[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 190243
TCDAT1

TROPICAL STORM FIONA DISCUSSION NUMBER   9
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL062016
1100 PM AST THU AUG 18 2016

A sizable burst of deep convection has been ongoing since late this
afternoon, and recent microwave imagery suggests that the low-level
center may be a little closer to the thunderstorm activity than it
was for most of the day.  Still, Dvorak estimates are unchanged at
T3.0/45 kt from TAFB and T2.5/35 kt from SAB, and the initial
intensity remains 40 kt.

Fiona's window for any strengthening is narrowing.  SHIPS guidance
is currently analyzing 10-15 kt of south-southwesterly shear, while
the UW-CIMSS analysis has the shear as high as 20-25 kt.  Despite
the discrepancy, these values suggest the shear is already
increasing, and it is expected to increase further and become more
westerly during the next three days.  Therefore, if the current
burst of convection continues, Fiona has a chance to strengthen
slightly during the next 12 hours and then gradually weaken through
days 3 and 4.  The forecast trends in the intensity models suggest
that Fiona could weaken to a tropical depression, which is now
shown in the official forecast.  Environmental conditions appear to
be a little less hostile by the end of the forecast period, and the
NHC forecast allows for the possibility that Fiona could
restrengthen after the shear lightens up.  The GFDL continues to
be an outlier by intensifying Fiona more than the other models, and
the official intensity forecast is therefore near or just below the
intensity consensus for most of the forecast period.

Microwave fixes indicate that Fiona has been moving more slowly
toward the northwest, or 305/7 kt.  The cyclone is moving toward a
break in the subtropical ridge located between 40W-55W, and an
amplifying deep-layer trough over the north Atlantic is expected to
maintain this break for the next several days.  By continuing on a
northwestward heading, Fiona is expected to reach the axis of the
subtropical ridge and turn northward by day 5.  The GFDL is still
well to the northeast of the rest of the track guidance since it
carries a much stronger system, but the other models are otherwise
in very good agreement.  Therefore, no significant changes were
made to the NHC track forecast, which remains close to the TVCN
multi-model consensus.

FORECAST POSITIONS AND MAX WINDS

INIT  19/0300Z 17.1N  41.8W   40 KT  45 MPH
 12H  19/1200Z 17.8N  43.1W   45 KT  50 MPH
 24H  20/0000Z 18.7N  44.9W   40 KT  45 MPH
 36H  20/1200Z 19.8N  46.9W   35 KT  40 MPH
 48H  21/0000Z 20.9N  48.9W   35 KT  40 MPH
 72H  22/0000Z 23.0N  53.2W   30 KT  35 MPH
 96H  23/0000Z 25.0N  56.5W   30 KT  35 MPH
120H  24/0000Z 27.5N  57.0W   35 KT  40 MPH

$$
Forecaster Berg
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