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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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