[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 200842
TCDAT1

TROPICAL STORM FIONA DISCUSSION NUMBER  14
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL062016
500 AM AST SAT AUG 20 2016

Fiona is looking ragged this morning with only a small area of deep
convection on the northern side of the apparent center of the
cyclone. Dvorak estimates are a bit lower than earlier, but I've
elected to keep Fiona a tropical storm for this advisory given the
earlier ASCAT data, perhaps generously. The cyclone is likely to
gradually weaken for the next few days with strong shear, upper-
level convergence, and dry mid-level air in its path. After
considering the unfavorable environment ahead, the NHC intensity
forecast is reduced from the previous one, and now shows Fiona as a
remnant low in 48 hours and dissipating by day 5. This forecast
could be conservative since the ECMWF and UKMET now open up this
system into a trough within 3 days.

The initial motion estimate is about the same as before, 295/11,
mostly based on extrapolation.  A subtropical ridge over the
central Atlantic should keep Fiona moving west-northwestward for
the next few days.  The long-range track forecast remains highly
contingent on Fiona's status as a tropical cyclone, a remnant low,
or a trough.  A deeper cyclone would likely take a more northern
turn at the end of the period due to a mid-level trough, while a
weaker cyclone would probably just continue moving to the
west-northwest (and not feel the trough).  Since the cyclone is now
expected to be a more shallow system in a few days, it makes sense
to be on the western side of the guidance envelope, which requires
the new NHC track forecast to be shifted leftward beyond day 2.


FORECAST POSITIONS AND MAX WINDS

INIT  20/0900Z 19.2N  46.7W   35 KT  40 MPH
 12H  20/1800Z 20.0N  48.4W   30 KT  35 MPH
 24H  21/0600Z 21.1N  50.8W   30 KT  35 MPH
 36H  21/1800Z 22.1N  53.4W   30 KT  35 MPH
 48H  22/0600Z 23.1N  56.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  23/0600Z 25.0N  61.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  24/0600Z 27.0N  63.5W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  25/0600Z...DISSIPATED

$$
Forecaster Blake
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