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Tropical Storm Iselle Discussion Number 3
NWS National Hurricane Center Miami FL EP142020
800 PM PDT Wed Aug 26 2020
Satellite imagery indicates that a convective burst has formed a
little closer to the center of Iselle during the past few hours,
although the circulation remains at least somewhat elongated.
Various subjective and objective satellite intensity estimates are
near 35 kt, so that remains the initial intensity. The imagery, as
well as model analyses, show that Iselle continues to be undergoing
strong easterly shear.
Iselle is embedded in a large monsoon gyre, and the southwesterly
flow on the south side of the gyre should be the main steering
influence for the next 60 h or so. This should result in the
cyclone continuing its current slow motion toward the northeast.
After that time, the cyclone should separate from the gyre, allowing
a mid-level ridge to the north and northeast of the system to become
the dominant steering mechanism. This should cause Iselle to turn
to the north, then northwest, then eventually to the west. The
track guidance is in good agreement with the general scenario,
although there is some spread on when and where there turns will
occur. The new forecast track is close to the HCCA corrected
consensus mode and has only minor adjustments from the previous
forecast.
Present indications are that the current strong shear will persist
for several days, and after 72 h the forecast track takes the
cyclone over cooler sea surface temperatures. Based on this,
significant strengthening appears unlikely, although short-lived
spin ups due to convective bursts are possible. The new intensity
forecast is unchanged from the previous forecast in showing little
change of strength for 72 h, followed by weakening to a depression
and a remnant low. An alternative scenario is that the shear causes
Iselle to weaken to a trough at any time during the next 120 h.
FORECAST POSITIONS AND MAX WINDS
INIT 27/0300Z 16.2N 116.2W 35 KT 40 MPH
12H 27/1200Z 16.7N 115.8W 35 KT 40 MPH
24H 28/0000Z 17.3N 115.3W 35 KT 40 MPH
36H 28/1200Z 17.9N 114.7W 35 KT 40 MPH
48H 29/0000Z 18.6N 114.1W 35 KT 40 MPH
60H 29/1200Z 19.4N 113.6W 35 KT 40 MPH
72H 30/0000Z 20.7N 113.7W 30 KT 35 MPH
96H 31/0000Z 22.5N 115.5W 30 KT 35 MPH...POST-TROP/REMNT LOW
120H 01/0000Z 22.5N 116.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
$$
Forecaster Beven
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