[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 222051
TCDAT2

TROPICAL STORM KARL DISCUSSION NUMBER  34
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122016
ISSUED BY THE NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
500 PM AST THU SEP 22 2016

Air Force Reserve and NOAA Hurricane Hunter aircraft have been
flying in Karl, and the data indicate that the cyclone has
re-intensified to a 35-kt tropical storm.  The maximum 850-mb
flight-level wind so far is 45 kt, which equates to an intensity of
about 35 kt.  This is also supported by SFMR observations.  In
addition, dropsonde data suggest lowering the central pressure down
to 1005 mb. Combined with the aircraft data, scatterometer winds
within the convective bands indicate that tropical-storm-force winds
extend 120 nm to the north of the center of circulation.

Karl has continued its northwestward progression at 305 degrees
with a speed of 14 knots.  The synoptic pattern around this system
features a mid-level high off to its northeast while a mid-level
low is positioned to the north of Puerto Rico.  Karl is forecast to
move around the western extent of the anticyclone which will make
it turn more northward by 36 hours before accelerating
northeastward into the mid-latitude westerlies by 48 hours.  Based
on the adjustment of the initial position provided by the aircraft,
the track guidance has shifted a bit left of the positions indicated
in the previous advisory.  As a result, the updated official
forecast has been adjusted westward during the first 48 hours, lying
a bit closer to the GFS/GEFS mean clustering while giving some
credit to the TVCN multi-model consensus.  Later in the period, the
ECMWF solution was deemed too slow and was not weighted as heavily
at that point.

Karl lies right along the edge of a zone of higher shear to its
west, and the various analyses are showing about 15 to 20 knots of
southeasterly shear over the system.  The global models continue to
show the shear decreasing at least some during the next couple of
days, but there are differences.  For example, GFS fields show the
shear decreasing to around 5 knots or less by Friday morning
through evening, while the ECMWF only shows the shear decreasing to
around 10 knots during the same period.  This continues to make the
intensity forecast difficult.  At this point, the intensity
forecast was generally left unchanged which allows Karl to become a
Category 1 hurricane by Sunday.

Based on the latest forecast, the western edge of the
tropical-storm-force wind field could begin to affect Bermuda
during the next 24 to 36 hours.  The Bermuda Weather Service has
therefore issued a Tropical Storm Warning for the island.

FORECAST POSITIONS AND MAX WINDS

INIT  22/2100Z 25.5N  63.2W   35 KT  40 MPH
 12H  23/0600Z 26.8N  64.4W   40 KT  45 MPH
 24H  23/1800Z 28.8N  65.1W   45 KT  50 MPH
 36H  24/0600Z 30.8N  64.2W   50 KT  60 MPH
 48H  24/1800Z 32.7N  61.0W   60 KT  70 MPH
 72H  25/1800Z 39.7N  47.0W   70 KT  80 MPH
 96H  26/1800Z 49.0N  29.0W   55 KT  65 MPH...POST-TROP/EXTRATROP
120H  27/1800Z...DISSIPATED

$$
Forecaster Campbell/Rubin-Oster/Berg
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