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Tropical Storm LOWELL Forecast Discussion

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WTPZ42 KNHC 210840

200 AM PDT THU AUG 21 2014

Lowell continues to exhibit a rather large, but ragged eye in
microwave and geostationary satellite imagery. The latest Dvorak
intensity estimates are 55 kt from SAB and 65 kt from TAFB.  A
couple of recent ASCAT passes shows a large area of 50 to 55 kt
winds.  Based on a blend of the satellite estimates and the ASCAT
data the initial wind speed is increased to 60 kt.  Although not
explicitly shown in the official forecast, Lowell could become a
hurricane during the next 6 to 12 hours while it remains over 26
degree Celsius water and in a low-shear environment. After that
time, the shear is expected to remain low, but the cyclone will be
moving over progressively cooler waters and into a more stable air
mass that should cause weakening. Given the large size of Lowell's
wind field, the rate of weakening is likely to be slower than the
typical spin down of eastern Pacific tropical cyclones that move
over cool water.  In fact, Lowell is likely to become a
convective-free post-tropical cyclone with gale-force winds in
a few days.

The initial motion estimate is 315/3.  A mid- to upper-level trough
extending southwestward from southern California will weaken and
move eastward today.  This will allow the subtropical ridge to the
north and east of Lowell to rebuild, which will result in a slightly
faster northwestward motion during the next several days.  The track
guidance is in good agreement during the first 2 to 3 days, and the
NHC track is near the middle of the tightly clustered model envelope
during that time. Later in the period the spread of the models
increases, but for now the NHC track will remain near the model
consensus at days 4 and 5.


INIT  21/0900Z 19.7N 121.9W   60 KT  70 MPH
 12H  21/1800Z 20.3N 122.5W   60 KT  70 MPH
 24H  22/0600Z 21.3N 123.6W   55 KT  65 MPH
 36H  22/1800Z 22.3N 124.8W   50 KT  60 MPH
 48H  23/0600Z 23.4N 126.3W   45 KT  50 MPH
 72H  24/0600Z 25.3N 129.0W   35 KT  40 MPH...POST-TROPICAL
 96H  25/0600Z 27.0N 131.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  26/0600Z 28.3N 134.5W   25 KT  30 MPH...POST-TROP/REMNT LOW

Forecaster Brown

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Page last modified: Thursday, 21-Aug-2014 08:40:45 UTC