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Potential Tropical Cyclone Twenty-Six Forecast Discussion Number 1
2020-10-04 23:02:19| National Hurricane Center (Atlantic)
Issued at 500 PM EDT Sun Oct 04 2020 000 WTNT41 KNHC 042102 TCDAT1 Potential Tropical Cyclone Twenty-Six Discussion Number 1 NWS National Hurricane Center Miami FL AL262020 500 PM EDT Sun Oct 04 2020 Visible satellite imagery and earlier scatterometer data indicated that the circulation associated with the area of low pressure in the central Caribbean Sea has gradually become better defined. The associated deep convection does not yet have enough organization to classify the system as a tropical depression, but there has been some increase in convection just south of the estimated center. The earlier ASCAT data revealed peak winds of around 30 kt over the northeastern portion of the circulation, and that is the basis for the initial intensity. The disturbance is located over warm waters and in a moist environment, but there is some modest northeasterly shear over the system. The global models indicate that the shear will decrease overnight, and the oceanic and atmospheric environment is expected to quite favorable for both the development of a tropical cyclone and subsequent strengthening of the system over the northwestern Caribbean Sea during the next couple of days. The intensity guidance is quite aggressive, but also assumes that the system already has a tropical cyclone structure. Therefore, the NHC intensity forecast is a little below the intensity consensus during the first 24-48 hours, but does show the system at or near hurricane strength by the time is near western Cuba on Tuesday. Environmental conditions are expected to remain favorable for strengthening over the southeastern Gulf of Mexico, and additional strengthening is predicted during that time. Late in the period, conditions are forecast to become less conducive as the vertical wind shear increases and the system nears the cooler shelf waters of the northern Gulf of Mexico. The disturbance is moving west-northwestward or 290/9 kt. A mid- to upper-level ridge over the western Atlantic is forecast to build westward over the next few days, which should continue to steer the system west-northwestward to northwestward over the northwestern Caribbean Sea and southeastern Gulf of Mexico. The forward speed of the system is likely to increase in 48 to 72 hours while it moves between the ridge and Tropical Storm Gamma to its southwest. After 72 hours, the cyclone is forecast to slow down and turn northward around the western portion of the ridge and a mid- to upper-level trough over the south-central United States. The track guidance is in relatively good agreement during the first 48 to 72 hours, but there is increasing spread thereafter. Users are reminded that the average 4- and 5-day NHC track forecast errors are about 160 to 200 miles at those time periods. Key Messages: 1. Tropical storm conditions are expected in the Cayman Islands beginning late Monday, and a Tropical Storm Warning is in effect. 2. Dangerous storm surge and hurricane conditions are possible in portions of western Cuba and the Isle of Youth by Tuesday afternoon, and a Hurricane Watch is in effect. 3. Heavy rainfall will affect portions of Hispaniola, Jamaica, the Cayman Islands, and western Cuba during the next few days and could lead to life-threatening flash floods and mudslides. 4. The system is forecast to approach the northern Gulf Coast late this week as a hurricane. While there is large uncertainty in the track and intensity forecasts at these time ranges, there is a risk of dangerous storm surge, wind, and rainfall hazards along the coast from Louisiana to the western Florida Panhandle. Residents in these areas should monitor the progress of the system and check for updates to the forecast during the week. FORECAST POSITIONS AND MAX WINDS INIT 04/2100Z 16.7N 76.6W 30 KT 35 MPH...POTENTIAL TROP CYCLONE 12H 05/0600Z 17.1N 77.7W 30 KT 35 MPH...TROPICAL DEPRESSION 24H 05/1800Z 18.0N 79.1W 35 KT 40 MPH 36H 06/0600Z 19.0N 80.5W 45 KT 50 MPH 48H 06/1800Z 21.0N 82.8W 60 KT 70 MPH 60H 07/0600Z 23.1N 85.4W 70 KT 80 MPH 72H 07/1800Z 24.7N 87.9W 80 KT 90 MPH 96H 08/1800Z 26.5N 90.3W 85 KT 100 MPH 120H 09/1800Z 28.6N 90.3W 85 KT 100 MPH $$ Forecaster Brown
