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Tropical Storm Aletta Discussion Number   5
NWS National Hurricane Center Miami FL       EP022018
900 PM MDT Wed Jun 06 2018

Deep convection has been bursting just to the northeast of the
center during the past few hours, although a recent SSMI/S overpass
suggests that the low-level circulation continues to consist of
multiple swirls rotating around a common center.  Because of the
increase in convection, subjective satellite intensity estimates
from TAFB and SAB have gone up to T3.0, so the initial intensity is
set at 45 kt.

There is about 15 kt of deep-layer southwesterly shear impinging on
Aletta according to analyses from UW-CIMSS, which can be seen in the
motions of the high cirrus emanating away from additional convection
to the southwest.  The global models aren't picking up on this shear
very well, with the SHIPS diagnostics currently showing shear just
under 10 kt.  It now appears that some westerly shear could persist
over Aletta for the next few days, and thus only modest
intensification is expected over waters that are 27-29 degrees
Celsius.  The NHC official forecast still calls for Aletta
to reach hurricane strength in a couple of days, and the updated
forecast is close to the HCCA guidance and the intensity consensus
through 48 hours.  The intensity models are showing a faster
weakening rate starting on day 3 due to cooler waters and higher
shear, and although the NHC forecast has been trended downward to
account for this, it still lies above much of the guidance later in
the forecast period.

The latest fixes indicate that Aletta is slowing down, and the
initial motion estimate is 275/6 kt.  A mid-level ridge centered
over northwestern Mexico should continue to steer the cyclone slowly
westward for a day or two.  A continued slow motion to the
west-northwest or northwest is then expected starting in 48 hours
once Aletta reaches a break in the subtropical ridge.  The ECMWF
remains on the southern side of the guidance envelope, while the
GFS, HMON, and HWRF lie on the northern edge. Since the upper-level
patterns among these models are similar, the discrepancies among
their respective tracks appear related to how strong they each make
the cyclone, as the models with a stronger cyclone forecast a
more northerly track. The new NHC forecast is closest to the HCCA
guidance and is not too different from the previous official
forecast.

FORECAST POSITIONS AND MAX WINDS

INIT  07/0300Z 14.3N 108.4W   45 KT  50 MPH
 12H  07/1200Z 14.5N 109.4W   50 KT  60 MPH
 24H  08/0000Z 14.7N 110.4W   55 KT  65 MPH
 36H  08/1200Z 14.9N 111.4W   60 KT  70 MPH
 48H  09/0000Z 15.2N 112.3W   65 KT  75 MPH
 72H  10/0000Z 16.3N 113.8W   75 KT  85 MPH
 96H  11/0000Z 17.5N 115.5W   55 KT  65 MPH
120H  12/0000Z 18.0N 117.0W   35 KT  40 MPH

$$
Forecaster Berg

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