[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 172042
TCDAT1

TROPICAL STORM FIONA DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL062016
500 PM AST WED AUG 17 2016

During the past 6 hours, the tropical cyclone has made a transition
from a large outer banding pattern to more of a CDO-type feature.
Also, a 1635Z SSMI overpass confirmed the tight inner-core structure
noted in the two earlier ASCAT passes. Dvorak satellite intensity
estimates are T2.5/35 kt from TAFB and SAB, and a 1225Z ASCAT-A
overpass indicated winds of near 35 kt at that time. Based on these
data, the initial intensity is increased to 35 kt and the cyclone is
upgraded to Tropical Storm Fiona.

The initial motion estimate is 305/14 kt. The latest model guidance
has come into much better agreement with the previous track
forecast, with the GFS model shifting a little farther south and the
ECMWF making a significant shift to the north due to that model not
weakening Fiona as much now as compared to previous forecast cycles.
These model changes have resulted in a northward shift in the
consensus models TVCN and GFEX. Given the much better agreement in
the NHC model guidance, the new track forecast was only shifted
slightly southward of the previous advisory track after 72 hours in
order to move closer to the consensus models.

There are no significant changes to the previous intensity forecast.
Fiona is a compact tropical cyclone with a radius of maximum winds
of 15 nmi or less, which makes the storm susceptible to sharp
fluctuations in intensity. The aforementioned SSMI satellite pass
also indicated that the small inner-core region of Fiona now appears
to be insulated from the very dry mid-level air that had earlier
eroded the cyclone's convection. That, along with low shear
conditions and marginal SSTs near 27C, should allow for some gradual
strengthening for the next 36-48 hours, assuming that the cyclone
can mix out any occasional intrusions of dry air. By 72 hours, the
vertical shear is forecast to increase to around 20 kt from the
southwest, which should act to arrest the intensification process
and perhaps even induce some slight weakening. However, by 96 hours
and beyond, the shear is forecast to steadily decrease when the
cyclone will be moving over SSTs greater than 28C. These conditions
could allow for the compact cyclone to maintain a steady intensity
despite being embedded in mid-level humidity values of near 50
percent. The official intensity forecast is similar to the previous
advisory and closely follows the intensity consensus model IVCN
through 48 hours, and then lies just below the Decay-SHIPS model at
72-120 hours.

FORECAST POSITIONS AND MAX WINDS

INIT  17/2100Z 15.1N  37.8W   35 KT  40 MPH
 12H  18/0600Z 16.0N  39.0W   40 KT  45 MPH
 24H  18/1800Z 17.0N  40.6W   45 KT  50 MPH
 36H  19/0600Z 17.8N  42.1W   50 KT  60 MPH
 48H  19/1800Z 18.7N  43.8W   50 KT  60 MPH
 72H  20/1800Z 20.3N  47.4W   45 KT  50 MPH
 96H  21/1800Z 22.6N  51.3W   45 KT  50 MPH
120H  22/1800Z 24.6N  55.0W   45 KT  50 MPH

$$
Forecaster Stewart
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