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TROPICAL STORM FIONA DISCUSSION NUMBER 8
NWS NATIONAL HURRICANE CENTER MIAMI FL AL062016
500 PM AST THU AUG 18 2016
After a brief convective hiatus, deep convection with cloud tops of
-70C to -75C has redeveloped into a small CDO feature directly
over the previously exposed low-level circulation center. Satellite
classifications are T3.0/45 kt and T2.5/35 kt from TAFB and SAB,
respectively, and UW-CIMSS ADT values have decreased to T3.0/45 kt.
A blend of these values support an initial intensity of 40 kt.
Fiona is moving a little faster toward the northwest, or 300/09 kt.
Other than minor fluctuations in the forward speed of the cyclone
due to intermittent periods of convective re-organization like the
most recent episode, the latest model guidance continues to be in
strong agreement on Fiona moving west-northwestward to northwestward
toward a break in the Bermuda-Azores High for the next 120 hours.
Given the tightly packed model guidance about the previous few
forecast tracks, the new NHC track forecast is just an extension of
the previous advisory, and lies close to but a little slower than
the consensus model, TVCN.
There is no significant change to the previous intensity forecast or
rationale. Despite the earlier sharp decrease in deep convection,
the inner-core wind field of the compact cyclone has remained quite
robust based on the lack of no arc cloud lines or outflow boundaries
seen emanating outward from the center in visible satellite today.
As result, Fiona should be able to generate additional convection in
the short term and strengthen some during the next 12-24 hours.
After that time, the global and regional models remain in good
agreement on the cyclone moving through a band of strong
southwesterly vertical wind shear of 20-25 kt from 36-72 hours,
which is expected to induce weakening. However, the amount of
weakening remains uncertain due to continued mixed dynamic and
thermodynamic conditions. Although Fiona will be propagating through
significant shear and into a drier airmass, the small cyclone will
also be moving over warmer SSTs of 28C-29C and into a region of much
cooler upper-tropospheric temperatures, which will produce greater
instability and generate fairly strong convection that could help
offset the unfavorable shear conditions. Given these mixed signals,
the official intensity forecast remains an average of the various
intensity models, which at 72 hours still ranges from hurricane
strength in the GFDL model to a 25-kt remnant low in the ECMWF and
FORECAST POSITIONS AND MAX WINDS
INIT 18/2100Z 17.0N 41.3W 40 KT 45 MPH
12H 19/0600Z 17.7N 42.5W 45 KT 50 MPH
24H 19/1800Z 18.5N 44.3W 45 KT 50 MPH
36H 20/0600Z 19.6N 46.1W 35 KT 40 MPH
48H 20/1800Z 20.8N 48.0W 35 KT 40 MPH
72H 21/1800Z 23.0N 51.7W 35 KT 40 MPH
96H 22/1800Z 25.0N 55.0W 35 KT 40 MPH
120H 23/1800Z 27.2N 56.6W 35 KT 40 MPH