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Tropical Storm Marco Forecast Discussion Number 11

2020-08-23 04:45:54| National Hurricane Center (Atlantic)

Issued at 1000 PM CDT Sat Aug 22 2020 000 WTNT44 KNHC 230245 TCDAT4 Tropical Storm Marco Discussion Number 11 NWS National Hurricane Center Miami FL AL142020 1000 PM CDT Sat Aug 22 2020 Marco has taken on distinctly sheared appearance. Reports from an Air Force Reserve Hurricane Hunter plane, microwave imagery, and radar imagery from Cuba all indicate that deep convection is limited to the east side of the tropical storm and that it no longer has a nearly closed eyewall. The degradation of Marco's structure appears to be primarily due to strong upper-level southwesterly flow. Despite the shear, the plane still measured SFMR winds near 55 kt and the intensity is held at that value. Marco is a small tropical storm and will be susceptible to rapid changes in structure and intensity until it reaches the northern Gulf Coast. Such systems are often not very resilient in a high-shear environment, however even a brief relaxation of the shear could result in quick strengthening. It would not be surprising if Marco's intensity evolves in step-wise fashion consisting of periods of arrested development followed by fast strengthening if/when the shear relaxes. While the statistical models still show Marco becoming a hurricane within 24 h, the run-to-run consistency of the dynamical guidance remains poor. The latest HWRF, HMON and GFS forecasts show Marco weakening as it approaches the northern Gulf Coast, and this remains a distinct possibility if the shear remains consistently high. The NHC intensity forecast has not been changed substantially, in large part due to the low confidence of the forecast, and is consequently above all of the guidance at 36 and 48 h when Marco is forecast to be near the northern Gulf Coast. Additional adjustments to the forecast are likely on Sunday. In sharp contrast to earlier today, no large changes were made to the track forecast, though that should not be interpreted as an increase in forecast confidence. Marco is forecast to move north-northwestward and approach the northern Gulf Coast on Monday. As it moves inland and weakens, a turn toward the west at a slower forward speed is anticipated. This turn could occur before or after Marco moves inland, and will be tied in part to exactly when Marco begins to weaken since a stronger, deeper storm should continue to feel the affects of the upper-level southwesterly flow and move farther north while a weaker system will be steered westward by a low- to mid-level ridge extending over the southeastern US. The NHC forecast is nearly on top of the multi-model consensus, but the spread in the guidance is still higher than normal. Key Messages: 1. Tropical storm conditions will continue over portions of extreme western Cuba for a few more hours. Heavy rainfall is also expected overnight in the eastern portions of the Mexican states of Quintana Roo and Yucatan, and across far western Cuba, which could result in flash flooding. 2. Marco is expected to be at or near hurricane strength when it approaches the central Gulf Coast as a hurricane on Monday. Hurricane conditions, life-threatening storm surge, and heavy rainfall are possible along portions of the Gulf Coast beginning on Monday, and Hurricane and Storm Surge watches have been issued. Interests in these areas should follow any advice given by local government officials. 3. Tropical Storm Laura could bring additional storm surge, rainfall, and wind impacts to portions of the U.S. Gulf Coast by the middle of next week. This could result in a prolonged period of hazardous weather for areas that may also be affected by Marco. Interests there should monitor the progress of Marco and Laura and updates to the forecast during the next few days. FORECAST POSITIONS AND MAX WINDS INIT 23/0300Z 22.8N 86.3W 55 KT 65 MPH 12H 23/1200Z 24.3N 87.1W 65 KT 75 MPH 24H 24/0000Z 26.3N 87.9W 70 KT 80 MPH 36H 24/1200Z 28.3N 88.9W 70 KT 80 MPH 48H 25/0000Z 29.6N 90.4W 55 KT 65 MPH...INLAND 60H 25/1200Z 30.5N 92.0W 35 KT 40 MPH...INLAND 72H 26/0000Z 30.9N 93.5W 30 KT 35 MPH...INLAND 96H 27/0000Z...DISSIPATED $$ Forecaster Zelinsky

