580
WTPZ44 KNHC 231446
TCDEP4

Tropical Storm Iselle Discussion Number   1
NWS National Hurricane Center Miami FL       EP092026
800 AM MST Sun Aug 23 2026

The area of low pressure that we have been monitoring for the past
several days (EP92) has had gale-force winds and sufficiently
well-organized convection for the past 12 h or so.  The ASCAT-C pass
from 23/0517Z showed winds of 40 kt, and that the system did not yet
have a well-defined circulation, although it was closer than it was
on the previous pass from 12 h earlier.  An OSCAT pass a couple
hours later around 23/0716Z suggested that the surface circulation
was continuing to become better-defined.  Since that time, satellite
imagery indicates continued consolidation of deep convection over
the low-level center, and the system very likely has a well-defined
circulation by now.  Thus, EP92 is upgraded to Tropical Storm
Iselle, and the current intensity is estimated at 40 kt based on the
scatterometer data.

There have been no other recent microwave passes, and the confidence
in the initial position is low. The initial motion is estimated to
be west-northwestward, or 300 degrees at 10 kt.  A northwestward
motion is expected through Monday while Iselle is steered by ridging
over the southwestern United States and northwestern Mexico. As the
ridge builds in a bit more to the north of the cyclone, a gradual
turn toward the west-northwest is expected on Tuesday and Wednesday.
 The NHC track forecast closely follows the HFIP Corrected Consensus
(HCCA) model and the Google DeepMind (GDMI), which is on the
northern side of the guidance envelope.  There is a healthy amount
of spread in the track guidance, and confidence in the track
forecast is slightly below average.

Iselle will be traveling over warm water and within a moist
environment with weak vertical wind shear for the next couple of
days.  Only slow intensification is forecast during the next 12 h as
the cyclone attempts to consolidate and form a core.  Given the
favorable environment, steady strengthening is forecast tonight and
Monday, and the NHC forecast shows Iselle reaching hurricane
strength Monday night.  By Tuesday night, a modest increase in
west-southwesterly vertical wind shear and decreasing sea-surface
temperatures along Iselle`s path should lead to gradual weakening.
The NHC intensity forecast lies near the intensity consensus and the
hurricane regional model solutions.


FORECAST POSITIONS AND MAX WINDS

INIT  23/1500Z 15.3N 114.2W   40 KT  45 MPH
 12H  24/0000Z 16.5N 115.6W   45 KT  50 MPH
 24H  24/1200Z 17.9N 117.2W   55 KT  65 MPH
 36H  25/0000Z 19.3N 118.7W   65 KT  75 MPH
 48H  25/1200Z 20.5N 120.3W   65 KT  75 MPH
 60H  26/0000Z 21.4N 121.9W   60 KT  70 MPH
 72H  26/1200Z 22.0N 123.6W   55 KT  65 MPH
 96H  27/1200Z 23.0N 127.2W   45 KT  50 MPH
120H  28/1200Z 23.9N 130.8W   35 KT  40 MPH

$$
Forecaster Hagen