000
WTNT42 KNHC 180231
TCDAT2

TROPICAL STORM KARL DISCUSSION NUMBER  15
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122016
1100 PM AST SAT SEP 17 2016

Karl continues to struggle.  The tropical storm is producing a few
patches of deep convection to the north and east of the exposed
center, but the cyclone lacks banding features.  An ASCAT-B pass
just prior to 0000Z captured a portion of the circulation and showed
that the winds were lower there than they were in the previous pass.
Based on that data and the Dvorak classifications, the initial wind
speed is lowered a little to 35 kt.

The poor structure of Karl is likely due to the combined effects of
southwesterly shear and dry mid-level air.  These unfavorable
atmospheric conditions and marginally warm sea surface temperatures
should keep Karl relatively steady state for the next day or so.
After that time, lower shear, slightly more moisture, and warmer sea
surface temperatures should allow the cyclone to strengthen.  The
intensity models are a bit higher this cycle at the longer range,
and the NHC forecast has been nudged upward.  This prediction lies
near the lower end of the guidance, in best agreement with the SHIPS
model.

The center of Karl is moving south of due west at about 10 kt.  A
general westward motion at a slightly faster pace is expected
during the next day or two while Karl is steered by the flow on the
south side of a subtropical ridge.  A turn to the west-northwest is
predicted beyond a couple of days as Karl moves on the southwestern
periphery of the ridge and toward a weakness.  There is not a
significant amount of spread in the models, or the ensemble members
of the GFS and ECMWF, and the official forecast is largely an
update of the previous one.

FORECAST POSITIONS AND MAX WINDS

INIT  18/0300Z 17.8N  41.3W   35 KT  40 MPH
 12H  18/1200Z 17.9N  43.3W   35 KT  40 MPH
 24H  19/0000Z 18.1N  46.0W   40 KT  45 MPH
 36H  19/1200Z 18.6N  48.6W   45 KT  50 MPH
 48H  20/0000Z 19.4N  51.1W   50 KT  60 MPH
 72H  21/0000Z 21.2N  56.1W   55 KT  65 MPH
 96H  22/0000Z 23.7N  61.4W   65 KT  75 MPH
120H  23/0000Z 25.8N  65.2W   75 KT  85 MPH

$$
Forecaster Cangialosi