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Hurricane SANDRA Forecast Discussion

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WTPZ42 KNHC 252033

200 PM MST WED NOV 25 2015

Sandra's intensification has continued at a rapid pace.  The
hurricane now has a clearer eye in infrared satellite imagery that
is completely surrounded by a white ring on the Dvorak enhancement
curve.  Dvorak estimates from TAFB and SAB were T5.5/102 kt and
T5.0/90 kt, respectively, although data-T numbers were up to 6.0.
The UW-CIMSS ADT scheme has been flopping between different scene
types, but when it has used an eye pattern it has yielded estimates
in the 5.5-6.0 range.  Based on these numbers, the initial intensity
is raised to 100 kt, making Sandra a category 3 hurricane.  There
have now been 9 major hurricanes over the eastern North Pacific
Ocean during the 2015 season, which is a record for that basin.

Some additional strengthening is anticipated since Sandra will be
moving over sea surface temperatures between 29 and 30 degrees
Celsius and within a low-shear environment for another 24 hours or
so.  However, it is hard to tell how much longer rapid
intensification (RI) will last.  The SHIPS RI index has fallen a
bit since earlier today, but it still shows about a 50 percent
chance of a 30-kt increase in winds over the next 24 hours.
Strengthening is still shown in the NHC forecast for another 12
hours, with some weakening beginning in 24-36 hours once the shear
increases.  Southwesterly shear increases to well over 30 kt beyond
48 hours, and rapid weakening is expected as Sandra approaches the
Baja California peninsula and western mainland Mexico.  The NHC
intensity forecast is a little higher than the guidance for the
first 12-24 hours, but then it is very similar to the IVCN intensity
consensus thereafter.

Sandra appears to have turned northwestward with an initial motion
of 305/7 kt.  The hurricane is beginning to move around the western
extent of a subtropical ridge located over southern Mexico, and it
should recurve to the north and northeast during the next several
days as a mid-/upper-level trough pushes the ridge eastward.  The
track guidance remains in relatively good agreement through 24
hours, with some divergence thereafter as the ECMWF model (on
the western edge of the guidance) takes a wider and slower turn
while the GFS (on the eastern edge) take a sharper and faster turn.
The difference in these models is due to their depiction of
Sandra's vertical integrity once the shear increases.  The ECMWF
shows a strongly tilted and shallower system by day 3 while the GFS
maintains a deeper vertical circulation even in the face of 50 kt
of shear.  The NHC track forecast remains close to the model
consensus aids, but it is a little slower than the previous
forecast due to the latest ECMWF forecast, which seems like a more
realistic solution at this time.

Although Sandra is forecast to weaken, it is too soon to know
exactly how it will affect portions of the southern Baja California
peninsula or the west coast of mainland Mexico.  Since the forecast
has slowed down a bit, a watch may not be required until tonight or
on Thursday for portions of Mexico.  Regardless, interests in those
areas should continue to monitor the progress of Sandra.


INIT  25/2100Z 13.0N 109.9W  100 KT 115 MPH
 12H  26/0600Z 14.0N 110.6W  110 KT 125 MPH
 24H  26/1800Z 15.7N 111.1W  105 KT 120 MPH
 36H  27/0600Z 17.5N 111.2W   90 KT 105 MPH
 48H  27/1800Z 19.3N 110.8W   75 KT  85 MPH
 72H  28/1800Z 23.1N 108.6W   40 KT  45 MPH
 96H  29/1800Z 27.0N 107.0W   20 KT  25 MPH...POST-TROP/INLAND
120H  30/1800Z...DISSIPATED

Forecaster Berg