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Tropical Storm Isaias Forecast Discussion Number 31

2020-08-04 22:57:11| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Tue Aug 04 2020 000 WTNT44 KNHC 042057 TCDAT4 Tropical Storm Isaias Discussion Number 31 NWS National Hurricane Center Miami FL AL092020 500 PM EDT Tue Aug 04 2020 NOAA Doppler radar data from Islip, New York (KOKX), indicated 70-kt winds just off the surface south of Long Island earlier this afternoon, along with buoy reports from New York Harbor Inlet (44065) that supported lowering the intensity to 55 kt, so that value is used for this advisory. The motion remains north-northeastward or 020/35 kt. Isaias will continue moving north-northeastward through tonight, accompanied by a gradual decrease in forward speed. In 12 hours or, the cyclone is expected to interact with a larger extratropical over southeastern Canada and degenerate into a post-tropical cyclone near western Maine. By 24 hours, Isaias is expected to transition to an extratropical low, and dissipate or become absorbed by the aforementioned larger extratropical low. The new NHC track forecast is just an extension of the previous advisory, and lies very close to the tightly clustered consensus track models. Isaias will continue to gradually spin down and lose its tropical characteristics. The precipitation has decreased in size and also has become more stratiform in appearance, which are indications that the cyclone is gradually weakening. This downward trend should continue until Isais becomes an extratropical low in 12-24 hours. There also remains a risk of tornadoes across portions of New England this afternoon and evening Key Messages: 1. Tropical storm force winds that could cause tree damage and power outages will continue to spread across New England tonight. 2. Heavy rainfall near the path of Isaias, through the Hudson River Valley, is likely to result in flash flooding, particularly through urban areas and the surrounding terrain of the Catskills, Adirondack and Green Mountain Ranges through Tuesday night. Scattered minor to moderate river flooding is likely across portions of the Mid-Atlantic. Quick-responding rivers in the Northeast will also be susceptible to minor and possible moderate river flooding. 3. The threat of tornadoes will spread into New England this afternoon and evening. FORECAST POSITIONS AND MAX WINDS INIT 04/2100Z 42.7N 74.2W 55 KT 65 MPH 12H 05/0600Z 46.3N 72.4W 35 KT 40 MPH...POST-TROP/INLAND 24H 05/1800Z 50.7N 71.1W 30 KT 35 MPH...POST-TROP/EXTRATROP 36H 06/0600Z 53.3N 70.0W 25 KT 30 MPH...POST-TROP/EXTRATROP 48H 06/1800Z 54.1N 67.8W 20 KT 25 MPH...POST-TROP/EXTRATROP 60H 07/0600Z...DISSIPATED $$ Forecaster Stewart

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Tropical Storm Isaias Forecast Discussion Number 27

2020-08-03 22:59:24| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Mon Aug 03 2020 000 WTNT44 KNHC 032059 TCDAT4 Tropical Storm Isaias Discussion Number 27 NWS National Hurricane Center Miami FL AL092020 500 PM EDT Mon Aug 03 2020 Isaias is undergoing its daily reorganization phase, with a ragged eye-like feature now showing up in NOAA Doppler weather radars from Charleston, Wilmington, and Jacksonville. The last Air Force Reserve reconnaissance flight reported a peak SFMR-derived surface wind speed of 62 kt in the southwest quadrant around 1630Z while Isaias was experiencing a center reformation further to the east. Since then, Doppler velocities of 70-75 kt have been observed between 10,000-15,000 ft in the northern semicircle, but within reflectivity regions less than 35 dBZ, which suggests that a the reduction factor is likely less than the 90-percent value typically used. Thus the intensity is being held at 60 kt, which means that Isaias is very near hurricane status. Another Air Force Reserve reconnaissance will be investigating the cyclone around 0000Z this evening. Isaias is now moving north-northeastward 015/14 kt. The new NHC model guidance remains in excellent agreement on Isaias continuing to gradually accelerate north-northeastward to northeastward for the next 36 hours ahead of a powerful deep-layer trough and associated cold front. The cyclone should make landfall later this evening near the South Carolina-North Carolina border, and then accelerate north-northeastward at 25-30 kt across eastern North Carolina early Tuesday, eastern Virginia and the Delmarva peninsula Tuesday afternoon, and into New England Tuesday night into early Wednesday. The new NHC track forecast is just an update and extension of the previous advisory track, and lies very close to a blend of the tightly packed multi-model consensus aids TVCA, GFEX, TVCX, and NOAA-HCCA. Satellite animation and special 1800Z upper-air soundings indicate that the vertical shear across Isaias has weakened and has also become more southwesterly, which better aligns with the forecast track. Given this and the improved structure of the system, Isaias is still expected to strengthen and regain hurricane status before making landfall, and most of the intensity guidance shows a 60-65 kt system at that time. 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, Isaias is forecast to only slowly weaken due to interaction with an unusually strong 100-120 kt jetstream. The expected strong baroclinic forcing will keep Isaias' circulation intact and also produce very strong wind gusts along the mid-Atlantic coast tomorrow. As a result, the gust factors at 24-48 h have been increased above the standard 20 percent in the Forecast/Advisory (TCMAT4). The cyclone is forecast to be absorbed by a larger extratropical low over Canada in 3-4 days. Key Messages: 1. There is the danger of life-threatening storm surge inundation along portions of the immediate coastline and adjacent waterways of northeastern South Carolina and the North Carolina coast, including portions of Pamlico and Albemarle Sounds. Residents in these areas should follow advice given by local emergency officials. 2. Isaias is expected to be at or near hurricane strength when it reaches the coast of northeastern South Carolina and southern North Carolina in a few hours, and hurricane conditions are expected in the Hurricane Warning area this evening. 3. Isaias is expected to bring widespread sustained tropical storm force winds and wind gusts to hurricane force to the mid-Atlantic coast, including portions of the Chesapeake Bay region, on Tuesday, which could cause tree damage and power outages. Tropical storm conditions are expected to spread across New England late Tuesday into early Wednesday. 4. Heavy rainfall along the East Coast near the path of Isaias will result in flash and urban flooding, some of which may be significant in the eastern Carolinas, mid-Atlantic and northeast through Wednesday. Widespread minor to moderate river flooding is possible across portions of the Carolinas and the Mid-Atlantic. Quick- responding rivers in the northeast will also be susceptible to minor river flooding. FORECAST POSITIONS AND MAX WINDS INIT 03/2100Z 32.0N 79.4W 60 KT 70 MPH 12H 04/0600Z 34.7N 78.1W 60 KT 70 MPH...INLAND 24H 04/1800Z 39.3N 75.3W 55 KT 65 MPH...INLAND 36H 05/0600Z 44.5N 71.8W 45 KT 50 MPH...INLAND 48H 05/1800Z 48.8N 68.6W 30 KT 35 MPH...POST-TROP/EXTRATROP 60H 06/0600Z 52.0N 67.1W 25 KT 30 MPH...POST-TROP/EXTRATROP 72H 06/1800Z 54.3N 63.4W 25 KT 30 MPH...POST-TROP/EXTRATROP 96H 07/1800Z...DISSIPATED $$ Forecaster Stewart