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Tropical Storm Marco Forecast Discussion Number 10

2020-08-22 22:55:16| National Hurricane Center (Atlantic)

Issued at 400 PM CDT Sat Aug 22 2020 104 WTNT44 KNHC 222055 TCDAT4 Tropical Storm Marco Discussion Number 10 NWS National Hurricane Center Miami FL AL142020 400 PM CDT Sat Aug 22 2020 Cutting to the chase, there have been some big changes among the model guidance, and subsequently the NHC forecast, for Marco this afternoon. While at this point it's a little speculative, the data collected by this morning's NOAA G-IV flight in the environment around Marco and across the Gulf of Mexico may have played a key role in the significant eastward shift seen in nearly all the 12z models. This isn't to say that the uncertainty in the eventual track has diminished. In fact, various ensemble members from some of the global models still show a potential risk to the coast anywhere from Texas to Alabama, and it's entirely possible that the volatile shifts seen in the models could continue. That being said, the new NHC track forecast has been shifted significantly eastward and now shows the center of Marco reaching southeastern Louisiana in about 2 days, which is the scenario currently shown by the GFS, ECMWF, HCCA, Florida State Superensemble, and the TVCN multi-model consensus. After Marco reaches the coast, the western Atlantic ridge is expected to build westward and should cause the cyclone to move more slowly toward the west-northwest across southern portions of Louisiana. As far as the intensity is concerned, the last fix made by this morning's reconnaissance flight indicated that the pressure had leveled off, and no higher winds had been observed from what was measured earlier in the flight. The radar presentation from Cuban radar has also degraded a bit, so Marco's initial intensity is held at 55 kt. Marco is beginning to move into a zone of moderate southwesterly shear, but otherwise favorable conditions of warm ocean water and some upper-level divergence are expected to foster strengthening during the next day or so. With the exception of the HWRF, the other intensity models show Marco reaching hurricane strength, and the NHC foreast continues to show that possibility while Marco moves over the central Gulf. The shear is still expected to strengthen in 36-48 hours when the system is approaching the northern Gulf Coast, but with the shift in the forecast track, now there may not be enough time for Marco to weaken below hurricane intensity before it reaches land. The new NHC intensity forecast is near or just above the HCCA and Florida State Superensemble models and holds Marco as a hurricane until it reaches the coast. The forecast track changes now bring tropical storm force winds to the coast in 36-48 hours, which necessitates the issuance of Hurricane and Storm Surge Watches for a portion of the northern Gulf Coast. These watches will likely need to be upgraded to warnings later tonight. Key Messages: 1. Tropical storm conditions will continue over portions of extreme western Cuba through this evening. Heavy rainfall is also expected in the eastern portions of the Mexican states of Quintana Roo and Yucatan, and across far western Cuba, which could result in flash flooding. 2. Marco is expected to approach the central Gulf Coast as a hurricane on Monday. Hurricane conditions, life-threatening storm surge, and heavy rainfall are possible along portions of the Gulf Coast beginning on Monday, and Hurricane and Storm Surge watches have been issued. Interests in these areas should follow any advice given by local government officials. 3. Tropical Storm Laura could bring additional storm surge, rainfall, and wind impacts to portions of the U.S. Gulf Coast by the middle of next week. This could result in a prolonged period of hazardous weather for areas that could be affected by Marco. Interests there should monitor the progress of Marco and Laura and updates to the forecast during the next few days. FORECAST POSITIONS AND MAX WINDS INIT 22/2100Z 21.9N 85.7W 55 KT 65 MPH 12H 23/0600Z 23.3N 86.5W 65 KT 75 MPH 24H 23/1800Z 25.3N 87.3W 75 KT 85 MPH 36H 24/0600Z 27.5N 88.3W 75 KT 85 MPH 48H 24/1800Z 29.3N 89.6W 65 KT 75 MPH...ON THE COAST 60H 25/0600Z 30.4N 91.1W 50 KT 60 MPH...INLAND 72H 25/1800Z 31.1N 92.7W 40 KT 45 MPH...INLAND 96H 26/1800Z 32.3N 95.6W 25 KT 30 MPH...INLAND 120H 27/1800Z...DISSIPATED $$ Forecaster Berg

