[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 200246
TCDAT1

TROPICAL STORM FIONA DISCUSSION NUMBER  13
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL062016
1100 PM AST FRI AUG 19 2016

A 2346 UTC ASCAT-B overpass helped to locate the center, which is
beneath the cirrus canopy but along the southern edge of the deep
convection.  The scatterometer pass suggested that the maximum
winds could be as high as 40 kt, but the highest vectors had
questionable directions and were on the edge of the pass.
Therefore, the intensity remains 35 kt, which is supported by
subjective and objective satellite intensity estimates.

Southwesterly vertical shear has increased to about 20 kt and is
forecast to go up further during the next 48 hours or so.  Given the
increasing shear, a relatively dry middle troposphere, and Fiona's
small size, the cyclone is expected to gradually degrade during the
next few days.  The intensity and global models either maintain
Fiona's intensity or show weakening through the forecast period, and
the ECMWF even opens the system up into a trough by day 3 or 4.
Given that Fiona will likely struggle to maintain deep convection
as environmental conditions worsen, the NHC forecast now shows the
cyclone degenerating into a remnant low by day 3.  Although a
remnant low is carried through day 5, dissipation is possible before
the end of the forecast period.

The initial motion remains west-northwestward but a little faster
at 295/10 kt.  The track models are in good agreement on Fiona
accelerating a bit along this heading during the next 48 hours, and
no significant changes to the NHC forecast was required during that
time.  After 48 hours, however, the track forecast is highly
contingent on Fiona's status as a tropical cyclone, a remnant low,
or a trough.  The ECMWF, which dissipates the cyclone, has a track
well to the south and west of the other models, while the GFS, GFS
ensemble mean, and Florida State Superensemble still show a
northward turn by day 5.  Regardless, the entire guidance envelope
has shifted westward significantly after 48 hours.  The NHC
official forecast has been moved in that direction, but it still
lies east of the various consensus models.

FORECAST POSITIONS AND MAX WINDS

INIT  20/0300Z 18.7N  45.5W   35 KT  40 MPH
 12H  20/1200Z 19.4N  47.2W   35 KT  40 MPH
 24H  21/0000Z 20.5N  49.5W   35 KT  40 MPH
 36H  21/1200Z 21.6N  52.0W   30 KT  35 MPH
 48H  22/0000Z 22.6N  54.7W   30 KT  35 MPH
 72H  23/0000Z 24.6N  59.1W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  24/0000Z 26.5N  62.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  25/0000Z 29.0N  63.0W   25 KT  30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Berg
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