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Tropical Storm Isaias Forecast Discussion Number 26

2020-08-03 16:57:19| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT Mon Aug 03 2020 000 WTNT44 KNHC 031457 TCDAT4 Tropical Storm Isaias Discussion Number 26 NWS National Hurricane Center Miami FL AL092020 1100 AM EDT Mon Aug 03 2020 Isaias continues to undergo strong bursting and then weakening convective phases, with the cyclone currently in the latter mode. Satellite and radar data indicate that Isaias' convective organization has become a little disheveled since the previous advisory, but this is to be expected since the tropical cyclone has been undergoing bursting periods about every 8 hours or so beginning overnight around 0600-0900Z. Thus, the system is due for another re-organization phase shortly if it holds true to form. That may already be underway based on recent radar and recon data showing a slight eastward shift in the center position. The highest 700-mb flight-level wind observed has been 63 kt, which reduces to about a 57-kt surface wind. Jacksonville Doppler radar velocities north and north-northwest of the center have been around 60 kt at 9,000 ft, which equals about 54 kt surface winds. The initial intensity will be held at 60 kt, perhaps a little generously, for this advisory despite the recent rise in the central pressure to around 998 mb. Isaias is still moving northward but a little faster at 360/11 kt. The track forecast reasoning remains essentially the same as the previous few advisories, with Isaias expected to gradually turn toward the north-northeast later today and begin to accelerate by this evening. The latest NHC model guidance is more tightly packed about the previous advisory track, so only minor tweaks were required. The new NHC track forecast lies very close to a blend of the multi-model consensus aids TVCA, GFEX, TVCX, and NOAA-HCCA. Current westerly vertical wind shear of 20-25 kt is expected to decrease somewhat during the next 12 h and also become more southwesterly, which will be in better alignment with Isaias' forward motion vector. The decrease in the shear should result in less tilt to the cyclone, allowing Isaias to strengthen and regain hurricane status just before making landfall, and most of the intensity guidance shows a 60-65 kt system at that time. After landfall, Isaias is forecast to only slowly weaken due to interaction with an unusually strong winter-type jetstream that will be possessing winds of 100-120 kt. Expected strong baroclinic forcing will keep Isaias' circulation intact and also produce very strong wind gusts along the Mid-Atlantic states tomorrow. As a result, the gust factors at 24-48 h have been increased above the standard 20 percent in the Forecast/Advisory (TCMAT4). The cyclone is forecast to be absorbed by a larger extratropical low over Canada in 3-4 days. Key Messages: 1. There is the danger of life-threatening storm surge inundation along portions of the immediate coastline and adjacent waterways of northeastern South Carolina and the North Carolina coast in the Storm Surge Warning area. Residents in these areas should follow advice given by local emergency officials. 2. Isaias is forecast to regain hurricane strength before it reaches the coast of northeastern South Carolina and southern North Carolina, and hurricane conditions are expected in the Hurricane Warning area by this evening. Preparations should be rushed to completion. 3. Isaias is expected to bring widespread sustained tropical storm force winds and wind gusts to hurricane force to the mid-Atlantic coast Tuesday, which could cause tree damage and power outages. Tropical storm conditions are expected to reach southern New England on late Tuesday and are possible along the northern New England coast Tuesday night and early Wednesday. 4. Heavy rainfall will result in flash and urban flooding, some of which may be significant in the eastern Carolinas and the mid-Atlantic, through Tuesday night near the path of Isaias up the East Coast of the United States. Widespread minor to moderate river flooding is possible across portions of the Carolinas and the Mid-Atlantic. FORECAST POSITIONS AND MAX WINDS INIT 03/1500Z 30.7N 80.1W 60 KT 70 MPH 12H 04/0000Z 33.1N 79.2W 65 KT 75 MPH 24H 04/1200Z 37.2N 77.0W 55 KT 65 MPH...INLAND 36H 05/0000Z 42.0N 73.8W 50 KT 60 MPH...INLAND 48H 05/1200Z 46.7N 70.2W 35 KT 40 MPH...INLAND 60H 06/0000Z 50.2N 67.2W 30 KT 35 MPH...POST-TROP/EXTRATROP 72H 06/1200Z 53.5N 64.2W 25 KT 30 MPH...POST-TROP/EXTRATROP 96H 07/1200Z...DISSIPATED $$ Forecaster Stewart

