10:32:46| National Hurricane Center (Atlantic)

Issued at 800 AM CVT Sat Aug 01 2020 000 WTNT45 KNHC 010832 TCDAT5 Tropical Depression Ten Discussion Number 3 NWS National Hurricane Center Miami FL AL102020 800 AM CVT Sat Aug 01 2020 Satellite images indicate that only a small area of deep convection remains with the depression, and the center is partially exposed on the southeastern edge of the thunderstorms. The initial wind speed is kept 30 kt based on persistence. It seems like the chance for the depression to become a tropical storm has passed with it now moving over cooler waters into a drier air mass. Thus weakening is forecast, and the system will likely degenerate into a remnant low later today. The depression continues moving northwestward, and that course should persist today before turning towards the west-northwest around a larger cyclonic gyre on Sunday. A slight northward adjustment has been made to the forecast, a bit south of the GFS model. All of the model guidance dissipate the small low within 36 hours, so this is indicated in the new forecast. FORECAST POSITIONS AND MAX WINDS INIT 01/0900Z 17.8N 21.6W 30 KT 35 MPH 12H 01/1800Z 19.2N 23.2W 25 KT 30 MPH...POST-TROP/REMNT LOW 24H 02/0600Z 20.5N 26.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 36H 02/1800Z...DISSIPATED $$ Forecaster Blake

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Tropical Depression Ten Forecast Discussion Number 2

2020-08-01 04:48:41| National Hurricane Center (Atlantic)

Issued at 200 AM CVT Sat Aug 01 2020 000 WTNT45 KNHC 010248 TCDAT5 Tropical Depression Ten Discussion Number 2 NWS National Hurricane Center Miami FL AL102020 200 AM CVT Sat Aug 01 2020 The small depression maintained a large burst of convection in its northwest quadrant for several hours after the last advisory, though cloud tops have warmed steadily since around 00Z. An ASCAT-B pass near 2300 UTC showed max winds near 35 kt, but these winds coincided with the coldest cloud tops in Meteosat imagery at that time, indicating they may have been rain inflated. Data from ASCAT-C valid about 40 minutes earlier only showed peak winds closer to 30 kt, also near the deepest convection. The highest wind retrievals in the east half of the circulation, where you would often expect the highest winds for a system moving north or northwest, were between 25 and 30 kt in both passes. Subjective Dvorak intensity estimates were 30 kt and 25 kt from TAFB and SAB, respectively, at 00Z. It is possible that the cyclone briefly had tropical-storm-strength winds at one point, but given the observed warming of cloud tops since 00Z, the intensity is maintained at 30 kt for this advisory. There is no real change to the forecast reasoning. The depression is moving northwestward and a turn toward the west-northwest is expected later today as it moves around a larger cyclonic gyre. Conditions do not appear to be favorable for any strengthening, and the dynamical models indicate that deep convection will dissipate later today due to cool underlying SSTs and a drier surrounding environment. The NHC track and intensity forecast is based on the TVCA and IVCN consensus aids and has not been changed significantly from the previous advisory. FORECAST POSITIONS AND MAX WINDS INIT 01/0300Z 17.1N 21.1W 30 KT 35 MPH 12H 01/1200Z 18.4N 22.5W 30 KT 35 MPH 24H 02/0000Z 19.3N 25.1W 25 KT 30 MPH...POST-TROP/REMNT LOW 36H 02/1200Z 20.0N 28.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 48H 03/0000Z...DISSIPATED $$ Forecaster Zelinsky

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Hurricane Isaias Forecast Discussion Number 16

2020-08-01 04:45:42| National Hurricane Center (Atlantic)

Issued at 1100 PM EDT Fri Jul 31 2020 000 WTNT44 KNHC 010245 TCDAT4 Hurricane Isaias Discussion Number 16 NWS National Hurricane Center Miami FL AL092020 1100 PM EDT Fri Jul 31 2020 Isaias has a somewhat asymmetrical appearance on satellite images, with lots of deep convection over the eastern semicircle and not as much over the western part of the circulation. Reports from an Air Force Hurricane Hunter aircraft indicate that the central pressure has dropped a few mb, and flight-level and SFMR-observed surface wind observations from the aircraft indicate an intensity of near 70 kt. The hurricane is experiencing some westerly shear, and this is likely to limit intensification. However, some strengthening is still possible on Saturday. When Isais moves farther north in a couple of days, the shear should cause a gradual weakening trend to begin. The official intensity forecast is about the same as the previous one, and a little above the model consensus. The hurricane continues moving northwestward at a bit slower pace, or 310/13 kt. The general track forecast philosophy has not changed. Isais should continue to move northwestward on the southwest side of a mid-level ridge through Saturday and begin to turn north-northwestward through a weakness in the ridge on Sunday. Thereafter, a turn toward the north-northeast and northeast with an increase in forward speed is likely during days 3 to 5. The official track forecast is close to the previous one and also very close to the latest dynamical model consensus. The hurricane warning has been extended northward along the Florida east coast. Key Messages: 1. Hurricane conditions and dangerous storm surge are expected in portions of the Bahamas through Saturday, and Hurricane Warnings are in effect. 2. Hurricane conditions are expected along portions of the Florida east coast late Saturday and Saturday night, and a Hurricane Warning has been issued. Preparations to protect life and property should be rushed to completion. 3. Dangerous storm surge is possible along the Florida east coast from Jupiter Inlet to Ponte Vedra Beach where water rises of 2 to 4 feet above ground level are possible along the immediate coastline and adjacent waterways. Residents in these areas should follow advice given by local emergency officials. 4. Isaias will produce heavy rains and potentially life-threatening flash and urban flooding, especially in low-lying and poorly drained areas, across South to east-Central Florida, and across the Carolinas to the mid Atlantic. Isolated minor river flooding is possible across the eastern Carolinas and into Virginia early next week. 5. There is a risk of impacts from winds, heavy rainfall, and storm surge spreading along much of the the U.S. east coast through early next week, and interests there should monitor the progress of Isaias and updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 01/0300Z 23.3N 76.4W 70 KT 80 MPH 12H 01/1200Z 24.5N 77.8W 75 KT 85 MPH 24H 02/0000Z 25.9N 79.1W 75 KT 85 MPH 36H 02/1200Z 27.1N 80.0W 70 KT 80 MPH 48H 03/0000Z 28.5N 80.5W 65 KT 75 MPH 60H 03/1200Z 30.2N 80.5W 60 KT 70 MPH 72H 04/0000Z 33.1N 78.9W 60 KT 70 MPH 96H 05/0000Z 41.0N 72.5W 55 KT 65 MPH 120H 06/0000Z 47.5N 64.0W 45 KT 50 MPH...POST-TROP/EXTRATROP $$ Forecaster Pasch

