[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 190245
TCDAT2

Potential Tropical Cyclone Two Discussion Number   2
NWS National Hurricane Center Miami FL       AL022017
1100 PM AST Sun Jun 18 2017

The convective cloud pattern associated with the disturbance has
continued to improve since the previous advisory, with a curved
convective band now wrapping more than halfway around the alleged
center. As a result, the initial intensity has been increased to 35
kt, consistent with a TAFB Dvorak estimate of T2.5/35 kt.  The
overall environment is expected to be conducive for some additional
development for the next 24-36 hours before southwesterly vertical
wind shear increases to more than 20 kt by 48 hours and beyond
across the eastern Caribbean Sea. The disturbance is expected to
be a tropical storm before reaching the Windward Islands in 36 hours
or so. The system is forecast to become a remnant low by 72 hours
and dissipate by day 4 due to quite hostile upper-level winds. The
official intensity forecast is similar to the previous advisory and
closely follows the consensus model IVCN.  An Air Force Reserve
reconnaissance aircraft is scheduled to investigate the disturbance
Monday afternoon.

The National Hurricane Center now has the option to issue advisories
on disturbances that are not yet tropical cyclones, but which pose
the threat of bringing tropical storm or hurricane conditions to
land areas within 48 hours. Under previous policy this was not
possible. These systems are known as Potential Tropical Cyclones in
advisory products and are numbered from the same list as
depressions. Because of the threat to the Windward Islands,
advisories have been initiated on Potential Tropical Cyclone Two and
the appropriate warnings have been issued by the respective
governments in the Windward Islands. Advisory packages will continue
until the threat of tropical-storm-force winds for land areas
sufficiently diminishes, although if the system becomes a tropical
cyclone, the normal rules for discontinuing advisories would apply.
Users should be aware that forecast uncertainty for disturbances is
generally larger than for tropical cyclones, especially beyond 48-72
hours.

The initial motion estimate remains 280/20 kt. The disturbance is
expected to be steered westward to west-northwestward at around 20
kt for the next couple of days along the southern periphery of a
strong deep-layer ridge located across most of the subtropical and
tropical Atlantic. The new NHC track forecast is close to the
previous advisory and the consensus model TVCN through 48 hours, and
then lies a little south of the previous track at 72 hours due to
expected weakening and decoupling of the low- and upper-level
circulations. Given that tropical storm conditions are expected to
extend well north of the center, users should not focus on the exact
track of the center, and the track forecast is of lower certainty
than usual.

FORECAST POSITIONS AND MAX WINDS

INIT  19/0300Z  7.9N  52.4W   35 KT  40 MPH...POTENTIAL TROP CYCLONE
 12H  19/1200Z  8.7N  55.2W   40 KT  45 MPH...TROPICAL CYCLONE
 24H  20/0000Z  9.9N  58.8W   45 KT  50 MPH
 36H  20/1200Z 11.1N  62.3W   45 KT  50 MPH
 48H  21/0000Z 12.1N  65.9W   40 KT  45 MPH
 72H  22/0000Z 13.5N  73.4W   30 KT  35 MPH...POST-TROP/REMNT LOW
 96H  23/0000Z...DISSIPATED

$$
Forecaster Stewart
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