[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 201436
TCDAT2

TROPICAL STORM KARL DISCUSSION NUMBER  25
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122016
ISSUED BY THE NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1100 AM AST TUE SEP 20 2016

Karl's center remains exposed to the southwest of the deep
convection, as it appears that the cyclone has been unable to
escape the influence of a mid-/upper-level low to its west.  Dvorak
Current Intensity estimates remain 3.0 from TAFB and 2.5 from SAB,
and the latest UW-CIMSS ADT estimate is T2.5.  The initial
intensity is therefore held at 35 kt.

SHIPS and UW-CIMSS shear analyses indicate that the vertical shear
that has been affecting Karl has not yet decreased, but it is now
southerly at around 15 kt.  The global models continue to indicate
that the shear should decrease during the next few days, but this
probably won't happen until Karl becomes more separated from the
upper-level low.  The thermodynamic environment has improved
slightly since yesterday, as the cyclone appears to be associated
with a large moisture envelope with mid-level relative humidities
increasing into the 50 percent range.  Little change in strength is
likely during the next 24 hours, but after that time intensification
is expected due to warm SSTs, more moisture, and lower shear.  The
official intensity forecast continues to show Karl as a hurricane in
the 3-5 day range, but the updated forecast has been nudged downward
slightly based on the latest guidance.  The new forecast is very
close to SHIPS guidance and is near the upper end of the guidance
envelope.

Karl's center has been straddling the 20th parallel, but the
12-hour motion estimate is 275/15 kt.  Karl should be reaching the
western extent of the subtropical high during the next 48 hours,
and its motion should therefore turn northwestward by day 2 and
then northward by day 4.  After that time, the cyclone is expected
to accelerate toward the northeast when it gets picked up by the
mid-latitude westerlies ahead of a trough.  The cross-track spread
in the guidance has decreased since yesterday for the time period
when Karl recurves, although there are significant speed
differences after recurvature.  Most notably, the ECMWF is
significantly slower than the other models and doesn't show Karl
being picked up as quickly by the mid-latitude trough.  To split
the difference, the updated track forecast is very similar to the
previous one and is close to a consensus of the GFS and ECMWF.

FORECAST POSITIONS AND MAX WINDS

INIT  20/1500Z 20.0N  54.3W   35 KT  40 MPH
 12H  21/0000Z 20.7N  56.0W   35 KT  40 MPH
 24H  21/1200Z 21.9N  58.3W   35 KT  40 MPH
 36H  22/0000Z 23.5N  60.5W   45 KT  50 MPH
 48H  22/1200Z 25.1N  62.7W   55 KT  65 MPH
 72H  23/1200Z 27.6N  65.2W   70 KT  80 MPH
 96H  24/1200Z 31.0N  63.5W   85 KT 100 MPH
120H  25/1200Z 36.5N  54.5W   90 KT 105 MPH

$$
Forecaster Berg/Krekeler
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