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Tropical Storm Gamma Forecast Discussion Number 12
2020-10-05 10:50:29| National Hurricane Center (Atlantic)
Issued at 400 AM CDT Mon Oct 05 2020 000 WTNT45 KNHC 050850 TCDAT5 Tropical Storm Gamma Discussion Number 12 NWS National Hurricane Center Miami FL AL252020 400 AM CDT Mon Oct 05 2020 Gamma is a strongly sheared tropical storm that has been meandering just north of the northern coast of the Yucatan Peninsula of Mexico. Strong convective cloud tops colder than -80 deg C are located more than 60 nmi north and northeast of the well-defined and fully exposed low-level circulation center. An Air Force Reserve reconnaissance aircraft investigating the cyclone found maximum 850-mb flight-level winds of only 44 kt along with peak SFMR surface winds of 47 kt. In addition, a 0329Z ASCAT-A pass revealed peak surface winds of 41 kt. Allowing for undersampling by the scatterometer instrument and blending it with the SFMR winds supports lowering the initial intensity to 45 kt. The central pressure of 999 mb is based on a center dropsonde reports of 1000 mb and 10-12 kt winds. The initial motion estimate is an uncertain 240/02 kt. Gamma's future track remains highly uncertain after day 2. For the next 48 hours or so, the cyclone is forecast to move slowly southwestward to west-southwestward and weaken due to strong southerly vertical wind shear. Thereafter, TD-26 to the east is forecast to strengthen into a hurricane, accompanied by a corresponding increase in the size of that cyclone's circulation, which is expected to result in some binary interaction between the two tropical cyclones. Due to Gamma's expected smaller size on days 3 and 4, the cyclone is forecast to be pulled eastward and then northward, making landfall on and then moving over the northwestern Yucatan Peninsula. By day 5, Gamma is expected to degenerate into a remnant low and then be absorbed by the larger circulation of the expected central Gulf of Mexico hurricane that is currently TD-26. The new NHC track forecast is similar to but a little slower than the previous advisory track, and lies close to the southern edge of the guidance envelope. Strong southerly wind shear, combined with drier and more stable air just to the west of Gamma, is expected to weaken the cyclone further over the next few days. If land interaction occurs sooner than expected, then the cyclone could weaken and dissipate sooner than currently forecast. For now, the intensity forecast maintains continuity with the previous advisory and shows dissipation by 120 hours as Gamma interacts with what is expected to be a significant hurricane over the central Gulf of Mexico by 96 hours, which could end up absorbing the smaller Gamma tropical cyclone. Due to the uncertainty in the forecast at this time, future advisories may feature larger-than-normal changes to the track or intensity forecast. It is also worth noting that several dynamical model trackers appear to lose Gamma and jump to tracking Tropical Depression Twenty-Six when it reaches the Gulf of Mexico, resulting in unrealistic depictions of the model intensity and track forecasts. KEY MESSAGES: 1. Gamma is expected to produce heavy rainfall through midweek over portions of southeast Mexico, including the Mexican states of Yucatan, Campeche, and Tabasco. This rainfall could result in significant flash flooding. 2. Even though Gamma is just offshore, tropical storm conditions are likely along portions of the northern coast of the Yucatan Peninsula later today. FORECAST POSITIONS AND MAX WINDS INIT 05/0900Z 22.7N 87.5W 45 KT 50 MPH 12H 05/1800Z 22.5N 88.0W 40 KT 45 MPH 24H 06/0600Z 22.1N 88.9W 40 KT 45 MPH 36H 06/1800Z 21.4N 89.8W 35 KT 40 MPH 48H 07/0600Z 20.8N 90.4W 35 KT 40 MPH 60H 07/1800Z 20.3N 90.5W 30 KT 35 MPH...INLAND YUCATAN 72H 08/0600Z 20.7N 90.1W 30 KT 35 MPH...INLAND YUCATAN 96H 09/0600Z 22.5N 90.0W 30 KT 35 MPH...OVER WATER 120H 10/0600Z...DISSIPATED $$ Forecaster Stewart
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Tropical Storm Gamma Graphics
2020-10-05 07:48:01| National Hurricane Center (Atlantic)
5-Day Uncertainty Track last updated Mon, 05 Oct 2020 05:48:01 GMT Wind Speed Probabilities last updated Mon, 05 Oct 2020 03:25:26 GMT
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Summary for Tropical Storm Gamma (AT5/AL252020)
2020-10-05 07:46:23| National Hurricane Center (Atlantic)
...RECONNAISSANCE AIRCRAFT FINDS GAMMA A LITTLE WEAKER WHILE IT CONTINUES TO MEANDER JUST NORTH OF THE YUCATAN PENINSULA... As of 1:00 AM CDT Mon Oct 5 the center of Gamma was located near 22.8, -87.4 with movement W at 2 mph. The minimum central pressure was 999 mb with maximum sustained winds of about 50 mph.