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Tropical Storm Laura Forecast Discussion Number 12

2020-08-22 22:52:19| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sat Aug 22 2020 000 WTNT43 KNHC 222052 TCDAT3 Tropical Storm Laura Discussion Number 12 NWS National Hurricane Center Miami FL AL132020 500 PM AST Sat Aug 22 2020 A couple of hours ago an observing site on the southeast coast of Puerto Rico, Las Mareas, reported sustained winds of 52 kt. These winds were apparently associated with a mesocyclone embedded within the larger-scale circulation and not representative of the intensity of the tropical storm. Scatterometer data and WSR-88D Doppler velocities from San Juan support an intensity of 45 kt. Since the center should be moving over land for the next 48 hours or so, no additional intensification is anticipated until Monday night when the center moves over the southeastern Gulf of Mexico. Over warm waters, with anticipated weak vertical shear, and anticyclonic flow aloft, Laura will likely strengthen into a hurricane before it reaches the northern Gulf of Mexico coast. The NHC intensity forecast is close to the multi-model consensus, but given the possibility of a favorable upper-air environment over the Gulf, this forecast could be conservative. At this time it does not seem likely that Marco, which is forecast to make landfall on the north-central Gulf coast a day or two earlier than Laura, should have much of an influence on the latter system. Center locations from earlier scatterometer data, low-cloud motions, and Dvorak fixes from both TAFB and SAB, give a motion of about 280/16 kt. Laura should move west-northwestward along the southern side of a mid-tropospheric anticyclone centered near the southeastern U.S. coast through 72 hours. Then, the cyclone is expected to turn northwestward to northward on the western side of the high. The official track forecast is on the right side of the track guidance suite. Given the predicted track and wind radii, a Tropical Storm Watch has been issued for the Florida Keys and the Dry Tortugas. Key Messages: 1. Tropical storm conditions are expected to continue across portions of the U.S. Virgin Islands and Puerto Rico through this evening. Tropical storm conditions are also expected across portions of the Dominican Republic and Haiti, the Turks and Caicos, the southeastern Bahamas, and central and eastern Cuba through Sunday. Heavy rainfall is likely across these areas and could cause mudslides and flash and urban flooding through Sunday, with widespread river flooding possible in Puerto Rico. 2. Tropical storm conditions are possible over portions of the central Bahamas and Andros Island Sunday night and Monday, and in the Florida Keys on Monday. 3. The details of the long-range track and intensity forecasts remain uncertain since Laura is forecast to move near or over portions of the Greater Antilles through Monday. However, Laura is forecast to strengthen over the Gulf of Mexico and could bring storm surge, rainfall, and wind impacts to portions of the U.S. Gulf Coast by the middle of next week. This could result in a prolonged period of hazardous weather for areas that are likely to be affected by Tropical Storm Marco earlier in the week. Interests there should monitor the progress of Laura and Marco and updates to the forecast during the next few days. FORECAST POSITIONS AND MAX WINDS INIT 22/2100Z 18.0N 68.1W 45 KT 50 MPH 12H 23/0600Z 18.9N 70.6W 45 KT 50 MPH...INLAND 24H 23/1800Z 20.1N 74.3W 45 KT 50 MPH...INLAND 36H 24/0600Z 21.6N 78.1W 45 KT 50 MPH...INLAND 48H 24/1800Z 23.0N 81.6W 45 KT 50 MPH...INLAND 60H 25/0600Z 24.3N 84.5W 55 KT 65 MPH...OVER WATER 72H 25/1800Z 25.7N 87.3W 65 KT 75 MPH 96H 26/1800Z 29.0N 90.7W 75 KT 85 MPH 120H 27/1800Z 33.0N 91.0W 30 KT 35 MPH...INLAND $$ Forecaster Pasch

