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TROPICAL STORM FIONA DISCUSSION NUMBER  17
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL062016
1100 PM AST SAT AUG 20 2016

Somewhat surprisingly, two recent ASCAT passes showed that Fiona is
producing 45-kt winds in its northeastern quadrant.  This intensity
is also supported by the latest ADT estimate.  Despite the increase
in maximum winds, the next 36 hours will be critical for Fiona's
survival as a tropical cyclone.  During this period, westerly shear
of 30 kt will be at its strongest, and mid-level relative humidities
will be at their lowest.  Therefore, gradual weakening is
anticipated, and it's entirely possible that Fiona could soon
struggle to maintain organized deep convection.  For now, the NHC
official forecast shows Fiona becoming a remnant low by 72 hours.
However, there is a lot of uncertainty in this forecast.  If Fiona
can survive the next 36 hours, it might persist as a weak tropical
cyclone through the end of the forecast period since environmental
conditions do become a little more favorable in a couple of days. It
should be noted that most of the global models hang onto a weak low
for at least the next 5 days, with the exception of the ECMWF which
shows the low opening up into a trough by 48 hours.  For now, the
NHC forecast continues to show dissipation by day 5 as a compromise.

Fiona's motion has been wavering between west-northwest and
northwest, and the latest estimate is 305/13 kt.  Low-level ridging
should keep Fiona on this general trajectory for the next 72 hours
or so, followed by a northwestward turn with a decrease in speed by
day 4 when the cyclone approaches a stalled frontal boundary off
the east coast of the United States.  The NHC official forecast has
been nudged south and west to be closer to the TVCN multi-model
consensus, especially toward the end of the forecast period.

FORECAST POSITIONS AND MAX WINDS

INIT  21/0300Z 21.7N  50.1W   45 KT  50 MPH
 12H  21/1200Z 22.4N  52.0W   45 KT  50 MPH
 24H  22/0000Z 23.3N  54.6W   40 KT  45 MPH
 36H  22/1200Z 24.2N  57.3W   40 KT  45 MPH
 48H  23/0000Z 24.9N  59.5W   35 KT  40 MPH
 72H  24/0000Z 26.6N  63.3W   30 KT  35 MPH...POST-TROP/REMNT LOW
 96H  25/0000Z 29.0N  66.0W   30 KT  35 MPH...POST-TROP/REMNT LOW
120H  26/0000Z...DISSIPATED

$$
Forecaster Berg

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