[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 170250
TCDAT2

TROPICAL STORM KARL DISCUSSION NUMBER  11
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122016
1100 PM AST FRI SEP 16 2016

The deep convection with Karl is limited to the northeastern
quadrant because of the 20 kt westerly vertical shear currently
afflicting the tropical storm.  Despite this, a 0007 UTC ASCAT
scatterometer pass indicated maximum winds of about 40 kt, which is
the initial intensity.

Karl should continue to struggle for the next day or so due to the
hostile vertical shear.  The shear should relax as Karl reaches the
west side of an upper-level trough in about 36 to 48 hours.  From
that point in time onward, Karl should be experiencing more
conducive conditions: gradually warming SSTs, a moist low to middle
troposphere, and low vertical shear.  The NHC intensity forecast is
for Karl to weaken slightly within the next day and then steadily
strengthen starting in about two days onward.  This is quite
similar to the previous advisory and is based upon the IVCN
multi-model consensus.  However, confidence in Karl actually
becoming a hurricane in five days is low because of the spread in
the guidance.

Karl is moving toward the west at 13 kt.  The Bermuda-Azores
high will be building in north of the tropical storm, helping to
propel Karl on a general westward track for the next couple of
days.  Beginning in about three days, Karl will reach the western
periphery of the Bermuda-Azores high and bend toward the
west-northwest.  The reliable track models have substantial spread
- especially during days one to three - with some solutions
showing a west-southwesterly track, which is rarely seen in
this part of the Atlantic basin.  The NHC track forecast is
slightly south of the previous advisory, and is based upon
the TVCN multi-model consensus.

The initial size of Karl is determined by the recent scatterometer
pass, which showed a large area of tropical-storm-force winds in the
northern semicircle, but none to the south.  The NHC wind radii
forecast is based upon the RVCN consensus technique.


FORECAST POSITIONS AND MAX WINDS

INIT  17/0300Z 18.5N  37.0W   40 KT  45 MPH
 12H  17/1200Z 18.5N  38.8W   35 KT  40 MPH
 24H  18/0000Z 18.5N  41.5W   35 KT  40 MPH
 36H  18/1200Z 18.5N  44.1W   35 KT  40 MPH
 48H  19/0000Z 18.5N  46.8W   40 KT  45 MPH
 72H  20/0000Z 20.0N  52.0W   50 KT  60 MPH
 96H  21/0000Z 22.0N  57.0W   60 KT  70 MPH
120H  22/0000Z 25.0N  61.5W   70 KT  80 MPH

$$
Forecaster Landsea
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