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Tropical Storm Guillermo Discussion Number   7
NWS National Hurricane Center Miami FL       EP072021
500 PM HST Sun Jul 18 2021
 
Guillermo's convective pattern has eroded significantly since the 
previous advisory due to northwesterly vertical wind shear and 
entrainment of drier and cooler air. Earlier ASCAT showed the system 
was a solid 50-kt tropical cyclone, thus, only slight weakening is 
indicated on this advisory to allow for some spin down of the 
vortex. Deep convection will likely return near the center later 
tonight during the nocturnal convective maximum period, especially 
since the cyclone is still going to be moving over sea-surface 
temperatures (SST) near 27 deg C, which should result in some slight 
re-strengthening or at least hold a steady intensity for the next 24 
h or so. By 36 h and beyond, however, Guillermo will be passing over 
sub-25C SSTs and into a stronger northwesterly wind shear 
environment, which should induce slow but steady weakening through 
the rest of the forecast period, with the cyclone becoming a 
post-tropical remnant low by 120 hours. The new NHC intensity 
forecast is similar to the previous advisory and closely follows the 
NOAA-HCCA and IVCN consensus models.
 
The initial motion estimate is now westward, or 280/13 kt. Guillermo 
made a due west lurch at about 18 kt after the central deep 
convection dissipated, which resulted in the vortex becoming a 
little more vertically shallow. However, the motion over the past 
few hours has been fairly steady at around 13 kt. The latest model 
guidance is tightly packed about the previous advisory track, so 
no significant changes were required since the global and regional 
models maintain the strong subtropical ridge to the north of 
Guillermo for the next 120 hours. The official forecast track 
closely follows a blend of the TVCE and GFEX simple consensus 
models, and the HCCA corrected-consensus model.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  19/0300Z 18.9N 115.6W   45 KT  50 MPH
 12H  19/1200Z 19.1N 117.6W   45 KT  50 MPH
 24H  20/0000Z 19.2N 120.4W   45 KT  50 MPH
 36H  20/1200Z 19.1N 123.2W   40 KT  45 MPH
 48H  21/0000Z 19.1N 126.1W   35 KT  40 MPH
 60H  21/1200Z 18.9N 128.9W   30 KT  35 MPH
 72H  22/0000Z 18.6N 131.6W   30 KT  35 MPH
 96H  23/0000Z 18.3N 136.7W   25 KT  30 MPH
120H  24/0000Z 18.3N 141.7W   20 KT  25 MPH...POST-TROP/REMNT LOW
 
$$
Forecaster Stewart
 
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