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Tropical Storm Guillermo Discussion Number   3
NWS National Hurricane Center Miami FL       EP072021
900 PM MDT Sat Jul 17 2021
 
The overall structure of Guillermo has changed little since this 
afternoon, with the only notable difference being a small burst of 
deep convection just to the southwest of the center. Recent 
satellite imagery suggests that the system is tilted, with the 
mid-level circulation offset a bit to the northeast from the 
low-level center. Guillermo is a large tropical cyclone, and the 
sprawling circulation of the storm is producing broken banding over 
an area over 400 n mi wide. The latest T-numbers from TAFB and the 
UW-CIMSS ADT have also changed little since 6 h ago, which suggest 
that the Guillermo's initial intensity remains 35 kt.
 
A fortuitous AMSR2 microwave overpass helped to confirm the
location of Guillermo's low-level center a few hours ago, and the
initial motion estimate is 290/11 kt. The cyclone is expected to 
turn to the west on Sunday as it becomes embedded in easterly flow 
to the south of a mid-level ridge. This ridge should remain in 
place through the early part of next week. By 72 h, the guidance is 
suggesting that Guillermo will become a shallow cyclone, and a 
large low-level ridge to its northwest will become the primary 
steering mechanism, resulting a slightly south-of-west motion. The 
official NHC forecast is near the previous one through 60 h, but is 
a little faster and slightly to the south of it thereafter. 

Due to the large size of Guillermo's circulation, it may take 
longer than normal for the cyclone to develop a tight wind field, 
despite being in an environment of low vertical wind shear and over 
warm waters. Therefore, only gradual strengthening is indicated 
over the next 36 h. After 36 h, the forecast track of the cyclone 
takes it over cooler waters with SSTs less than 25C by 60 h. There 
are also indications that Guillermo will encounter moderate 
northwesterly shear in a few days, which would entrain stable air 
located to its north into the circulation. These factors should 
cause steady weakening beginning in a couple of days. And although 
not shown in the official forecast, several of the convection 
allowing models show the cyclone losing most, if not all, of its 
deep convection and becoming a remnant low by the end of the 
forecast period. Since Guillermo's forecast track keeps it fairly 
close to a tight SST gradient, with warmer waters just south, it is 
possible that a small deviation of the track to the south could 
prevent the cyclone from becoming a remnant low so soon. The 
latest NHC intensity forecast is slightly lower than the previous 
one due to a decrease in the overall intensity guidance, yet remains 
on the high end of the guidance through 36 h.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  18/0300Z 17.5N 111.1W   35 KT  40 MPH
 12H  18/1200Z 17.9N 112.6W   40 KT  45 MPH
 24H  19/0000Z 18.3N 114.8W   50 KT  60 MPH
 36H  19/1200Z 18.5N 117.4W   50 KT  60 MPH
 48H  20/0000Z 18.7N 120.1W   45 KT  50 MPH
 60H  20/1200Z 18.6N 122.9W   40 KT  45 MPH
 72H  21/0000Z 18.4N 125.5W   35 KT  40 MPH
 96H  22/0000Z 17.9N 130.7W   30 KT  35 MPH
120H  23/0000Z 17.5N 136.0W   30 KT  35 MPH
 
$$
Forecaster Latto
 
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