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Tropical Storm Marco Forecast Discussion Number 9

2020-08-22 16:54:43| National Hurricane Center (Atlantic)

Issued at 1000 AM CDT Sat Aug 22 2020 000 WTNT44 KNHC 221454 TCDAT4 Tropical Storm Marco Discussion Number 9 NWS National Hurricane Center Miami FL AL142020 1000 AM CDT Sat Aug 22 2020 Reconnaissance data, geostationary and microwave satellite imagery, and radar data from Cuba all indicate that Marco is strengthening quickly this morning. The Air Force Reserve Hurricane Hunters measured a peak 850-mb flight-level wind of 69 kt, and there were several SFMR measurements of 50-55 kt to the northeast of the center. These data support raising Marco's initial intensity to 55 kt, and the central pressure based on dropsonde data has fallen to 992 mb. The crew on the plane reported the formation of a partial eyewall, which agrees with what we've seen on recent microwave and radar images. The track forecast has been complicated by the fact that the plane has fixed Marco's center to the east of the previous forecast track, and that makes the current motion north-northwestward, or 340/10 kt. The subtropical ridge currently located over the southwestern Atlantic is forecast to build westward along the northern Gulf Coast during the next few days, and this expanding ridge is expected to push Marco northwestward and then eventually westward while the cyclone moves across the Gulf of Mexico. This general thinking has not changed, but the adjusted initial position ended up shifting the track guidance to the north and east on this cycle. In response, the new NHC track forecast has been adjusted eastward and northward during the first 3 days and is generally between the HCCA and TVCN consensus aids. Marco has finally tapped into the favorable conditions over the northwestern Caribbean Sea, and the cyclone's overall small size and small radius of maximum winds makes it susceptible to quick changes in intensity. The tropical storm is just beginning to move into a higher zone of shear to its north, but conditions should remain conducive enough for Marco to intensity to a hurricane during the next 24 hours. After that time, southwesterly shear is expected to increase over 20 kt by day 2 and then over 30 kt by day 3, and those conditions, along with the cyclone's small size, should cause weakening as Marco gets closer to the central and northwestern Gulf coast. The updated NHC intensity forecast has been increased during the first 2 days and lies above the HCCA/Florida State Superensemble solutions but below the SHIPS/LGEM scenarios. The intensity forecast comes back in line with the previous forecast by day 3 during the expected weakening phase, and Marco is ultimately expected to dissipate over Texas by the end of the forecast period. The updated track forecast suggests that watches could be required for a portion of the central Gulf Coast later today. Key Messages: 1. Marco is forecast to strengthen to a hurricane as it moves into the southeastern Gulf of Mexico by tonight, and tropical storm conditions are expected over the northeastern coast of the Yucatan Peninsula and in extreme western Cuba. Heavy rainfall is also expected in the eastern portions of the Mexican states of Quintana Roo and Yucatan, and across far western Cuba, which could result in flash flooding. 2. Marco is expected to move across the central Gulf of Mexico as a hurricane Sunday and approach the central Gulf Coast on Monday. There is an increasing risk of impacts from storm surge, winds, and heavy rainfall from the upper Texas coast to Louisiana early next week, and interests there should monitor the progress of Marco, as storm surge, tropical storm, and/or hurricane watches could be issued later today. FORECAST POSITIONS AND MAX WINDS INIT 22/1500Z 20.9N 85.3W 55 KT 65 MPH 12H 23/0000Z 22.1N 86.0W 65 KT 75 MPH 24H 23/1200Z 23.9N 87.1W 75 KT 85 MPH 36H 24/0000Z 25.6N 88.4W 75 KT 85 MPH 48H 24/1200Z 27.2N 89.9W 70 KT 80 MPH 60H 25/0000Z 28.5N 91.6W 60 KT 70 MPH 72H 25/1200Z 29.1N 93.6W 50 KT 60 MPH 96H 26/1200Z 29.3N 96.6W 35 KT 40 MPH...INLAND 120H 27/1200Z...DISSIPATED $$ Forecaster Berg