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Tropical Storm Gamma Public Advisory Number 11A
2020-10-05 07:46:23| National Hurricane Center (Atlantic)
Issued at 100 AM CDT Mon Oct 05 2020 000 WTNT35 KNHC 050546 TCPAT5 BULLETIN Tropical Storm Gamma Intermediate Advisory Number 11A NWS National Hurricane Center Miami FL AL252020 100 AM CDT Mon Oct 05 2020 ...RECONNAISSANCE AIRCRAFT FINDS GAMMA A LITTLE WEAKER WHILE IT CONTINUES TO MEANDER JUST NORTH OF THE YUCATAN PENINSULA... SUMMARY OF 100 AM CDT...0600 UTC...INFORMATION ---------------------------------------------- LOCATION...22.8N 87.4W ABOUT 180 MI...290 KM ENE OF PROGRESO MEXICO ABOUT 165 MI...265 KM NNW OF COZUMEL MEXICO MAXIMUM SUSTAINED WINDS...50 MPH...85 KM/H PRESENT MOVEMENT...W OR 270 DEGREES AT 2 MPH...4 KM/H MINIMUM CENTRAL PRESSURE...999 MB...29.50 INCHES WATCHES AND WARNINGS -------------------- CHANGES WITH THIS ADVISORY: None. SUMMARY OF WATCHES AND WARNINGS IN EFFECT: A Tropical Storm Warning is in effect for... * North and west of Cancun to Dzilam Mexico A Tropical Storm Watch is in effect for... * West of Dzilam to Campeche Mexico A Tropical Storm Warning means that tropical storm conditions are occurring somewhere within the warning area. A Tropical Storm Watch means that tropical storm conditions are possible within the watch area. For storm information specific to your area, please monitor products issued by your national meteorological service. DISCUSSION AND OUTLOOK ---------------------- At 100 AM CDT (0600 UTC), data from an Air Force Reserve Hurricane Hunter aircraft indicated that the center of Tropical Storm Gamma was located near latitude 22.8 North, longitude 87.4 West. The tropical storm is drifting toward the west near 2 mph (4 km/h). Gamma is forecast to gradually turn toward the southwest or west-southwest later today, and continue to move slowly in that direction through mid-week. On the forecast track, the center of Gamma will continue to meander offshore of the northern Yucatan Peninsula this morning, and pass near or just offshore of the northern coast of the Yucatan Peninsula late today and on Tuesday. Data from the reconnaissance aircraft indicate that maximum sustained winds have decreased to near 50 mph (85 km/h) with higher gusts. Further weakening is forecast during the next 48 hours. Afterward, little change in strength is forecast. Tropical-storm-force winds extend outward up to 160 miles (260 km) from the center. The estimated minimum central pressure based on measurements from the hurricane hunter plane is 999 mb (29.50 inches). HAZARDS AFFECTING LAND ---------------------- Key messages for Tropical Storm Gamma can be found in the Tropical Cyclone Discussion under AWIPS header MIATCDAT5, WMO header WTNT45 KNHC, and on the web at www.hurricanes.gov/text/MIATCDAT5.shtml. RAINFALL: Gamma is expected to produce an additional 3 to 6 inches of rainfall, with isolated maximum amounts of 8 inches, across portions of the Mexican states of Yucatan, Campeche and Tabasco. This rainfall may enhance any ongoing flash flooding and result in new areas of flash flooding into the middle of the week. WIND: Tropical storm conditions are likely in portions of the Tropical Storm Warning area along the northern coast of the Yucatan Peninsula today. Tropical Storm conditions are possible within the Tropical Storm Watch area late today and on Tuesday. NEXT ADVISORY ------------- Next complete advisory at 400 AM CDT. $$ Forecaster Stewart
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Tropical Storm Gamma Forecast Discussion Number 11
