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TROPICAL STORM FIONA DISCUSSION NUMBER  18
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL062016
500 AM AST SUN AUG 21 2016

After the earlier convective bursting episode and resultant spin up
of the low-level wind field, strong westerly shear of at least 30 kt
has stripped away the convective cloud shield, leaving a fully
exposed low-level circulation center that is easy to locate.
Assuming some spin down of the circulation due to the loss of
convection, the initial intensity has been lowered to 40 kt.

Now that Fiona's circulation has become a little more shallow, the
low-level center has turned more westward and the cyclone is now
moving 295/14 kt. The global models are in good agreement on a
mid-level ridge to the north and northeast of Fiona remaining in
place for the next few days, which should keep the small cyclone
moving in a general west-northwest direction through 72 hours. After
that time, the western portion of the ridge is expected to weaken
due to an approaching frontal system, which should allow Fiona or
its remnant circulation to slow down and turn northwestward. The new
track forecast is essentially just an update of the previous
advisory track, and lies close to the TVCN consensus model forecast.

During the next 24 hours or so, Fiona is forecast to traverse
through a band of strong westerly to northwesterly vertical wind
shear of 25-30 kt and dry mid-level air with humidity values near 40
percent. As a result, gradual weakening is expected due to the small
cyclone likely not being able to sustain deep convection for long
periods of time. The new intensity forecast holds onto the previous
trend of Fiona weakening into a remnant low by 72 hours. However, a
lot of uncertainty remains with this intensity forecast. Strong
instability that will be present due to very warm SSTs of 29C-30C
and a cool upper-level troposphere, which could produce periodic
bursts of intense convection that could sustain the low-level vortex
until environmental become more favorable for strengthening on days
4 and 5. Most of the global models continue to depict a weak low for
at least the next 5 days, except for the ECMWF model, which weakens
Fiona to an open trough by 48 hours. The NHC intensity remains a
compromise of these extremes and shows dissipation by day 5.

FORECAST POSITIONS AND MAX WINDS

INIT  21/0900Z 22.2N  51.7W   40 KT  45 MPH
 12H  21/1800Z 22.9N  53.7W   35 KT  40 MPH
 24H  22/0600Z 23.8N  56.3W   35 KT  40 MPH
 36H  22/1800Z 24.5N  58.6W   35 KT  40 MPH
 48H  23/0600Z 25.3N  60.7W   30 KT  35 MPH
 72H  24/0600Z 27.0N  64.2W   30 KT  35 MPH...POST-TROP/REMNT LOW
 96H  25/0600Z 29.7N  66.3W   30 KT  35 MPH...POST-TROP/REMNT LOW
120H  26/0600Z...DISSIPATED

$$
Forecaster Stewart

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