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Tropical Storm Zeta Forecast Discussion Number 3
2020-10-25 10:01:08| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Sun Oct 25 2020 000 WTNT43 KNHC 250900 TCDAT3 Tropical Storm Zeta Discussion Number 3 NWS National Hurricane Center Miami FL AL282020 500 AM EDT Sun Oct 25 2020 Earlier this morning, the tropical cyclone located over the northwestern Caribbean Sea was upgraded to Tropical Storm Zeta at 0600 UTC based on 0314Z ASCAT surface wind data of 32-33 kt located southeast of the broad center. Since then, a NOAA reconnaissance aircraft has been investigating Zeta and has found maximum 925-mb flight-level winds of about 40 kt and reliable SFMR surface winds of 33-36 kt. Recent satellite classifications from TAFB, SAB, and UW-CIMSS are 35 kt. Based on these data, the initial intensity is set at 35 kt. The initial motion estimate is stationary. Although Zeta has been steadily losing latitude during the past 12 hours, this has been primarily due to the broad low-level center reforming closer to the very intense convection located in the southern semicircle of the cyclone's large circulation. Satellite trends over the past 6 hours indicate that a mid-level circulation located about 90 nmi east of the low-level center is likely imparting a weak southerly component of motion on Zeta as well. Over the course of the next 72 hours, a weak shortwave trough currently located over the southeastern U.S. from the Tennessee Valley southward into the north-central Gulf of Mexico will continue to move eastward, allowing a low- to mid-level ridge to steadily build westward across the Bahamas, Florida, and the central Gulf of Mexico. This will gradually force Zeta on a northwestward track across or near the northeastern tip of the Yucatan Peninsula in about 48 h and into the central Gulf of Mexico by 72 h. The latest NHC model guidance is in very agreement on this developing 3-day track scenario. Thereafter, however, the models diverge fairly significantly due to uncertainty in the evolution of the next steering mechanism -- a trough over the northwestern U.S. that will dig southward and phase up/merge with a trough off the southern California and Baja California coasts. Phasing of northern and southern stream systems is always difficult to forecast, especially when one of those systems is outside the U.S. upper-air observing system like the system currently is off southern California. After the two systems phase in about 48 hours, a powerful mid/upper-level low is forecast to form by 72 h and eject eastward out of the southwestern U.S., causing the ridge over the Gulf of Mexico to erode eastward. This will allow Zeta to turn northward and then northeastward toward the north-central Gulf coast. The global models remain in poor agreement on the details of the timing of the eastward movement of the upper-low, resulting in model solutions ranging from Louisiana (ECMWF-UKMET-FSSE) to the Florida Panhandle (HWRF-HMON). The new NHC track forecast is a little left of the previous advisory track through 72 hours, and lies near the middle of the guidance envelope, close to the HCCA and TVCA multi-model consensus. However, given the current lack of motion of the system and the large model spread late in the period, the details of the track forecast are more uncertain than usual. Although the low- and mid-level circulations remain unaligned, the overall environment for the next 60-72 hours is expected to remain conducive for at least gradual intensification. After 72 hours, however, the deep-layer vertical wind shear is expected to increase while Zeta moves over cooler SSTs near the northern Gulf Coast, which should result in weakening before the cyclone moves inland. The new NHC intensity forecast is a tad higher than the previous advisory, and lies near the upper end of the intensity guidance. While the current NHC forecast indicates that the system should weaken below hurricane strength before landfall, users are reminded that strong tropical storms can still produce significant storm surge, rainfall, and wind impacts along the northern Gulf Coast. The cyclone should be absorbed into a frontal system by the end of the forecast period. KEY MESSAGES: 1. Zeta is expected to produce tropical storm conditions over extreme western Cuba on Monday. Tropical storm conditions are possible in the northern Yucatan Peninsula of Mexico Monday night and early Tuesday. 2. Through Wednesday, heavy rainfall is expected from Tropical Storm Zeta across portions of central and western Cuba, the Cayman Islands, Jamaica, the northeast Yucatan peninsula of Mexico, Southern Florida and the Keys. This rainfall may lead to flash flooding in urban areas. 3. Zeta is forecast to approach the northern Gulf Coast as a tropical storm Tuesday night and Wednesday, and could bring storm surge, rainfall, and wind impacts to areas from Louisiana to the Florida Panhandle. Residents in these areas should monitor the progress of Zeta and updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 25/0900Z 17.7N 83.5W 35 KT 40 MPH 12H 25/1800Z 18.2N 83.8W 40 KT 45 MPH 24H 26/0600Z 18.9N 84.4W 45 KT 50 MPH 36H 26/1800Z 19.8N 85.7W 55 KT 65 MPH 48H 27/0600Z 21.1N 87.4W 65 KT 75 MPH 60H 27/1800Z 22.9N 89.5W 70 KT 80 MPH 72H 28/0600Z 25.4N 90.7W 65 KT 75 MPH 96H 29/0600Z 32.0N 89.1W 35 KT 40 MPH...INLAND 120H 30/0600Z...DISSIPATED $$ Forecaster Stewart
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