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Tropical Storm Isaias Forecast Discussion Number 24
2020-08-03 04:43:45| National Hurricane Center (Atlantic)
Issued at 1100 PM EDT Sun Aug 02 2020 000 WTNT44 KNHC 030243 TCDAT4 Tropical Storm Isaias Discussion Number 24 NWS National Hurricane Center Miami FL AL092020 1100 PM EDT Sun Aug 02 2020 Isais has not become better organized on satellite and radar imagery at this time. The cyclone continues to produce vigorous deep convection but this convection continues to be displaced to the northeast of the center, and convective banding features are ill-defined. Air Force Hurricane Hunter observations show that the central pressure has not fallen since earlier today, and flight-level and SFMR-observed surface wind measurements from the aircraft indicate that the intensity remains near 60 kt. Although Isais will be moving through an environment of fairly strong southwesterly shear during the next day or so, it will also be traversing the warm waters of the Gulf Stream with its high oceanic heat content. The latter factor could result in the system becoming a hurricane near landfall. The official forecast is close to the intensity consensus, which keeps Isaias just below hurricane strength. It should be emphasized that there is little difference between a strong tropical storm or a category 1 hurricane, in terms of impacts. After landfall, the interaction with land and strong shear should result in steady weakening. Based on the global models, the cyclone is likely to become an extratropical cyclone after it moves into Canada and dissipate over the north Atlantic shortly thereafter. The motion continues to be slightly west of due north or 345/8 kt. There has been no significant change to the track forecast reasoning. Over the next couple of days, Isaias is expected to accelerate north-northeastward on the east side of a large 500 mb trough over the eastern United States and then turn northeastward in 3 to 4 days as it moves through the mid-latitude westerlies. The official track forecast is very close to the previous one and similar to the latest corrected dynamical model consensus. This is between the GFS, which shows a slightly faster motion and the ECMWF, which is slightly slower. 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 Maryland through early Tuesday. 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, is expected through midweek along and near the path of Isaias across the East Coast of the United States. Widespread minor to moderate river flooding is possible across portions of the Carolinas and the mid-Atlantic. 4. Tropical storm conditions are forecast to spread northward across the remainder of the U.S east coast on Tuesday and early Wednesday. A tropical storm watch is in effect through Long Island Sound and additional watches and warnings will likely be issued on Monday. FORECAST POSITIONS AND MAX WINDS INIT 03/0300Z 28.5N 79.8W 60 KT 70 MPH 12H 03/1200Z 29.7N 80.0W 60 KT 70 MPH 24H 04/0000Z 32.2N 79.3W 60 KT 70 MPH 36H 04/1200Z 35.9N 77.3W 55 KT 65 MPH...INLAND 48H 05/0000Z 40.5N 74.0W 40 KT 45 MPH...INLAND 60H 05/1200Z 45.5N 70.0W 35 KT 40 MPH...INLAND 72H 06/0000Z 50.0N 65.5W 30 KT 35 MPH...POST-TROP/EXTRATROP 96H 07/0000Z 55.5N 56.5W 25 KT 30 MPH...POST-TROP/EXTRATROP 120H 08/0000Z...DISSIPATED $$ Forecaster Pasch
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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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Tropical Storm Isaias Forecast Discussion Number 22
2020-08-02 16:59:10| National Hurricane Center (Atlantic)
Issued at 1100 AM EDT Sun Aug 02 2020 000 WTNT44 KNHC 021458 TCDAT4 Tropical Storm Isaias Discussion Number 22 NWS National Hurricane Center Miami FL AL092020 1100 AM EDT Sun Aug 02 2020 Another recent burst of deep convection has recently developed northeast of the center, which has resulted in Isaias making a slight northward jog. An impressive mid-level circulation has developed within the strongest thunderstorm cluster, along with rare reflectivity values of 55-60 dBZ for a tropical cyclone. The Melbourne, Florida, Doppler weather radar has been indicating patches of velocity values of 65-66 kt at around 10,000 ft just north of the center, which equates to 58-59 kt surface winds. For now, the intensity will remain at 55 kt since previous convective bursts have not persisted for more than a couple of hours at best. Another Air Force Reserve Hurricane Hunter aircraft is scheduled to investigate Isaias later today. Radar and aircraft fixes indicate that Isaias is now moving toward the north-northwest or 340/07 kt. A slow north-northwestward motion should continue for the next 24 hours or so as Isaias moves into a weakness that has developed in the Bermuda-Azores ridge over north Florida and off the Georgia coast seen in 02/1200Z upper-air data. A turn toward the north is forecast to occur by all of the global models by Monday morning, followed by a faster forward motion toward the northeast by Monday afternoon and evening when the cyclone will be influenced by southwesterly steering flow ahead of a strong eastward-moving mid- to upper-level trough. Continued northeastward acceleration across the mid-Atlantic and New England states is expected