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Tropical Storm Guillermo Discussion Number 2
NWS National Hurricane Center Miami FL EP072021
300 PM MDT Sat Jul 17 2021
Fragmented convective banding has gradually increased since the
previous advisory, with a very tight low-level vortex having become
evident over the center in high-resolution visible satellite
imagery. Low-cloud motion vectors in the inner-core region have
been around 45 kt, which equals approximately 34-kt surface winds if
the cloud motions represent the 925-mb level. Satellite current
intensity (CI) estimates remain T2.5/35 kt from TAFB and and SAB.
The latest UW-CIMSS ADT estimate is T2.2/32 kt, with the raw and
adjusted T-numbers at 37 kt and 36 kt, respectively. Based on these
data, the intensity has been increased to 35 kt, making the cyclone
Tropical Storm Guillermo.
The initial motion estimate is 295/12 kt. There are no significant
changes to the previous track forecast or reasoning. The global and
regional models remain in very good agreement on Guillermo moving
west-northwestward today, and then turning toward the west later
tonight or early Sunday, with a general westward motion continuing
through day 3. By days 4 and 5, the deep-layer ridge to the north of
Guillermo is forecast by the models to gradually build southward,
forcing the tropical storm west-southwestward. The latest NHC model
guidance has converged tightly around the previous forecast track
through 96 h, so no adjustments were made. The new official forecast
track lies close to the tightly packed consensus models, which are
about midway between the stronger GFS track forecast on the right
side of the guidance envelope and the weaker ECMWF solution on the
left or south side.
Guillermo is expected to remain within a low vertical wind shear
regime and a moist mid-level environment, and over sea-surface
temperatures (SST) of at least 27C for the next 48 hours or so.
These favorable conditions should allow for at least modest
strengthening to occur during that time, with the only hindering
factor being Guillermo's large circulation. By 60 hours and beyond,
the cyclone will be moving over sub-26C SSTs, which should induce
gradual weakening despite the low vertical wind shear pattern. The
new NHC intensity forecast is similar to the previous advisory, and
closely follows the NOAA-HCCA corrected consensus model, which is
above most of the available intensity guidance.
FORECAST POSITIONS AND MAX WINDS
INIT 17/2100Z 17.1N 110.1W 35 KT 40 MPH
12H 18/0600Z 17.6N 111.7W 45 KT 50 MPH
24H 18/1800Z 18.0N 113.9W 50 KT 60 MPH
36H 19/0600Z 18.3N 116.2W 55 KT 65 MPH
48H 19/1800Z 18.4N 118.7W 55 KT 65 MPH
60H 20/0600Z 18.5N 121.2W 50 KT 60 MPH
72H 20/1800Z 18.4N 123.8W 45 KT 50 MPH
96H 21/1800Z 18.1N 128.9W 35 KT 40 MPH
120H 22/1800Z 17.8N 134.1W 30 KT 35 MPH
$$
Forecaster Stewart
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