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TROPICAL STORM KARL DISCUSSION NUMBER  10
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122016
500 PM AST FRI SEP 16 2016

Convection is quite strong in the northeastward quadrant of Karl,
although the center has recently become exposed due to
moderate-to-strong westerly shear.  There has been little
change to the overall satellite presentation in the past several
hours, and the initial wind speed is kept at 40 kt.  A continuation
of the shear is forecast to keep Karl from strengthening much over
the next day or two. This shear is forecast to relax by all of the
models by early next week, and some strengthening should then take
place while Karl moves over warmer waters.  The amount of
intensification is a big question mark, with the GFS/ECMWF now
showing only a weaker tropical storm, while the rest of the standard
intensity guidance (HWRF, GFDL, SHIPS and LGEM models) have Karl as
a hurricane by the end of the period.  Because of the uncertainty,
the intensity forecast is kept the same as the previous one, not too
far from the model consensus.

The initial motion estimate is about the same as before, 280/12.
Model guidance has made a large northward shift today due to the
forecasted interaction of a mid/upper-level trough with Karl over
the next few days.  Initially, the tropical cyclone was expected to
be on the western side of the trough, which would have helped steer
the cyclone more to the west-southwest over the next couple of
days.  However, almost all of the guidance now show the trough
staying to the west of Karl, which would impart a west or
west-northwest motion during that time.  The lack of a
west-southwest track has large implications down the line, with the
cyclone gaining a lot more latitude since Karl is closer to a
weakness in the subtropical ridge.  Continuity dictates that the
NHC forecast not be shifted as much as the guidance suggests for
this package, since the guidance could still shift back to the
south.  A large northward adjustment is made to the NHC forecast at
all times, but it remains south of the model consensus and the
ECMWF solutions.


FORECAST POSITIONS AND MAX WINDS

INIT  16/2100Z 18.8N  35.6W   40 KT  45 MPH
 12H  17/0600Z 19.0N  37.3W   40 KT  45 MPH
 24H  17/1800Z 19.0N  39.7W   40 KT  45 MPH
 36H  18/0600Z 19.0N  42.5W   40 KT  45 MPH
 48H  18/1800Z 19.0N  45.3W   45 KT  50 MPH
 72H  19/1800Z 20.0N  50.5W   50 KT  60 MPH
 96H  20/1800Z 22.0N  55.5W   60 KT  70 MPH
120H  21/1800Z 25.0N  60.5W   70 KT  80 MPH

$$
Forecaster Blake

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