on days 3 and 4. The global models continue to show little cross-track difference, but still have significant along-track differences with the GFS being the fastest and the ECMWF being the slowest . Since the preponderance of the model guidance is slower than the GFS solution, the new official forecast track closely follows the consensus models TVCA and HCCA, and lies near the previous advisory track after 12 h. Isaias will remain over warm Gulfstream waters where water temperatures are near 30 deg C. Despite very unfavorable vertical shear conditions of 25-30 kt the past couple of days, the cyclone has managed to hold together, which is an indication that the system has a deep, well-formed vertical circulation. While some slight intensification is possible if the shear decreases, the official forecast calls for the intensity to remain steady until landfall occurs in the Carolinas in 36 hours or so. Some baroclinic interaction with the right-rear quadrant of an anticyclonically curved jet streak is expected to hold the intensity a little above what would normally be expected for a post-landfall tropical cyclone in 48-72 hours. The NHC intensity forecast is similar to the HFIP corrected consensus model and the IVCN intensity consensus model. Key Messages: 1. Tropical storm conditions will spread northward along the east coast of Florida within the warning area through early Monday and will reach the coasts of Georgia, South Carolina, and southern North Carolina within the warning area Monday and early Tuesday. 2. Dangerous storm surge is possible from Edisto Beach South Carolina to Cape Fear North Carolina where water rises of 2 to 4 feet above ground level are possible along the immediate coastline and adjacent waterways. A Storm Surge Warning may be needed for a portion of this area later today, and residents there should follow advice given by local emergency officials. 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 coastal Carolinas and Virginia, is expected through midweek along and near the path of Isaias along the U.S. East Coast. Widespread minor to isolated moderate river flooding is possible across portions of the Carolinas and Mid-Atlantic. 4. A Tropical Storm Watch is in effect for portions of the North Carolina coast, where tropical storm conditions are possible on Tuesday. Additional watches and warnings will likely be issued later today as Isaias is expected to move northward near or over the mid-Atlantic and northeast coasts Tuesday and Wednesday. FORECAST POSITIONS AND MAX WINDS INIT 02/1500Z 26.9N 79.6W 55 KT 65 MPH 12H 03/0000Z 28.0N 80.1W 55 KT 65 MPH 24H 03/1200Z 29.6N 80.5W 55 KT 65 MPH 36H 04/0000Z 32.2N 79.8W 55 KT 65 MPH 48H 04/1200Z 35.7N 77.7W 50 KT 60 MPH...INLAND 60H 05/0000Z 40.1N 74.3W 45 KT 50 MPH...INLAND 72H 05/1200Z 44.6N 70.4W 40 KT 45 MPH...INLAND 96H 06/1200Z 51.7N 63.2W 30 KT 35 MPH...POST-TROP/EXTRATROP 120H 07/1200Z...DISSIPATED $$ Forecaster Stewart
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Tropical Storm Isaias Forecast Discussion Number 21
2020-08-02 10:57:01| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Sun Aug 02 2020 000 WTNT44 KNHC 020856 TCDAT4 Tropical Storm Isaias Discussion Number 21 NWS National Hurricane Center Miami FL AL092020 500 AM EDT Sun Aug 02 2020 The burst of deep convection that developed to the northeast of the center last evening has not resulted in any significant improvement in the structure of Isaias according to recent reconnaissance aircraft data and radar imagery. The tropical cyclone is still being adversely affected by about 25 kt of 850-200 mb vertical wind shear. An Air Force Reserve reconnaissance aircraft that has provided several center fixes overnight has found peak 850-mb flight-level winds of 67 kt, and peak believable SFMR winds of 51 kt. These data, along with an earlier ASCAT overpass, indicate that Isaias' intensity is 55 kt. In addition, both the aircraft and ASCAT data suggest that the wind field over the western portion of the circulation is somewhat smaller than previous estimated. Since Isaias has not shown any signs of re-organizing overnight, it appears that the window of opportunity for it to re-strengthen is closing. The SHIPS guidance and global model fields do not show the shear abating during the next 24-36 hours while Isaias moves near the Florida coast. As a result, the new NHC intensity forecast shows little change in strength during this time. Later in the period, gradual weakening should occur as the storm moves northward along the east coast of the United States. The NHC intensity forecast is in good agreement with the HFIP corrected consensus model and the latest multi-model intensity consensus. Recent aircraft fixes and radar data show that Isaias is moving northwestward or 325/8 kt. A slow northwestward to north-northwestward motion around the western periphery of a deep-layer ridge centered near Bermuda should take the center of Isaias very near the east coast of Florida through Monday. After that, the tropical cyclone should begin to accelerate north-northeastward to northeastward as a trough slides eastward into the eastern United States. The dynamical models are tightly clustered, and the NHC track forecast lies between the typically reliable GFS and ECMWF models. The reduction in the initial and predicted intensity of Isaias has necessitated changes in warnings along the east coast of Florida. The Hurricane Warning along the east-central coast of Florida has been replaced with a Tropical Storm Warning. Key Messages: 1. Tropical storm conditions will continue across portions of the northwestern Bahamas through today. 