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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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