[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 180244
TCDAT1

TROPICAL STORM FIONA DISCUSSION NUMBER   5
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL062016
1100 PM AST WED AUG 17 2016

For a time late this afternoon, it seemed that Fiona might lose all
its deep convection.  However, a new burst developed around 2300 UTC
and has persisted since that time.  An 0026 UTC ASCAT-B pass
revealed that the cyclone is still producing a small area of
tropical-storm-force winds, and the intensity remains 35 kt.

Fiona has a tough road ahead.  Vertical shear right now is
relatively low, but it is expected to begin increasing within the
next 24 hours and then become belligerent out of the southwest by
days 3 and 4.  In addition, the cyclone is surrounded by dry air,
and any negative impacts that the dry air is already having on the
convection is likely to be compounded once the shear increases in a
few days.  With the exception of the GFDL, which makes Fiona a
hurricane by the end of the forecast period, the rest of the
intensity models respond to the unfavorable environment by showing
relatively little strengthening.  The NHC forecast follows this same
thinking, and it shows Fiona only modestly strengthening during the
next 36 hours, with some weakening probable by day 3.  Based on the
latest intensity guidance, the new NHC forecast has been adjusted
downward from the previous forecast, but it still lies above the
consensus at most forecast times.

The recent ASCAT data helped to fix the center, and the initial
motion estimate is now west-northwestward, or 300/14 kt.  A break
in the mid-tropospheric ridge to the north of Fiona is expected to
to migrate westward during the next few days, but the cyclone's
shallow nature is expected to help maintain a west-northwestward
motion for much of the forecast period.  The bulk of the track
models are tightly clustered on such a heading.  However, the GFDL
(which makes Fiona hurricane) shows a sharp northward turn, while
the HWRF (which essentially shows no intensification) takes a much
more southern track.  Since these two scenarios appear to cancel
each other out, the NHC official forecast closely follows the
consensus models, requiring a westward adjustment to the previous
forecast.

FORECAST POSITIONS AND MAX WINDS

INIT  18/0300Z 15.5N  39.0W   35 KT  40 MPH
 12H  18/1200Z 16.3N  40.4W   35 KT  40 MPH
 24H  19/0000Z 17.1N  41.9W   40 KT  45 MPH
 36H  19/1200Z 17.8N  43.5W   45 KT  50 MPH
 48H  20/0000Z 18.6N  45.2W   45 KT  50 MPH
 72H  21/0000Z 20.6N  49.1W   40 KT  45 MPH
 96H  22/0000Z 22.5N  53.0W   40 KT  45 MPH
120H  23/0000Z 24.5N  57.0W   40 KT  45 MPH

$$
Forecaster Berg
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