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Tropical Storm Zeta Forecast Discussion


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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