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TROPICAL STORM KARL DISCUSSION NUMBER  13
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122016
1100 AM AST SAT SEP 17 2016

Karl has been in a fairly steady state during the past day or more
with convection periodically firing near the center and a larger
area of more continuous thunderstorms far from the center.  Ship
BATFR18 reported sustained winds of about 40 kt at 1100 UTC, which
support keeping that wind speed for this advisory.  The shear that
has been affecting Karl should gradually diminish over the next
couple of days as it moves away from a nearby upper-level low.  A
small amount of weakening is possible in the short term since the
system is still struggling against the shear. Thereafter,
strengthening is anticipated due to Karl likely moving over
very warm waters concurrent with rather light shear. One question
mark is the amount of available moisture in the mid-levels, with
both the ECMWF and the GFS showing a somewhat dry environment. While
a lot of the tropical-cyclone-specific intensity guidance shows a
hurricane at long range, the marginal moisture environment forecast
by those two global models makes me think twice about going too
high.  The new forecast is very similar to the previous one and is
similar to a blend of the SHIPS/LGEM models.

Karl has been wobbling a little south of due west during the day,
depending on how much convection is near the center. A strengthening
low- to mid-level ridge over the central Atlantic is expected to
continue pushing Karl south of due west during the next 24 hours.
After that time, Karl should move more to the west-northwest around
the weakening ridge through day 4, well to the northeast of the
Leeward Islands, and possibly turn northwestward by 120 hours while
the cyclone approaches a weakness in the ridge.  The spread in the
models have increased since the last advisory, perhaps due to Karl's
interaction with the upper-low. The new model consensus and the
previous official forecast were very close to one another, so no
significant changes are made in the new NHC forecast.


FORECAST POSITIONS AND MAX WINDS

INIT  17/1500Z 18.3N  39.0W   40 KT  45 MPH
 12H  18/0000Z 18.1N  40.9W   35 KT  40 MPH
 24H  18/1200Z 17.8N  43.6W   35 KT  40 MPH
 36H  19/0000Z 17.8N  46.2W   40 KT  45 MPH
 48H  19/1200Z 18.3N  48.7W   45 KT  50 MPH
 72H  20/1200Z 20.2N  54.0W   50 KT  60 MPH
 96H  21/1200Z 22.5N  59.0W   60 KT  70 MPH
120H  22/1200Z 25.0N  63.0W   70 KT  80 MPH

$$
Forecaster Blake

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