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Tropical Storm Laura Forecast Discussion Number 11

2020-08-22 16:52:42| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sat Aug 22 2020 000 WTNT43 KNHC 221452 TCDAT3 Tropical Storm Laura Discussion Number 11 NWS National Hurricane Center Miami FL AL132020 1100 AM AST Sat Aug 22 2020 Laura is producing considerable deep convection but the cloud pattern remains disorganized. Surface observations indicate that the center is still not well defined, but the San Juan WSR-88D radar shows a mid-level center south of eastern Puerto Rico. The radar also shows heavy rains spreading over that island. The initial intensity estimate remains at 35 kt based on continuity. As noted in previous discussions, the intensity forecast for Laura is dependent on its future track. The official forecast, like the previous one, moves the center over the land masses of Hispaniola and Cuba for the next couple of days which should limit intensification at least through the weekend. After Laura moves into the Gulf of Mexico, warm waters should lead to strengthening and the system is likely to become a hurricane in 3 to 4 days. The official intensity forecast is close to the consensus guidance. At this time the influence of small tropical cyclone Marco, which should move over the northwestern Gulf about a day or two before Laura, is not expected to be significant. This could change in the coming days however. The initial motion estimate remains quite uncertain and is near 280/16 kt. No significant changes are being made to the official track forecast. Laura is likely to move on a generally west-northwestward track on the periphery of a mid-tropospheric high pressure area for the next few days. By day 4, the system should turn northwestward while moving along the southwestern side of the high. The official track forecast is only slightly north of the dynamical model consensus at 36-72 hours and essentially the same as the consensus otherwise. The GFS and its ensemble mean are significantly farther west of the official forecast at days 3 and beyond, which suggests relatively low confidence in the longer-range track of Laura. Key Messages: 1. Tropical storm conditions are expected across portions of the the Virgin Islands and Puerto Rico today. Tropical storm conditions are also expected along the northern coasts of the Dominican Republic and Haiti, and the Turks and Caicos and southeastern Bahamas Saturday into Sunday. Heavy rainfall is likely across these areas and could cause mudslides and flash and urban flooding through Sunday, with widespread river flooding possible in Puerto Rico. 2. Tropical storm conditions are possible over portions of the central Bahamas Sunday night, as well as portions of eastern and central Cuba Sunday and Sunday night. 3. The details of the long-range track and intensity forecasts remain more uncertain than usual since Laura is forecast to move near or over portions of the Greater Antilles through Monday. However, Laura is forecast to strengthen over the Gulf of Mexico and could bring storm surge, rainfall, and wind impacts to the northern U.S. Gulf Coast by the middle of next week. Interests there should monitor the progress of Laura and updates to the forecast during the next few days. FORECAST POSITIONS AND MAX WINDS INIT 22/1500Z 17.8N 66.8W 35 KT 40 MPH 12H 23/0000Z 18.6N 69.4W 40 KT 45 MPH...INLAND 24H 23/1200Z 19.4N 72.6W 40 KT 45 MPH...INLAND 36H 24/0000Z 20.9N 76.3W 45 KT 50 MPH...INLAND 48H 24/1200Z 22.4N 80.0W 45 KT 50 MPH...INLAND 60H 25/0000Z 23.7N 83.2W 55 KT 65 MPH...OVER WATER 72H 25/1200Z 25.2N 86.2W 65 KT 75 MPH 96H 26/1200Z 27.9N 90.6W 75 KT 85 MPH 120H 27/1200Z 31.6N 92.5W 40 KT 45 MPH...INLAND $$ Forecaster Pasch

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