2020-10-05 04:39:22| National Hurricane Center (Atlantic)
Issued at 1000 PM CDT Sun Oct 04 2020 000 WTNT45 KNHC 050239 TCDAT5 Tropical Storm Gamma Discussion Number 11 NWS National Hurricane Center Miami FL AL252020 1000 PM CDT Sun Oct 04 2020 Gamma has moved much farther northeast than previously anticipated. With little in the way of low or mid-level large-scale steering flow currently in place, it seems likely that upper-level southwesterlies, also responsible for the shear affecting the tropical storm, are causing this recent northeastward motion. This may be either through reformations of the center closer to the convection or a direct contribution to the net steering. An Air Force Reserve Hurricane Hunter aircraft is currently investigating Gamma and so far has found max SFMR winds of 47 kt but flight level winds of only 45 kt. Given the data so far, there is no indication that Gamma is any stronger than the 50 kt intensity, but it seems prudent to let the plane finish its pattern before lowering the winds at this time. Gamma's future track is highly uncertain. The track guidance spread is much higher than usual, and confidence in the forecast is accordingly low. Most of the global models indicate that Gamma will soon begin moving generally west-southwestward as a mid-level ridge briefly builds over the eastern and central Gulf of Mexico. Unfortunately, there is little agreement on exactly when this will happen or how fast Gamma will move once it turns. After about 48 h, Gamma could interact with Tropical Depression Twenty-Six, which is forecast to move over the northwestern Caribbean Sea at that time. Current dynamical models are notoriously bad at forecasting such interactions, but if the two systems do interact it will likely cause Gamma to move inland over the Yucatan. Whether such an interaction occurs or not, continued shear and proximity to land should cause the tropical storm to gradually weaken, and Gamma could dissipate if it moves over land (and stays there) in a few days, as shown by the GFS. The ECMWF shows the vortex remaining over water but dissipating nonetheless, while a couple other global models maintain the vortex longer and show it moving north over the central Gulf of Mexico late in the forecast period. The NHC forecast is a compromise between those solutions, showing a remnant low moving north over the southern Gulf of Mexico by day 5. In general, the NHC intensity forecast is similar to the previous one, but slightly lower to account for the system potentially moving inland. Due to the uncertainty in the forecast at this time, future advisories may feature larger than normal changes to the track or intensity forecast. It is also worth noting that several dynamical model trackers appear to lose Gamma and jump to tracking Tropical Depression Twenty-Six when it reaches the Gulf of Mexico, resulting in unrealistic depictions of the intensity and track forecasts. 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 are likely along portions of the northern coast of the Yucatan Peninsula on Monday. FORECAST POSITIONS AND MAX WINDS INIT 05/0300Z 22.8N 87.2W 50 KT 60 MPH 12H 05/1200Z 22.7N 87.7W 45 KT 50 MPH 24H 06/0000Z 22.3N 88.5W 45 KT 50 MPH 36H 06/1200Z 21.8N 89.5W 40 KT 45 MPH 48H 07/0000Z 21.3N 90.3W 40 KT 45 MPH 60H 07/1200Z 20.3N 90.7W 35 KT 40 MPH 72H 08/0000Z 20.2N 90.1W 30 KT 35 MPH...INLAND 96H 09/0000Z 22.0N 90.7W 30 KT 35 MPH...OVER WATER 120H 10/0000Z 24.0N 91.0W 30 KT 35 MPH...POST-TROP/REMNT LOW $$ Forecaster Zelinsky
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