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 943 
 WTPZ42 KNHC 070233
 TCDEP2
 
 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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