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Tropical Storm Gamma Forecast Discussion Number 10
2020-10-04 22:58:18| National Hurricane Center (Atlantic)
Issued at 400 PM CDT Sun Oct 04 2020 000 WTNT45 KNHC 042058 TCDAT5 Tropical Storm Gamma Discussion Number 10 NWS National Hurricane Center Miami FL AL252020 400 PM CDT Sun Oct 04 2020 The last aircraft reconnaissance fix, recorded a couple of hours ago, indicated that Gamma's center had wobbled a little to the right of the previous 6-hour motion and is located near the southeastern edge of the convective canopy. Flight-level winds adjusted to the surface, SFMR wind data, and a blend of Dvorak satellite intensity estimates from TAFB and SAB still support an initial intensity of 50 kt for this advisory. The UW-CIMSS shear analysis reveals stiff 20-25 kt southerly shear impinging on the south portion of the cyclone. Subsequently, the large-scale models and the ECMWF/GFS statistical-dynamical intensity guidance show the shear magnitude increasing during the next 24 hours. This inhibiting upper wind pattern should induce weakening soon. Although the shear is predicted to decrease somewhat around mid-period, Gamma should exhibit little change in strength through the remaining period which is in best agreement with all of the skilled intensity guidance. As mentioned in previous advisories, it cannot be stressed enough the relatively high level of uncertainty in this intensity forecast, and that it's primarily dependent on whether or not the cyclone moves back over the Yucatan Peninsula. Gamma should continue to move little, or meander over the southern Gulf of Mexico, near the northern coast of the Yucatan Peninsula through most of tomorrow. By Monday night, the global models are in good agreement with the western extent of a subtropical ridge reestablishing temporarily over the central Gulf of Mexico. This change in the synoptic steering pattern should cause the cyclone to turn and move west-southwestward through the 72 hour forecast period. After a brief reduction in forward speed late Wednesday, while Potential-Tropical Cyclone Twenty-Six moves into the central Gulf of Mexico, a turn toward the northwest is expected on Thursday as the ridge, once again, builds back over the central Gulf in response to PTC Twenty-Six approaching the northern Gulf coast. The official track forecast is adjusted to the left of the previous one through the 60 hour period to agree more with the HCCA consensus, and shifted to the right through day 5. KEY MESSAGES: 1. Gamma is expected to produce heavy rainfall for several days over portions of southeastern Mexico, the Yucatan Peninsula, Central America, and far western Cuba. This rainfall could result in life-threatening flash flooding and mudslides, particularly in the mountainous regions of southeastern Mexico and Central America. 2. Even though Gamma is just offshore, tropical storm conditions will continue across portions of the northern coast of the Yucatan Peninsula today. FORECAST POSITIONS AND MAX WINDS INIT 04/2100Z 22.4N 87.8W 50 KT 60 MPH 12H 05/0600Z 22.3N 88.1W 45 KT 50 MPH 24H 05/1800Z 22.1N 88.6W 40 KT 45 MPH 36H 06/0600Z 21.7N 89.4W 40 KT 45 MPH 48H 06/1800Z 21.3N 90.4W 40 KT 45 MPH 60H 07/0600Z 20.8N 91.1W 40 KT 45 MPH 72H 07/1800Z 20.1N 91.4W 40 KT 45 MPH 96H 08/1800Z 21.0N 92.1W 40 KT 45 MPH 120H 09/1800Z 22.2N 92.7W 40 KT 45 MPH $$ Forecaster Roberts
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Hurricane Marie Forecast Discussion Number 23