2. Tropical storm conditions are expected to spread northward along the east coast of Florida within the warning area through early Monday. These conditions are expected to spread northward along the coast of Georgia and South Carolina within the warning area on Monday. 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 there should follow advice given by local emergency officials. 4. Isaias will produce heavy rains and potentially life-threatening flash flooding across portions of the northwestern Bahamas, and flash and urban flooding, especially in low-lying and poorly drained areas, along the East Coast of the United States. Minor to isolated moderate river flooding is possible across portions of the Carolinas and Mid-Atlantic this week. 5. Tropical storm watches are in effect for portions of the coast of South and North Carolina. Additional watches and warnings will likely be issued later today as Isaias is expected to move northward near or over the southeast and mid-Atlantic coasts over the next couple of days. FORECAST POSITIONS AND MAX WINDS INIT 02/0900Z 26.3N 79.5W 55 KT 65 MPH 12H 02/1800Z 27.4N 80.1W 55 KT 65 MPH 24H 03/0600Z 28.8N 80.6W 55 KT 65 MPH 36H 03/1800Z 30.7N 80.6W 55 KT 65 MPH 48H 04/0600Z 33.7N 79.1W 55 KT 65 MPH...INLAND 60H 04/1800Z 37.8N 76.4W 50 KT 60 MPH...INLAND 72H 05/0600Z 42.9N 72.4W 40 KT 45 MPH...INLAND 96H 06/0600Z 50.0N 65.0W 30 KT 35 MPH...POST-TROP/INLAND 120H 07/0600Z...DISSIPATED $$ Forecaster Brown
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Tropical Storm Isaias Forecast Discussion Number 20
2020-08-02 04:56:57| National Hurricane Center (Atlantic)
Issued at 1100 PM EDT Sat Aug 01 2020 000 WTNT44 KNHC 020256 TCDAT4 Tropical Storm Isaias Discussion Number 20 NWS National Hurricane Center Miami FL AL092020 1100 PM EDT Sat Aug 01 2020 Deep convection has increased this evening in a cluster to the east and northeast of the aircraft-observed center position. Radar data suggests a mid-level center or vorticity maximum in that area, which is between Grand Bahama and Andros Island. Observations from the Air Force Hurricane Hunters indicate that Isaias has not restrengthened thus far, but there is still a small window of opportunity for it to regain hurricane intensity Sunday while passing over the Gulf Stream waters. Synoptic surveillance data from the NOAA Gulfstream-IV aircraft indicate that the upper-level west-southwesterly flow over Isais will begin to increase by late Sunday. The resulting increase in shear and interaction with land should cause weakening beyond 24 hours, and the official intensity forecast is near or above the intensity model consensus. Assuming that the aforementioned mid-level feature is not the dominant center of circulation, the aircraft fixes give a motion estimate of about 310/8 kt. Isais has been moving northwestward along the southwestern periphery of a mid-level high pressure area. An approaching trough near the Ohio Valley is expected to cause the tropical cyclone to turn northward within 48 hours. After that, Isaias should accelerate north-northeastward to northeastward ahead of the trough, and move near or over the east coast of the United States. The official track forecast remains close to both the simple and corrected dynamic model consensus tracks, TVCN and HCCA respectively. Key Messages: 1. Hurricane conditions and dangerous storm surge will continue in portions of the northwest Bahamas through Sunday morning. 2. Hurricane conditions are expected along portions of the Florida east coast on Sunday with tropical storm conditions expected to begin late tonight. 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 there should follow advice given by local emergency officials. 4. Isaias will produce heavy rains and potentially life-threatening flash flooding in the Bahamas, and flash and urban flooding, especially in low-lying and poorly drained areas, along the east coast of the United States. Minor river flooding and isolated moderate river flooding is possible across portions of the Carolinas and mid Atlantic early next week. 5. Tropical storm watches and warnings are in effect for portions of the United States East Coast from northeast Florida to South Carolina. Additional watches and warnings will likely be issued on Sunday as Isaias is expected to move northward near or over the southeast and mid-Atlantic coasts early next week. FORECAST POSITIONS AND MAX WINDS INIT 02/0300Z 25.5N 79.1W 60 KT 70 MPH 12H 02/1200Z 26.6N 79.9W 65 KT 75 MPH 24H 03/0000Z 28.0N 80.5W 65 KT 75 MPH...ON COAST 36H 03/1200Z 29.7N 80.8W 60 KT 70 MPH 48H 04/0000Z 32.0N 80.0W 55 KT 65 MPH 60H 04/1200Z 35.3N 78.1W 50 KT 60 MPH...INLAND 72H 05/0000Z 39.7N 74.8W 40 KT 45 MPH...INLAND 96H 06/0000Z 47.0N 66.0W 35 KT 40 MPH...POST-TROP/INLAND 120H 07/0000Z 51.5N 57.5W 30 KT 35 MPH...POST-TROP/INLAND $$ Forecaster Pasch
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