[Tropical] Tropical Cyclone Discussion

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WTNT45 KNHC 270244
TCDAT5

TROPICAL STORM ERIKA DISCUSSION NUMBER   9
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL052015
1100 PM AST WED AUG 26 2015

Erika is a very disorganized tropical storm. Although convection
has increased and is a little closer to the center tonight, data
from both NOAA and Air Force reconnaissance plane indicate that
the surface pressure is not falling, and the maximum winds remain
about 40 kt. These winds are confined to squalls to the north and
east of the center.

Erika will be moving through a hostile wind shear environment as
predicted by global models and the SHIPS guidance. On this
basis, the NHC forecast weakens Erika to a 35-kt tropical storm
and keeps that intensity for the next 48 hours. Erika, however,
could even degenerate into a trough during the next day or so. If
Erika survives the next 3 days and reaches the Bahamas, the
environment is expected to become quite favorable. In fact, global
models and the HWRF/GFDL pair forecast Erika to become a hurricane
by the end of the forecast period. The NHC forecast is a little
below the intensity consensus to reflect the possibility that the
cyclone could dissipate before it reaches the Bahamas, and then it
will be too late to take advantage of the more conducive environment
there.

Fixes from the reconnaissance planes indicate that Erika is moving
toward the west or 280 degrees at 14 kt. The cyclone is embedded
within well-established steering currents south of the Atlantic
subtropical ridge. This persistent pattern will likely keep the
cyclone or its remnants, in case it weakens, on a west to
west-northwest motion for the next 3 days. After that time, the
system will be in between the southwestern edge of the subtropical
ridge and a mid-level trough, which is forecast to be nearly
stationary along the east-central portion of the United States.
This will force the cyclone to turn more to the northwest or even
northward. Guidance shifted farther east tonight, and consequently,
the NHC track forecast was adjusted slightly eastward, and it is
very close to the consensus of the ECMWF and the GFS models.

One should remember to not focus on the exact forecast track,
especially at the long range where the average NHC track errors
during the past 5 years are about 180 miles at day 4 and 240
miles at day 5.


FORECAST POSITIONS AND MAX WINDS

INIT  27/0300Z 16.7N  60.2W   40 KT  45 MPH
 12H  27/1200Z 17.4N  62.3W   35 KT  40 MPH
 24H  28/0000Z 18.6N  65.0W   35 KT  40 MPH
 36H  28/1200Z 20.0N  67.9W   35 KT  40 MPH
 48H  29/0000Z 21.2N  70.5W   35 KT  40 MPH
 72H  30/0000Z 23.5N  75.0W   45 KT  50 MPH
 96H  31/0000Z 26.0N  78.3W   55 KT  65 MPH
120H  01/0000Z 28.5N  79.3W   65 KT  75 MPH

$$
Forecaster Avila
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