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TROPICAL STORM KARL DISCUSSION NUMBER  18
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122016
500 PM AST SUN SEP 18 2016

Deep convection has gradually been increasing and becoming a little
better organized since the previous advisory now that light
upper-level southeasterly flow has replaced the previous hostile
vertical wind shear conditions. Visible satellite imagery and ASCAT
surface wind data from 1200Z-1300Z indicate that Karl's circulation
remains well-defined, especially the inner-core wind field. The
ASCAT data also revealed a large patch of 35-kt winds 60-100 nmi
north and northeast of the center, and that is the intensity used
for this advisory. The ASCAT data supports trimming back the 34-kt
wind radius in the northeastern quadrant.

Karl continues to move westward or 275/13 kt. Karl is expected to
move westward along the southern periphery of the Bermuda-Azores
ridge for the next couple of days, followed by a west-northwestward
motion on days 3-5 as the cyclone approaches a weakness in the ridge
located between Bermuda and the Bahamas. The global and regional
models are in general agreement on this track scenario through 72
hours, After that time, the models diverge significantly with the
GFS, Navy COAMPS, and GFDL models taking Karl farther west as a
weaker and more shallow cyclone, whereas the ECMWF, UKMET, and HWRF
models move Karl more northwestward as a stronger and deeper system.
For now, the official forecast track remains a compromise between
these extremes, close to the consensus model solutions, due to the
lack of a stable, consistent run-to-run forecast by the models.

The GFS and ECMWF models continue to forecast the vertical wind
shear to decrease to less than 10 kt from now through the end of the
forecast period, with near-zero shear values occuring at times
between 72-120 hours. Such low shear values continue to favor a
strengthening trend, especially since Karl will be moving over 30C
SSTs by days 4 and 5.  However, a somewhat dry mid-level environment
is expected to temper the intensification process a little due to
occasional intrusions of dry air into the inner-core region of
Karl's circulation. Once the cyclone develops an eye feature, then
strengthening will become more robust due to the low wind shear
conditions and high sea-surface temperatures. Since these
parameters can not be forecast very far in advance, the official
intensity forecast continues to show slow strengthening throughout
the period and remains close to the intensity consensus model IVCN.

FORECAST POSITIONS AND MAX WINDS

INIT  18/2100Z 18.3N  45.0W   35 KT  40 MPH
 12H  19/0600Z 18.5N  47.0W   40 KT  45 MPH
 24H  19/1800Z 19.0N  49.7W   40 KT  45 MPH
 36H  20/0600Z 19.7N  52.2W   45 KT  50 MPH
 48H  20/1800Z 20.5N  54.7W   50 KT  60 MPH
 72H  21/1800Z 22.9N  59.7W   60 KT  70 MPH
 96H  22/1800Z 25.4N  64.2W   70 KT  80 MPH
120H  23/1800Z 28.0N  67.0W   80 KT  90 MPH

$$
Forecaster Stewart

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