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Hurricane Marco Forecast Discussion Number 14

2020-08-23 22:55:03| National Hurricane Center (Atlantic)

Issued at 400 PM CDT Sun Aug 23 2020 000 WTNT44 KNHC 232054 TCDAT4 Hurricane Marco Discussion Number 14 NWS National Hurricane Center Miami FL AL142020 400 PM CDT Sun Aug 23 2020 Shortly after the 10 AM CDT advisory was issued, the Air Force Reserve Hurricane Hunter aircraft investigating Marco measured SFMR winds of 65 to 69 kt for several minutes. A dropsonde around the same time also measured winds equivalent to 64 kt at the surface. Based on these data, it was evident that Marco had strengthened into a hurricane. Since that time, the convective pattern has changed little, and the final leg of the aircraft mission did not reveal any notable changes to the cyclone's intensity. Therefore, the initial intensity has been set to 65 kt, which is also in agreement with the latest Dvorak intensity estimate from SAB. Marco continues to move north-northwest at around 11 kt. Model guidance is in good agreement on a north-northwest and then northwest motion into Monday morning as Marco nears the northern Gulf coast. However, there has been a major shift in the track guidance beyond Monday morning, and the majority of the forecast models now keep Marco offshore of the northern Gulf coast for the next few days rather than moving it inland over Louisiana Monday afternoon. Since this shift was so abrupt, I would rather split the difference between the previous official forecast track and the latest consensus tracks until another round of model runs can confirm this new suggested track is higher confidence. Based on this shift in track, tropical storm conditions are expected farther west along the Louisiana coast, and a Tropical Storm Warning has been issued from Morgan City to Cameron, Louisiana. It should be noted that if the trend in the models are correct, some of the impacts over portions of the north-central Gulf coast could be lessened. The current shear analysis from UW-CIMSS shows that Marco is experiencing about 20-25 kt of southwesterly shear, yet Marco has been able to slowly strengthen in this environment today, likely due to the presence of very warm waters and plenty of atmospheric moisture. By late tonight and through Monday, the SHIPS guidance suggests this shear will increase to over 30 kt. This should limit any further intensification, and could cause Marco to weaken before it nears the northern Gulf coast Monday afternoon. By Monday night, the shear is forecast to increase to close to 40 kt, which should strip the convection away from the center of the cyclone, causing it to weaken. After 48 h, Marco is now expected to become a remnant low devoid of deep convection. The official NHC intensity forecast is similar to the previous one, but is at the high end of the guidance through 24 h. Key Messages: 1. Hurricane conditions, life-threatening storm surge, and heavy rainfall are expected from Marco along portions of the Gulf Coast beginning on Monday. Interests in these areas should follow any advice given by local government officials. 2. Tropical Storm Laura could bring additional storm surge, rainfall, and wind impacts to portions of the U.S. Gulf Coast by the middle of the 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 23/2100Z 25.8N 87.8W 65 KT 75 MPH 12H 24/0600Z 27.1N 88.6W 65 KT 75 MPH 24H 24/1800Z 28.5N 89.7W 65 KT 75 MPH 36H 25/0600Z 29.3N 91.4W 50 KT 60 MPH...INLAND 48H 25/1800Z 29.8N 93.2W 35 KT 40 MPH...INLAND 60H 26/0600Z 29.9N 95.1W 30 KT 35 MPH...POST-TROP/INLAND 72H 26/1800Z 29.7N 96.8W 25 KT 30 MPH...POST-TROP/INLAND 96H 27/1800Z...DISSIPATED $$ Forecaster Latto

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Tropical Storm Marco Forecast Discussion Number 13

2020-08-23 16:57:59| National Hurricane Center (Atlantic)

