ZCZC MIATCDAT1 ALL
TTAA00 KNHC DDHHMM

TROPICAL STORM FIONA DISCUSSION NUMBER   7
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL062016
1100 AM AST THU AUG 18 2016

Deep convection has decreased since the previous advisory due to
Fiona moving into the daytime convective minimum period, plus likely
entrainment of dry mid-level air. Satellite classifications remain
T3.0/45 kt and T2.0/30 kt from TAFB and SAB, respectively, but
UW-CIMSS ADT values have increased to T3.2/49 kt. However, the
recent erosion of the inner-core convection argues for maintaining
the initial intensity at 40 kt despite the higher ADT value.

The forward speed has decreased sharply since the previous advisory,
and Fiona is now moving at 300/07 kt. The recent decrease in forward
speed has been well advertised by the NHC track guidance for the
past couple of days. The latest model guidance is in much better
agreement on Fiona moving west-northwestward to northwestward toward
a break in the subtropical ridge between 50W-55W throughout the
forecast period, and is now converging tightly around the previous
forecast track. Therefore, the new official forecast track is just
an update of the previous advisory track, and lies a little to the
left of consensus track model, TVCN, due to a strong right bias
caused by the much stronger and vertically deeper GFDL model.

The intensity forecast is a little less straightforward than the
track forecast due to mixed dynamic and thermodynamic signals. On
one hand, shear conditions are expected to increase to more than 20
kt in 48-72 hours while the cyclone is moving into a much drier air
airmass consisting of near 40 percent mid-level humidity values.
This combination of negative parameters generally supports
significant weakening. However, Fiona will also be moving over
warmer SSTs of 28-29C and into a region of much cooler upper-level
temperatures, which will act to generate greater instability and
likely more vigorous and persistent convection despite the drier
mid-level environment. Given these mixed signals, the new NHC
forecast is an average of the various intensity models, which at 96
hours ranges from hurricane strength in the GFDL model to a 25-kt
remnant low in the Navy-CTCI and ECMWF models.

FORECAST POSITIONS AND MAX WINDS

INIT  18/1500Z 16.4N  40.5W   40 KT  45 MPH
 12H  19/0000Z 17.2N  41.7W   45 KT  50 MPH
 24H  19/1200Z 18.1N  43.3W   45 KT  50 MPH
 36H  20/0000Z 19.1N  45.1W   40 KT  45 MPH
 48H  20/1200Z 20.3N  47.0W   35 KT  40 MPH
 72H  21/1200Z 22.6N  50.6W   35 KT  40 MPH
 96H  22/1200Z 24.4N  53.7W   35 KT  40 MPH
120H  23/1200Z 26.4N  56.0W   35 KT  40 MPH

$$
Forecaster Stewart

NNNN