2020-10-04 22:32:49| National Hurricane Center (East Pacific)
Issued at 200 PM PDT Sun Oct 04 2020 000 WTPZ43 KNHC 042032 TCDEP3 Hurricane Marie Discussion Number 23 NWS National Hurricane Center Miami FL EP182020 200 PM PDT Sun Oct 04 2020 The center of Marie is now totally exposed, well to the south and southwest of a few remaining areas of deep convection. While Marie's convective pattern resembles a low-end tropical storm, it takes time for systems of Marie's magnitude to spin down, and the initial intensity of 65 kt is based on a blend of Final-T and CI Dvorak numbers from TAFB. A pair of ASCAT passes near 18Z explicitly showed winds near 55 kt, but it likely under sampled the peak winds of the hurricane's tight circulation. The ASCAT also showed that Marie's tropical-storm-force and 50-kt wind radii have not yet contracted very much. The position of Marie was adjusted a little to the west, and the track forecast has been adjusted in that direction, but is otherwise very similar to the previous advisory for the first 72 h. After that time, the most recent guidance suggests that Marie, then expected to be a remnant low, could turn due west, or even south of due west. This is a pretty big departure from previous solutions, so the NHC forecast has been adjusted in that direction, but not nearly as much as the model consensus. It should be noted that regardless of position, Marie is forecast to be a weak remnant low at 96 and 120 h. High shear, cool SSTs, and stable surrounding air should cause Marie to continue to quickly weaken for the next few days. Simulated satellite imagery suggests that the system could produce intermittent convection for up to 60 more hours. It isn't clear how well organized it will be, and given recent trends it is possible Marie will become a remnant low much sooner than currently forecast. The system will likely dissipate sometime near or just after 120 h. FORECAST POSITIONS AND MAX WINDS INIT 04/2100Z 20.3N 129.5W 65 KT 75 MPH 12H 05/0600Z 20.7N 130.4W 55 KT 65 MPH 24H 05/1800Z 21.4N 131.9W 45 KT 50 MPH 36H 06/0600Z 22.1N 133.4W 40 KT 45 MPH 48H 06/1800Z 22.7N 134.8W 35 KT 40 MPH 60H 07/0600Z 23.3N 135.8W 30 KT 35 MPH 72H 07/1800Z 23.7N 136.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 08/1800Z 24.0N 138.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 09/1800Z 24.0N 140.0W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Zelinsky
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Hurricane Marie Forecast Discussion Number 22
2020-10-04 16:45:13| National Hurricane Center (East Pacific)
Issued at 800 AM PDT Sun Oct 04 2020 000 WTPZ43 KNHC 041445 TCDEP3 Hurricane Marie Discussion Number 22 NWS National Hurricane Center Miami FL EP182020 800 AM PDT Sun Oct 04 2020 Conventional satellite imagery reveals an increasingly apparent asymmetric/vertical tilt due to persistent west-southwesterly shear. The surface center is beginning to become exposed and is now near the western edge of the deep convective mass. The initial intensity is lowered to 80 kt for this advisory and is based on a compromise of the available subjective and objective satellite intensity estimates. Weakening should continue rather quickly through the forecast period while the shear magnitude increases with time and the cyclone traverses cooler oceanic surface temperatures. Marie should degenerate to a post-tropical remnant low in 3 days, or less, and this is in agreement with the majority of the global/regional and statistical guidance. The intensity forecast is near the IVCN intensity consensus and above the ECMWF and GFS Decay SHIPS which actually show dissipation in less than 4 days. Maria's initial motion is estimated to be northwestward, or 315/6 kt, within the mid-tropospheric steering flow provided by a subtropical ridge anchored to the north of the cyclone. Marie should continue moving in either a west-northwestward or northwestward motion through the remainder of