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Tropical Storm Guillermo Discussion Number   6
NWS National Hurricane Center Miami FL       EP072021
300 PM MDT Sun Jul 18 2021
 
Guillermo's convective pattern has not changed significantly since
early this morning, but an ASCAT overpass just after 1600 UTC
revealed that its wind field has expanded. The maximum winds in the 
ASCAT data were a little above 45 kt, so the initial intensity has 
been increased to 50 kt accordingly. This is just above the latest 
Dvorak estimates from TAFB and SAB.
 
A slight additional increase in the maximum winds appears likely
during the next day or so, owing to favorable environmental
conditions. However, the highly asymmetric structure of Guillermo
and its relatively broad inner-core will continue to be limiting
factors. In about 24 to 36 h, Guillermo will begin to move over
waters colder than 26 deg C, and this should cause the tropical
storm to gradually weaken. After about 72 h, a drier and more stable
environment will likely also contribute to weakening. Despite those
factors, the GFS and HWRF models continue to indicate that Guillermo
could maintain at least some deep convection through the full 5-day
period. The NHC forecast therefore carries Guillermo as a tropical
cyclone through day 5, but it would not be surprising if it became a
remnant low sooner than that. The official intensity forecast is
slightly higher than the intensity consensus, near the SHIPS and GFS
forecasts.
 
The aforementioned ASCAT data indicated that Guillermo is centered 
slightly north of previous estimates and the estimated initial 
motion is slightly faster than the previous advisory, at 290/ 12 kt. 
The NHC track forecast has been adjusted northward and faster as a 
result, but the overall reasoning remains the same. A mid-level 
ridge to the north of Guillermo will likely steer the tropical storm 
westward beginning tonight. Guillermo should then continue generally 
westward for the next several days. A slight west-southwestward 
bend is possible late in the forecast period as the cyclone weakens 
and becomes primarily steered by low-level flow. The official track 
forecast is heavily based on the multi-model consensus.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  18/2100Z 18.7N 113.8W   50 KT  60 MPH
 12H  19/0600Z 19.0N 115.7W   55 KT  65 MPH
 24H  19/1800Z 19.1N 118.4W   55 KT  65 MPH
 36H  20/0600Z 19.0N 121.3W   55 KT  65 MPH
 48H  20/1800Z 19.0N 124.2W   50 KT  60 MPH
 60H  21/0600Z 18.9N 127.1W   45 KT  50 MPH
 72H  21/1800Z 18.6N 129.8W   40 KT  45 MPH
 96H  22/1800Z 18.2N 135.0W   35 KT  40 MPH
120H  23/1800Z 18.0N 140.0W   30 KT  35 MPH
 
$$
Forecaster Zelinsky
 
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