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Tropical Storm Isaias Forecast Discussion Number 25

2020-08-03 10:56:48| National Hurricane Center (Atlantic)

Issued at 500 AM EDT Mon Aug 03 2020 000 WTNT44 KNHC 030856 TCDAT4 Tropical Storm Isaias Discussion Number 25 NWS National Hurricane Center Miami FL AL092020 500 AM EDT Mon Aug 03 2020 Isaias continues to produce an area of vigorous convection near and to the northeast of its low-level center. Overnight radar data from Melbourne and Jacksonville have shown a transient mid-level eye feature that is located northeast of the low-level center. The earlier Air Force Reserve Hurricane Hunter aircraft flew near that feature before departing the storm and found 700-mb flight-level winds of 71 kt, with the highest SFMR winds still around 60 kt, which is the basis for the initial wind speed for this advisory. An Air Force Reserve and NOAA Hurricane Hunter aircraft will be investigating Isaias within the next several hours. The vertical wind shear that has been plaguing Isaias is forecast to abate slightly today as the storm turns north-northeastward. All of the intensity models shows some slight strengthening during the next 12 hours, and the global models also indicate some deepening. As a result, the updated NHC intensity forecast calls for Isaias to regain hurricane status before the system moves over the coast of the Carolinas. The new intensity forecast has necessitated the issuance of a hurricane warning for a portion of the coasts of northern South Carolina and southern North Carolina. After landfall, Isaias is forecast to gradually weaken, but given the fast forward motion of the storm, strong winds are expected to spread northward along the coast of the Mid-Atlantic and Northeast United States. The cyclone is forecast to be absorbed by a larger extratropical low over Canada in 3-4 days. Isaias is moving northward or 355/8 kt. The track forecast reasoning remains the same as before. The tropical storm is expected to accelerate north-northeastward to northeastward ahead of a large mid-level trough over the eastern United States during the next couple of days. The dynamical models are in good agreement except for some slight differences in the forward speed of Isaias. The NHC track forecast is near the multi-model consensus aids, which have once again trended slightly faster. Key Messages: 1. There is the danger of life-threatening storm surge inundation along portions of the immediate coastline and adjacent waterways of northeastern South Carolina and southern North Carolina. 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. Isaias is forecast to regain hurricane strength before it reaches the coast of northeastern South Carolina and southern North Carolina, and hurricane conditions are expected in the Hurricane Warning by this evening. Preparations should be rushed to completion. 3. Tropical storm conditions are expected to spread northward within the Tropical Storm Warning area from Florida to portions of southern New England through Tuesday afternoon. Tropical storm conditions are possible across other portions of New England within the Tropical Storm Watch area by early Wednesday. Additional watches or warnings may be required later today. 4. Heavy rainfall will result in flash and urban flooding, some of which may be significant in the eastern Carolinas and the mid-Atlantic, 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. FORECAST POSITIONS AND MAX WINDS INIT 03/0900Z 29.7N 79.9W 60 KT 70 MPH 12H 03/1800Z 31.3N 79.6W 65 KT 75 MPH 24H 04/0600Z 34.8N 78.1W 60 KT 70 MPH...INLAND 36H 04/1800Z 39.4N 75.2W 55 KT 65 MPH...INLAND 48H 05/0600Z 44.4N 71.5W 35 KT 40 MPH...INLAND 60H 05/1800Z 48.6N 68.0W 30 KT 35 MPH...POST-TROP/EXTRATROP 72H 06/0600Z 52.0N 65.0W 25 KT 30 MPH...POST-TROP/EXTRATROP 96H 07/0600Z...DISSIPATED $$ Forecaster Brown/Berg

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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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