Tropical Storm Gilma Forecast Discussion

109 
WTPZ42 KNHC 190844
TCDEP2

Tropical Storm Gilma Discussion Number   4
NWS National Hurricane Center Miami FL       EP072024
200 AM MST Mon Aug 19 2024

A large convective burst formed during the evening and overnight 
hours with cloud tops as cold as -80C, and the center of Gilma 
seems to be underneath the eastern edge of the deep convection. 
This represents a change from 6 h ago, when the center had been a 
bit more exposed near the eastern edge of a weaker area of central 
convection.  The convective burst has weakened a bit during the last 
hour or so, and moderate northeasterly shear still appears to be 
affecting Gilma.  A 19/0412 UTC ASCAT-B pass missed the center of 
the cyclone, but it did measure winds up to 40 kt to the east of 
the center.  It is uncertain whether these vectors are located near 
or outside of Gilma's radius of maximum winds.  Subjective 
intensity estimates are T-3.0/45 kt from TAFB and T-2.0/30 kt from 
SAB.  The objective intensity estimates from CIMSS range from 35 to 
43 kt, about a 5 kt increase from 6 h ago.  Based on the ASCAT data 
and latest satellite intensity estimates, the initial intensity is 
raised to 45 kt for this advisory.

Moderate-to-strong northeasterly vertical wind shear is expected to 
persist for at least another 36 h, and only slow intensification is 
forecast in the short term.  After that time, the shear is forecast 
to relax some, and Gilma should be in a more conducive environment 
for strengthening by Tuesday night or Wednesday.  By the end of the 
forecast period, Gilma is expected to move into a drier and more 
stable environment, which should begin to induce weakening.  The 
latest intensity forecast is slightly higher than the previous 
official forecast in the short term due to the higher initial 
intensity, but after that is similar to the previous forecast.  The 
intensity forecast is slightly lower than the model consensus during 
the first 48 h and is near the consensus thereafter.

Gilma is still moving westward along the south side of a mid-level 
ridge at about 280/11 kt.  A similar motion should continue for the 
next 24 h followed by a gradual turn to the west-northwest.  By 
Wednesday, Gilma should slow down a bit and turn more northwestward 
when a trough off the west coast of the United States erodes the 
ridge.  The model spread is quite large through the forecast period, 
with the GFS being the largest outlier, showing a track well to the 
north-northeast of the rest of the guidance.  Part of the reason for 
this is how the GFS model handles the interaction with the 
aforementioned trough, but part of the reason may also be because 
the GFS model shows a different interaction with a tropical 
disturbance currently located several hundred miles to the west of 
Gilma, mentioned in the latest Tropical Weather Outlook.  The 
official track is quite similar to the previous one and lies just a 
touch south of the consensus aids beyond 48 h.


FORECAST POSITIONS AND MAX WINDS

INIT  19/0900Z 15.0N 114.6W   45 KT  50 MPH
 12H  19/1800Z 15.2N 116.3W   45 KT  50 MPH
 24H  20/0600Z 15.5N 118.2W   50 KT  60 MPH
 36H  20/1800Z 16.0N 119.8W   50 KT  60 MPH
 48H  21/0600Z 16.7N 121.5W   55 KT  65 MPH
 60H  21/1800Z 17.3N 122.7W   65 KT  75 MPH
 72H  22/0600Z 17.8N 123.9W   70 KT  80 MPH
 96H  23/0600Z 18.6N 125.9W   70 KT  80 MPH
120H  24/0600Z 19.5N 128.3W   60 KT  70 MPH

$$
Forecaster Hagen/Pasch