Issued at 1000 AM CDT Sun Aug 23 2020 000 WTNT44 KNHC 231457 TCDAT4 Tropical Storm Marco Discussion Number 13 NWS National Hurricane Center Miami FL AL142020 1000 AM CDT Sun Aug 23 2020 Deep convection with cloud tops of -75 to -80 degrees C has persisted over the center of Marco for the past several hours. An Air Force Reserve Hurricane Hunter aircraft currently investigating the cyclone measured winds equivalent to 62 kt both with the SFMR and in a dropsonde in the northeastern eyewall. However, the aircraft reported peak 850-mb flight level winds of only 64 kt. Based on a blend of these data it appears that Marco is on the cusp of becoming a hurricane, but is not quite there yet. Therefore, the initial intensity remains 60 kt. Based on the SHIPS guidance, Marco has about a 12-24 hour window to intensify in an environment characterized by moderate southwesterly shear, very warm waters, and plenty of atmospheric moisture. After that time, the vertical wind shear is expected to increase and this should begin to dominate the cyclone's environment. The latest NHC intensity forecast is unchanged from the previous one, forecasting Marco to become a hurricane later today, and maintaining hurricane intensity up until landfall in agreement with the latest LGEM intensity guidance. While it is possible that Marco will weaken just prior to landfall due to the increasing shear, there is little difference in the impacts between a 60 and 65 kt system. Marco is moving north-northwestward or 340/12 kt. Although the overall guidance has not changed much since the previous advisory, there remains considerable spread in this guidance by the time Marco reaches the northern Gulf coast. This spread could be attributed to the varying ways the models handle the system as it encounters the more hostile environment near the coast. Since the track consensus aids have changed little through 36 h, the official forecast is essentially the same as the previous one through that time. The model guidance has shifted a little northward beyond 36 h, so the official forecast was nudged to the north during that time as well. Key Messages: 1. Hurricane conditions, life-threatening storm surge, and heavy rainfall are expected from Marco along portions of the Gulf Coast beginning on Monday, and Hurricane and Storm Surge Warnings have been issued. Interests in these areas should follow any advice given by local government officials. 2. Tropical Storm Laura could bring additional storm surge, rainfall, and wind impacts to portions of the U.S. Gulf Coast by the middle of the 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 23/1500Z 24.7N 87.3W 60 KT 70 MPH 12H 24/0000Z 26.4N 88.1W 65 KT 75 MPH 24H 24/1200Z 28.2N 89.1W 65 KT 75 MPH 36H 25/0000Z 29.5N 90.4W 55 KT 65 MPH...INLAND 48H 25/1200Z 30.5N 92.1W 35 KT 40 MPH...INLAND 60H 26/0000Z 31.2N 93.8W 30 KT 35 MPH...INLAND 72H 26/1200Z 31.8N 95.4W 25 KT 30 MPH...INLAND 96H 27/1200Z...DISSIPATED $$ Forecaster Latto

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Tropical Storm Laura Forecast Discussion Number 15

