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TROPICAL STORM ESTELLE DISCUSSION NUMBER  16
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP062016
200 AM PDT TUE JUL 19 2016

Estelle has changed little since the previous advisory. Passive
microwave satellite data indicate that the cyclone has a robust and
nearly symmetrical low-level wind field, but periodic intrusions of
dry mid-level air have continued to prevent the development of a
closed eyewall. The initial intensity remains at 55 kt for this
advisory based on a consensus CI-number of T3.5 from TAFB and SAB.

The initial motion estimate is 285/09 kt. There remains no change to
forecast reasoning from the past couple of days. Estelle is expected
to move a little faster to the west today, followed by a turn back
toward the west-northwest on Wednesday as the ridge to the north
begins to weaken. By Thursday and beyond, a large mid-/upper-level
trough is forecast to drop southwestward and erode the subtropical
ridge, allowing Estelle to turn northwestward and move over much
cooler water as result. The guidance remains in excellent agreement
on this track scenario through 72 h, and then it diverges somewhat
due to differences in the strength and vertical depth of the
cyclone. The new NHC track forecast is a little north of the
consensus model TVCE and is located along the northern edge of the
guidance closer to the ECMWF model solution.

Estelle's failure to intensify to a hurricane remains something of
an enigma given the overall favorable ocean, mid-level humidity, and
low shear conditions that the cyclone has been experiencing. Some
southerly mid-level shear along with intrusions of dry mid-level air
have apparently been disrupting the development of a persistent eye
feature as noted in microwave imagery even this morning. The center
of Estelle is currently moving over a SST thermal ridge were
temperatures are at least 0.5C/1F warmer than indicated by the SHIPS
model. That extra heat energy could finally allow the cyclone to
reach hurricane status later today or tonight. On Wednesday,
however, gradual weakening is forecast to begin as Estelle moves
over cooler waters with SSTs less than 26C, and into a drier and
more stable air mass. The official intensity forecast remains above
all of the guidance through 36 h, and then closely follows the
consensus model IVCN after that. Given that Estelle will be over
nearly 22C SSTs by 72 h, the transition to a remnant low could occur
sooner than currently forecast.

FORECAST POSITIONS AND MAX WINDS

INIT  19/0900Z 18.5N 117.6W   55 KT  65 MPH
 12H  19/1800Z 18.9N 119.2W   60 KT  70 MPH
 24H  20/0600Z 19.3N 121.4W   65 KT  75 MPH
 36H  20/1800Z 19.7N 123.9W   55 KT  65 MPH
 48H  21/0600Z 20.3N 126.3W   45 KT  50 MPH
 72H  22/0600Z 22.1N 131.6W   35 KT  40 MPH
 96H  23/0600Z 25.0N 136.4W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  24/0600Z 28.5N 139.5W   20 KT  25 MPH...POST-TROP/REMNT LOW

$$
Forecaster Stewart

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