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Tropical Storm FRED Forecast Discussion

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WTNT41 KNHC 050834

500 AM AST SAT SEP 05 2015

Fred continues to produce bursts of convection, although the bursts
are smaller and farther from the center than they were 24 hours
ago. This suggests that the ongoing 35-40 kt of westerly vertical
shear is causing a gradual weakening.  The initial intensity remains
35 kt based on a subjective satellite intensity estimate from TAFB,
as well as objective AMSU and satellite consensus estimates from
CIMSS and CIRA.  The central pressure has been lowered a little
based on additional observations from drifting buoy 13519, which
reported a minimum pressure of 1008.4 mb as the center of Fred
passed to the north.

The initial motion is 290/10.  Fred is approaching a large break in
the subtropical ridge caused by a mid- to upper-level trough and
associated surface low centered several hundred miles east of
Bermuda.  The track guidance is in good agreement that this
baroclinic system will move eastward for the next several days, with
Fred recurving to the northeast between it and the subtropical
ridge.  After recurvature, there is some spread in the forecast
forward speed, with the new track compromising between the faster
ECMWF/GFS models and the slower UKMET/Canadian models.  The new
track is nudged slightly to the north of the previous track from
36-96 hours, and then is nudged a little to the east of the previous
track at 120 hours.

The intensity forecast is low in confidence due to several possible
scenarios.  First, although it is not explicitly forecast, Fred
could become a remnant low at any time during the next 24 to 36
hours due to continued shear and dry air entrainment, followed by
regeneration to a tropical cyclone when the shear subsequently
decreases.  Second, the dynamical models have two scenarios for Fred
as it interacts with the above-mentioned trough and surface low
after recurvature. One possibility, supported by the GFS and the
ECMWF, is that a relatively weak Fred gets absorbed into the
baroclinic low and dissipates earlier than currently forecast.
Another possibility, supported by the UKMET and Canadian models, is
that Fred stays farther away from the baroclinic low and intensifies
more than is currently forecast. Given the uncertainty, the new
intensity forecast is similar to the previous forecast in calling
for slight weakening early in the forecast period followed by slight
intensification after 36 hours.


INIT  05/0900Z 23.2N  41.0W   35 KT  40 MPH
 12H  05/1800Z 23.7N  42.1W   30 KT  35 MPH
 24H  06/0600Z 24.7N  42.7W   30 KT  35 MPH
 36H  06/1800Z 26.3N  42.5W   30 KT  35 MPH
 48H  07/0600Z 28.2N  41.1W   35 KT  40 MPH
 72H  08/0600Z 31.5N  37.5W   35 KT  40 MPH
 96H  09/0600Z 35.0N  33.0W   35 KT  40 MPH
120H  10/0600Z 36.5N  30.0W   35 KT  40 MPH

Forecaster Beven

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Page last modified: Saturday, 05-Sep-2015 08:34:34 UTC