2020-08-23 16:54:57| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT Sun Aug 23 2020 000 WTNT43 KNHC 231454 TCDAT3 Tropical Storm Laura Discussion Number 15 NWS National Hurricane Center Miami FL AL132020 1100 AM EDT Sun Aug 23 2020 Laura continues to produce a large area of deep convection to the south and southeast of the estimated center location. The center has not been easy to locate this morning, but the low-cloud motions seen in GOES-16 one-minute visible imagery, along with surface observations, suggest that the center is just west of the west coast of Haiti. Data from a NOAA P-3 aircraft that has flown a tail-Doppler radar mission along the southern and northern coasts of Hispaniola this morning found maximum winds of 40-45 kt, so the initial intensity has been set at 45 kt. Little change in strength is expected during the next 36 hours while Laura moves near or over Cuba. When the center of Laura emerges over the Gulf of Mexico Monday night, the upper-level environment is expected to be conducive for strengthening, and once the circulation recovers from its trek over land, deepening is anticipated. Warm water and a very favorable upper-level wind pattern are expected to allow for steady intensification until Laura reaches the northern Gulf coast, and with landfall expected between the 72 and 96 h forecast points, the system could be somewhat stronger than explicitly indicated below. The NHC intensity forecast is a blend of the SHIPS and HFIP corrected consensus models. Although the center of Laura was been difficult to track while it passed over Hispaniola, the estimated motion is west-northwestward at about 18 kt. A strong deep-layer ridge over the western Atlantic should continue to steer Laura west-northwestward for the next couple of days. The track guidance has continued to nudge southward during the first 36 hours and the official forecast has been adjusted accordingly, taking the storm closer to the southern coast of Cuba. After that time, the ridge is forecast to build westward over the eastern Gulf of Mexico. This pattern should allow Laura to maintain a west-northwestward motion until it approaches the central Gulf, where a northwestward motion is expected to begin as the storm nears the western periphery of the ridge. The dynamical models have trended toward stronger ridging over the eastern Gulf, resulting in a westward shift in the guidance. The NHC track forecast has been moved westward at 72-96 hours, and lies between the GFS and ECMWF solutions. Users are reminded to not to focus on the exact details of the track forecast at the longer range as future adjustments will likely be required, and storm hazards will extend far from the center. Key Messages: 1. Tropical storm conditions are expected across portions of the Dominican Republic and Haiti, the Turks and Caicos, the southeastern Bahamas, and Cuba through Monday. Heavy rainfall is likely across these areas and could cause mudslides and life-threatening flash and urban flooding. 2. Tropical storm conditions are possible over the central Bahamas and Andros Island tonight and Monday, and in the Florida Keys on Monday. 3. While the details of the long-range track and intensity forecasts remain uncertain, Laura is forecast to strengthen over the Gulf of Mexico and there is an increasing risk of storm surge, rainfall, and wind impacts along portions of the U.S. Gulf Coast by the middle of the week. This could result in a prolonged period of hazardous weather for areas that are likely to be affected by Marco earlier in the week. Interests along the Gulf Coast should monitor the progress of Laura and Marco and updates to the forecast during the next few days. FORECAST POSITIONS AND MAX WINDS INIT 23/1500Z 19.2N 73.2W 45 KT 50 MPH 12H 24/0000Z 20.3N 76.0W 45 KT 50 MPH...INLAND 24H 24/1200Z 21.6N 79.9W 45 KT 50 MPH...NEAR CUBA 36H 25/0000Z 22.9N 83.2W 45 KT 50 MPH...INLAND 48H 25/1200Z 24.1N 86.2W 50 KT 60 MPH...OVER WATER 60H 26/0000Z 25.3N 88.9W 65 KT 75 MPH 72H 26/1200Z 26.8N 91.4W 80 KT 90 MPH 96H 27/1200Z 30.9N 94.0W 85 KT 100 MPH...INLAND 120H 28/1200Z 35.7N 92.8W 30 KT 35 MPH...INLAND $$ Forecaster Brown

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Tropical Storm Laura Forecast Discussion Number 14

2020-08-23 10:59:58| National Hurricane Center (Atlantic)

Issued at 500 AM EDT Sun Aug 23 2020 000 WTNT43 KNHC 230859 TCDAT3 Tropical Storm Laura Discussion Number 14 NWS National Hurricane Center Miami FL AL132020 500 AM EDT Sun Aug 23 2020 Laura has maintained an impressive convective pattern despite the center being located over extreme south-central Dominican Republic. Numerous cloud tops of -85C to -90C have been noted over the Barahona peninsula, an indication that extremely heavy rainfall has been occurring there. The center of Laura passed over or very near Santo Domingo around 0430Z based on a noticeable wind shift that was measured at the international airport. Laura's outflow pattern has also continue to improve in all quadrants. The initial intensity of 40 kt is based on earlier scatterometer and aircraft data, along with surface observations along the north coast of the Dominican Republic. Laura has continued to move west-northwestward and the initial motion estimate is now 285/16 kt. There has been a significant westward shift in the latest NHC model guidance, which appears to be due to most of the global models taking the center of Laura farther south over central or southern Hispaniola rather than emerging it off the north coast of Haiti like the GFS is and has been forecasting. Given that the most intense convection has persisted along the southern coast of Hispaniola, that is where the most likely area that a low-level and/or mid-level circulation is most probable to develop or persist. As a result, the new NHC track forecast favors a more southerly and westerly track solution similar to the preponderance of the track guidance. However, the new forecast track has not been shifted as far to the left as the consensus models in the event that the models shift back to the north. However, the latter scenario is appearing less likely based observed satellite trends since the previous advisory. Little if any significant change in strength is expected due to Laura moving pretty much down the spine of Hispaniola and Cuba during the the next 36 hours, with the strongest wind likely remaining over water in the northeast quadrant where the pressure gradient will be the tightest between the cyclone and the Bermuda High. By 48 hours and continuing until landfall, Laura is forecast to remain in a low shear and very favorable upper-level outflow environment while passing of extremely warm SSTs near 31C. This should allow for significant strengthening to occur once the cyclone regains a decent inner core after exiting Cuba. The new NHC intensity forecast is a blend of the intensity forecasts by the GFS and ECMWF global models and the corrected consensus models HCCA and FSSE. Key Messages: 1. Tropical storm conditions are expected across portions of the Dominican Republic and Haiti, the Turks and Caicos, the southeastern Bahamas, and Cuba through Monday. Heavy rainfall is likely across these areas and could cause mudslides and flash and urban flooding. 2. Tropical storm conditions are possible the central Bahamas and Andros Island tonight 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 23/0900Z 18.8N 70.9W 40 KT 45 MPH 12H 23/1800Z 19.8N 73.7W 40 KT 45 MPH...OVER WATER 24H 24/0600Z 20.9N 77.5W 40 KT 45 MPH...INLAND 36H 24/1800Z 22.2N 81.1W 40 KT 45 MPH...INLAND 48H 25/0600Z 23.5N 84.2W 45 KT 50 MPH...OVER WATER 60H 25/1800Z 24.9N 87.0W 60 KT 70 MPH 72H 26/0600Z 26.4N 89.4W 75 KT 85 MPH 96H 27/0600Z 29.9N 92.5W 85 KT 100 MPH 120H 28/0600Z 35.0N 91.4W 30 KT 35 MPH...INLAND $$ Forecaster Stewart

