[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 250840
TCDAT2

TROPICAL STORM KARL DISCUSSION NUMBER  44
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122016
500 AM AST SUN SEP 25 2016

Karl continues to produce a large mass of cold-topped convection
sheared to the northeast of the low-level center. Some of the cloud
tops are as cold as -83 deg C, which is unusual for that far north.
Earlier NASA/NOAA Global Hawk aircraft dropsondes measured surface
winds as high as 54 kt, and recent satellite intensity estimates
from TAFB and UW-CIMSS ADT are 60 kt and 57 kt, respectively. Based
on these data, and given that Karl is now moving at a forward speed
of at least 40 kt, the initial intensity has been conservatively
increased to 60 kt.

The initial motion estimate is now 055/40 kt.  Karl should continue
to move northeastward at 45-50 kt ahead of a broad deep-layer trough
until the cyclone is absorbed by a larger extratropical low in about
36 hours. The new official forecast track is just an update of the
previous advisory and remains near the middle of the tightly packed
guidance envelope, close to the consensus model TVCN and the
consensus of the GFS and ECMWF global models.

Karl is currently located over a ridge of anomalously warm water
with SSTs of 27C to 27.5C, which likely explains the unusually deep
and cold-topped convection associated with the cyclone. Karl has
about another 6 hours or so over SSTs greater than 26C, which could
allow the cyclone to strengthen into a tropical hurricane before it
reaches much colder waters. By 12 hours and beyond, Karl will be
moving over sea-surface temperatures colder than 20C north of 40N
latitude, which will result in the cyclone losing its deep
convection and tropical characteristics. However, interaction with
the aforementioned deep-layer trough and associated baroclinic
energy should help Karl to strengthen into a powerful post-tropical
low pressure system possessing hurricane-force winds. By 36 hours,
the system should be absorbed into a larger extratropical low to its
northwest. The new NHC intensity forecast and wind radii are based
on input from the NOAA Ocean Prediction Center.

FORECAST POSITIONS AND MAX WINDS

INIT  25/0900Z 37.5N  53.5W   60 KT  70 MPH
 12H  25/1800Z 41.3N  46.3W   70 KT  80 MPH...POST-TROPICAL
 24H  26/0600Z 48.3N  35.5W   60 KT  70 MPH...POST-TROPICAL
 36H  26/1800Z...ABSORBED BY LARGER EXTRATROPICAL LOW

$$
Forecaster Stewart
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