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Tropical Storm Hernan Discussion Number 3
NWS National Hurricane Center Miami FL EP132020
400 PM CDT Wed Aug 26 2020
Despite the rather poor presentation in satellite imagery, a recent
ASCAT overpass showed that tropical-storm-force winds partially
associated with a broader scale monsoon gyre are occurring over the
southern portion of the circulation about 50-90 n mi from the
center. Based on the wind data, the initial intensity has been
raised to 35 kt, and the depression has been upgraded to Tropical
Storm Hernan.
Hernan is a sheared tropical cyclone, with only a small area of deep
convection just to the south of the center. The SHIPS guidance
indicates that this shear will continue throughout the forecast
period. While the cyclone remains over very warm waters of about 29
degrees C for the next 24-36 h, there should be sufficient
convection to maintain Hernan's current intensity. However, after 36
h the cyclone will move over relatively cooler waters of about 26
C. These very marginal water temperatures combined with the ongoing
shear should cause Hernan to weaken, and the system is expected to
degenerate to a remnant low by 60 h, as indicated by the GFS
simulated satellite. The NHC intensity is a blend of the various
corrected consensus aids and the SHIPS intensity guidance.
Hernan appears to have made its anticipated turn to the northeast
and the initial motion is 040/5 kt. The cyclone is expected to turn
north then northwest over the next 24 h as it pivots around inside a
cyclonic gyre that includes Tropical Depression Fourteen-E to its
west. After that time, Hernan should be steered west-northwestward
to westward by a mid-tropospheric ridge anchored over the
southwestern U.S.. On this forecast track, Hernan and most of its
winds and convection should remain just offshore the southwestern
coast of Mexico today and Thursday, and pass near or just south of
the southern tip of Baja California Sur on Friday and Saturday. The
official NHC forecast is little changed from the previous one, and
is near the various consensus aids.
FORECAST POSITIONS AND MAX WINDS
INIT 26/2100Z 17.7N 105.8W 35 KT 40 MPH
12H 27/0600Z 18.4N 105.7W 35 KT 40 MPH
24H 27/1800Z 19.5N 106.1W 35 KT 40 MPH
36H 28/0600Z 20.7N 107.3W 35 KT 40 MPH
48H 28/1800Z 21.7N 108.9W 30 KT 35 MPH
60H 29/0600Z 22.2N 110.7W 30 KT 35 MPH...POST-TROP/REMNT LOW
72H 29/1800Z 22.5N 112.7W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 30/1800Z 22.4N 115.1W 20 KT 25 MPH...POST-TROP/REMNT LOW
120H 31/1800Z...DISSIPATED
$$
Forecaster Latto
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