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Tropical Storm Marco Forecast Discussion Number 12

2020-08-23 10:58:57| National Hurricane Center (Atlantic)

Issued at 400 AM CDT Sun Aug 23 2020 000 WTNT44 KNHC 230858 TCDAT4 Tropical Storm Marco Discussion Number 12 NWS National Hurricane Center Miami FL AL142020 400 AM CDT Sun Aug 23 2020 Thunderstorm bursts continue to fire near the center of Marco, then weaken an hour or two later due to persistent shear. Overall, the storm's appearance hasn't changed with nearly all of the deep convection very near or northeast of the center. Although the satellite presentation is somewhat lacking at the moment, the earlier Air Force Reserve Hurricane Hunter mission found believable SFMR values to 60 kt, so that value remains the initial wind speed. The intensity forecast remains tricky with Marco due to its small size and marginal environment. There are some models that briefly relax the shear today, which will likely be enough of a change to allow Marco to reach hurricane strength. Later on, while the cyclone is near the coast of Louisiana, the shear is forecast to increase, but it is unknown exactly how close to landfall this will occur. Our best forecast at this time is that the strongest winds will be confined to the coast, and that Marco will then weaken faster than most hurricanes do over the swamps of Louisiana due to the shear. No significant changes were made to the intensity forecast, which is very close to the model consensus. The new forecast necessitates the issuance of hurricane warnings for portions of southeastern Louisiana. Marco continues moving north-northwestward or 335/11 kt. This general track and speed is likely today, with a turn to the northwest and decrease in forward speed expected as the storm weakens late Monday. While the track forecast is essentially unchanged from the previous one, there is still a fair bit of spread in the model guidance, likely tied to the intensity forecast. The stronger guidance is near or northeast of the new model consensus, due to the upper-level flow, and the NHC forecast leans in that direction, close to the previous official prediction. Key Messages: 1. Hurricane conditions, life-threatening storm surge, and heavy rainfall are expected from Marco along portions of the Gulf Coast beginning on Monday, and Hurricane and Storm Surge Warnings have been issued. Interests in these areas should follow any advice given by local government officials. 2. 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 23/0900Z 23.7N 87.0W 60 KT 70 MPH 12H 23/1800Z 25.3N 87.6W 65 KT 75 MPH 24H 24/0600Z 27.3N 88.6W 65 KT 75 MPH 36H 24/1800Z 29.0N 89.7W 65 KT 75 MPH 48H 25/0600Z 29.9N 91.5W 45 KT 50 MPH...INLAND 60H 25/1800Z 30.5N 93.1W 30 KT 35 MPH...INLAND 72H 26/0600Z 31.0N 95.0W 25 KT 30 MPH...INLAND 96H 27/0600Z...DISSIPATED $$ Forecaster Blake

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