Tropical Storm Gilma Forecast Discussion

000
WTPZ42 KNHC 200840
TCDEP2

Tropical Storm Gilma Discussion Number   8
NWS National Hurricane Center Miami FL       EP072024
200 AM PDT Tue Aug 20 2024

Gilma continues to struggle with moderate to strong northeasterly 
vertical wind shear. The center is exposed more than 30 n mi to the 
northeast of the main convective overcast area. A 20/0129 UTC SSMIS 
pass and a pair of more recent ASCAT passes were helpful in placing 
the center well outside of the convective area. The highest 
reliable ASCAT vectors were only about 40 kt. UW-CIMSS objective 
intensity estimates range from 34-42 kt. Subjective current
intensity estimates from TAFB and SAB were T-3.5/55 kt and T-3.0/45 
kt from SAB. Based on the degraded structure on satellite imagery, 
the ASCAT data and the objective intensity estimates from UW-CIMSS, 
the initial intensity is set to 45 kt for this advisory.

The aforementioned microwave and ASCAT data indicate that the 
center of the cyclone is a bit farther north than previously 
estimated. It appears that Gilma has slowed down, and the motion is 
estimated to be west-northwestward, or 295/7 kt. A mid-level ridge 
to the north of Gilma should steer the cyclone generally 
west-northwestward at a similar forward speed through the forecast 
period. Although the track guidance is gradually coming into better 
agreement, there is still more spread than normal among the various
track aids. The official track forecast is quite similar to the 
previous NHC track prediction. The NHC forecast track is closer to 
the ECMWF model than the GFS model through 72 h, and is close to an 
average of those two models beyond 72 h.

Moderate to strong northeasterly vertical wind shear will continue 
to affect Gilma for another 12 h or so. The SHIPS guidance indicates 
that the cyclone will reach a much lower wind shear regime by 24 h. 
Therefore, little change in strength is expected during the next 
12-24 h, following by steady intensification. The official intensity 
forecast was lowered in the short term due to the weaker initial 
intensity. However, the guidance has become more aggressive after 
the cyclone reaches the weaker shear, and the NHC forecast follows 
suit, now showing a peak of 85 kt in 60-72 hr. The new NHC intensity 
forecast is near the middle of the guidance envelope, in between the 
weaker statistical guidance and the stronger regional hurricane 
model dynamical guidance. SSTs along Gilma's path should decrease to 
about 26C in 72 h, and remain at that value through 96 h before 
dropping to below 26C. Weakening is forecast during that time 
period.


FORECAST POSITIONS AND MAX WINDS

INIT  20/0900Z 15.7N 118.3W   45 KT  50 MPH
 12H  20/1800Z 16.0N 119.8W   45 KT  50 MPH
 24H  21/0600Z 16.4N 121.5W   50 KT  60 MPH
 36H  21/1800Z 16.8N 123.0W   60 KT  70 MPH
 48H  22/0600Z 17.3N 124.2W   70 KT  80 MPH
 60H  22/1800Z 17.7N 125.3W   85 KT 100 MPH
 72H  23/0600Z 18.2N 126.4W   85 KT 100 MPH
 96H  24/0600Z 19.0N 128.6W   75 KT  85 MPH
120H  25/0600Z 20.0N 131.2W   60 KT  70 MPH

$$
Forecaster Hagen/Pasch