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Tropical Storm Iselle Discussion Number 7
NWS National Hurricane Center Miami FL EP142020
800 PM PDT Thu Aug 27 2020
Iselle continues to produce bursts of deep convection in an
environment of moderate easterly shear, with the latest round of
convection completely covering the low-level center. Since
satellite classifications haven't changed much since the previous
advisory, the initial intensity is left at 45 kt based on the
earlier ASCAT data. There is some uncertainty in exactly how strong
the shear currently is, with SHIPS diagnostics showing nearly 30 kt
while UW-CIMSS is analyzing a little under 20 kt. A recent SSMIS
pass showed some defined curved banding, so I'm inclined to think
the shear is closer to the lower estimate. With that said, the
SHIPS diagnostics do show the shear relaxing just a bit during the
next 12-24 hours, which could allow Iselle to strengthen a little,
and this scenario is supported by the latest HWRF, HMON, COAMPS-TC,
HCCA, and GFS model solutions. After that time, the shear is
expected to strengthen again, and then Iselle will be moving over
cooler waters. Therefore, the new NHC intensity forecast is very
close to the previous one from 24 hours and beyond. Iselle is
likely to lose all of its convection and become a remnant low in
about 3 days and then open up into a trough by day 5.
Iselle is embedded in an elongated mid-level trough which extends
northeastward to Tropical Storm Hernan near the coast of Mexico,
and its initial motion is north-northeastward, or 030/3 kt. The
interaction between the two tropical storms is expected to cause
Iselle to continue moving north-northeastward or northward during
the first 2 days. Once Hernan dissipates and Iselle weakens, a
low-level ridge over northern Mexico should cause Iselle to turn
toward the northwest and west on days 3 and 4. The track models
have shifted a bit to the northeast after 48 hours, leaving the
previous forecast near the left side of the guidance envelope, so
the new NHC forecast was adjusted closer to the multi-model
consensus aids during that period.
FORECAST POSITIONS AND MAX WINDS
INIT 28/0300Z 17.6N 115.5W 45 KT 50 MPH
12H 28/1200Z 18.2N 115.0W 50 KT 60 MPH
24H 29/0000Z 19.1N 114.5W 45 KT 50 MPH
36H 29/1200Z 20.1N 114.2W 40 KT 45 MPH
48H 30/0000Z 21.7N 114.2W 35 KT 40 MPH
60H 30/1200Z 23.2N 114.7W 30 KT 35 MPH
72H 31/0000Z 23.9N 115.7W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 01/0000Z 23.9N 117.0W 20 KT 25 MPH...POST-TROP/REMNT LOW
120H 02/0000Z...DISSIPATED
$$
Forecaster Berg
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