forecast period with some reduction in forward speed, day 3 and beyond. The NHC forecast track has been adjusted a little bit south of the previous advisory after the 24 hour period in order to be closer to the various multi-model consensus aids and to conform more with a motion typical of a shallower, vertically limited system moving through the trade wind flow. FORECAST POSITIONS AND MAX WINDS INIT 04/1500Z 20.3N 128.6W 80 KT 90 MPH 12H 05/0000Z 20.7N 129.5W 65 KT 75 MPH 24H 05/1200Z 21.3N 131.0W 55 KT 65 MPH 36H 06/0000Z 22.0N 132.3W 45 KT 50 MPH 48H 06/1200Z 22.7N 133.9W 35 KT 40 MPH 60H 07/0000Z 23.4N 135.2W 30 KT 35 MPH 72H 07/1200Z 24.0N 136.0W 30 KT 35 MPH...POST-TROP/REMNT LOW 96H 08/1200Z 24.8N 136.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 09/1200Z 25.3N 137.9W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Roberts
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Tropical Storm Gamma Forecast Discussion Number 9
2020-10-04 16:43:47| National Hurricane Center (Atlantic)
Issued at 1000 AM CDT Sun Oct 04 2020 000 WTNT45 KNHC 041443 TCDAT5 Tropical Storm Gamma Discussion Number 9 NWS National Hurricane Center Miami FL AL252020 1000 AM CDT Sun Oct 04 2020 An Air Force Reserve reconnaissance aircraft has found the center of Gamma over the far southern Gulf of Mexico, just north of the Yucatan Peninsula. Early morning visible satellite imagery has revealed decent banding structure over the northern and western portions of the circulation. The aircraft has measured a peak 850 mb flight-level wind of 66 kt and SFMR winds of 47 kt. A blend of these data support an initial intensity of 50 kt, which is also in line with the latest Dvorak classifications of T3.0 and T3.5 from SAB and TAFB, respectively. Gamma appears to be slowing down as anticipated, with an initial motion estimate of 360/3 kt. Gamma is expected to meander over the southern Gulf of Mexico, just north of the northern coast of the Yucatan peninsula during the next 12-24 hours. By late Monday, the western portion of a subtropical ridge is expected to build westward over the northern Gulf of Mexico. This should cause Gamma to turn westward to west-southwestward through the remainder of the forecast period. The dynamical model guidance is in general agreement on this scenario, but there are some differences in how close Gamma tracks to the northwestern portion of the Yucatan peninsula. The NHC track splits these differences and is a blend TCVA multi-model consensus and the HFIP corrected consensus. The new track forecast is very close to the previous official forecast. Although Gamma has re-strengthened this morning, increasing southerly wind shear is expected over the cyclone by tonight. This unfavorable upper-level environment is likely to cause some gradual weakening by late tonight and Monday. Since the vertical shear expected to remain moderate to strong through the latter portion of the forecast period, little overall change in strength is shown after 48 hours. KEY MESSAGES: 1. Gamma is expected to produce heavy rainfall for several days over portions of southeastern Mexico, the Yucatan Peninsula, Central America, and far western Cuba. This rainfall could result in life-threatening flash flooding and mudslides, particularly in the mountainous regions of southeastern Mexico and Central America. 2. Even though Gamma is offshore, tropical storm conditions will continue across portions of the northern coast of the Yucatan Peninsula today. FORECAST POSITIONS AND MAX WINDS INIT 04/1500Z 22.2N 88.2W 50 KT 60 MPH 12H 05/0000Z 22.6N 88.2W 50 KT 60 MPH 24H 05/1200Z 22.6N 88.5W 45 KT 50 MPH 36H 06/0000Z 22.3N 89.3W 40 KT 45 MPH 48H 06/1200Z 22.0N 90.2W 40 KT 45 MPH 60H 07/0000Z 21.5N 91.0W 40 KT 45 MPH 72H 07/1200Z 21.0N 91.6W 40 KT 45 MPH 96H 08/1200Z 20.4N 92.5W 40 KT 45 MPH 120H 09/1200Z 20.4N 93.0W 40 KT 45 MPH $$ Forecaster Brown
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