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Hurricane Isaias Forecast Discussion Number 15

2020-07-31 22:50:07| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Fri Jul 31 2020 000 WTNT44 KNHC 312050 TCDAT4 Hurricane Isaias Discussion Number 15 NWS National Hurricane Center Miami FL AL092020 500 PM EDT Fri Jul 31 2020 Deep convection, with occasional overshooting cloud tops of -85C to -90C just north of the center, has continued to develop during the normal diurnal convective minimum period, which is quite impressive. The most recent Air Force Reserve recon flight-level wind data, along with ASCAT surface wind data, indicate that the inner-core and outer wind field have both contracted in size. Furthermore, radar data from the Bahamas and an 1810Z AMSR-2 microwave pass also indicate that a small 10-nmi-wide mid-level eye is forming. The last recon central pressure was 991 mb and the 700 mb height had decreased by 30 meters since the earlier maximum height around 1230Z. These data indicate that Isaias is getting better organized. The initial intensity remains 65 kt based on an earlier 700-mb flight-level wind speed of 72 kt, which reduces to a 65-kt surface wind speed using a 90-percent adjustment factor. The initial motion remains northwestward or 305/12 kt. The 12Z global models have once again made a westward shift due to the ridge to the north of Isaias not weakening as quickly as expected. This is partly due to the ridge being stronger than expected and a shortwave trough over the central United States moving a little slower into the southeastern U.S. than previously indicated. The UKMET and ECMWF explicitly show Isaias making landfall in 36-48 hours along the southeast Florida coast, but appear to weaken the system below hurricane strength. The GFS similarly brings the cyclone close to the southeast and east-central Florida coasts, but also as a somewhat weaker system. In the 48 to 60-hour period, the cyclone is forecast to move slowly north-northwestward and northward through a break in the subtropical ridge extending westward from the Atlantic across Florida and into the northern Gulf of Mexico. By that time, however, Isaias is expected to weaken below hurricane strength due to the combination of strong southwesterly vertical wind shear and interaction the Florida peninsula. Around 72 hours, the cyclone should accelerate northeastward and possibly strengthen some before passing over eastern North Carolina on day 4, and across eastern New England on day 5. The NHC track forecast lies close to a blend of the consensus models TVCA and NOAA-HCCA and is east of the UKMET and ECMWF with the system forecast to be stronger than those models indicate. Due to the westward shift in the NHC forecast track, a Hurricane Warning and Storm Surge Watch have been issued for portions of the Florida east coast. The center of Isaias is now located in the center of an expanding CDO feature. The improved inner-core wind field and aforementioned convective structure, along with very warm SSTs near 30C, should support some strengthening overnight and early Saturday morning. However, increasing southwesterly vertical wind shear is expected to cause a gradual decrease in intensity by Sunday and continue into early next week. The new official intensity forecast is a little lower than the previous advisory and is near the higher end of the intensity guidance. Key Messages: 1. Hurricane conditions and dangerous storm surge are expected in portions of the Bahamas through Saturday, and Hurricane Warnings are in effect. 2. Hurricane conditions are expected along portions of the Florida east coast late Saturday and Saturday night, and a Hurricane Warning has been issued. Preparations to protect life and property should be rushed to completion. 3. Dangerous storm surge is possible along the Florida east coast from Jupiter Inlet to Ponte Vedra Beach where water rises of 2 to 4 feet above ground level are possible along the immediate coastline and adjacent waterways. Residents in these areas should follow advice given by local emergency officials. 4. Isaias will produce heavy rains and potentially life-threatening flash and urban flooding, especially in low-lying and poorly drained areas across south to east-central Florida, and across the Carolinas to the mid Atlantic. Isolated minor river flooding is possible across the eastern Carolinas and into Virginia early next week. 5. There is a risk of impacts from winds, heavy rainfall, and storm surge spreading along much of the the U.S. east coast through early next week, and interests there should monitor the progress of Isaias and updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 31/2100Z 22.6N 75.7W 65 KT 75 MPH 12H 01/0600Z 23.9N 77.2W 70 KT 80 MPH 24H 01/1800Z 25.4N 78.8W 75 KT 85 MPH 36H 02/0600Z 26.8N 79.9W 70 KT 80 MPH 48H 02/1800Z 28.3N 80.4W 65 KT 75 MPH 60H 03/0600Z 30.0N 80.4W 60 KT 70 MPH 72H 03/1800Z 32.5N 79.4W 60 KT 70 MPH 96H 04/1800Z 39.0N 74.5W 55 KT 65 MPH 120H 05/1800Z 45.4N 65.9W 50 KT 60 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart

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