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TROPICAL STORM SANDRA DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP222015
300 AM CST TUE NOV 24 2015

The tropical cyclone has continued to become better organized during
the past 6 hours, with a large curved convective cloud band with
tops colder than -80C and considerable lightning activity having
developed in the northern semicircle. In addition, a small CDO-like
feature has recently developed over the low-level center. The
initial intensity has been increased to 40 kt based on blend of
satellite intensity estimates of T2.5/35 kt from TAFB and T2.9/43 kt
from UW-CIMMS ADT at 0600 UTC, and the improved convective
appearance since that time. This makes Sandra the 18th named storm
of the 2015 eastern North Pacific hurricane season.

Sandra's initial motion is now 275/11 kt. During the next 3-4 days,
Sandra is forecast to gradually slow down as it moves around the
western periphery of a deep-layer subtropical ridge that extends
from the southwestern Gulf of Mexico westward across central Mexico.
On days 4 and 5, the cyclone is expected to turn northeastward and
accelerate ahead of a strong mid-latitude trough that will be moving
eastward toward the western coasts of the United States and Baja
California. The NHC model guidance remains good agreement on this
developing steering flow scenario, and the official forecast track
lies close to the consensus model, TVCE.

The intensity forecast is a little less straightforward than the
track forecast. Only the HWRF model shows Sandra reaching an
intensity of 75 kt; all of the other guidance is below that value.
This is something of an enigma given that the cyclone will be (a)
moving over SSTs greater than 29C, (b) in a moist mid-level
environment consisting of humidity values greater than 75 percent,
and (c) in a very low vertical shear environment of less than 5 kt.
These three factors typically would result in rapid intensification,
especially since it appears that Sandra is in the process of
developing a tighter inner-core wind field. The GFS-based SHIPS
intensity model indicates that the main inhibiting factors are the
shear direction and warmer-than-average 200 mb temperatures.
However, the latter doesn't appear to be a factor given the very
cold cloud tops currently noted. The global models are all
forecasting a pronounced upper-level anticyclone and a favorable
outflow pattern to develop over Sandra in the 24-48 hour period,
which should negate the negative shear direction parameter. As a
result, at least climatological development is forecast through 48
hours. By day 4, strong southwesterly shear in excess of 30 kt is
expected to induce rapid weakening as Sandra recurves into the
mid-latitude westerlies.  The NHC intensity forecast is similar to
the previous advisory, but now lies above all of the available
intensity guidance.

FORECAST POSITIONS AND MAX WINDS

INIT  24/0900Z 10.8N 105.0W   40 KT  45 MPH
 12H  24/1800Z 11.3N 106.7W   50 KT  60 MPH
 24H  25/0600Z 12.0N 108.5W   60 KT  70 MPH
 36H  25/1800Z 12.9N 110.0W   70 KT  80 MPH
 48H  26/0600Z 14.1N 110.7W   80 KT  90 MPH
 72H  27/0600Z 17.7N 110.7W   75 KT  85 MPH
 96H  28/0600Z 22.0N 108.8W   55 KT  65 MPH
120H  29/0600Z 25.5N 106.2W   20 KT  25 MPH...INLAND

$$
Forecaster Stewart

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