[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 200858
TCDAT2

TROPICAL STORM KARL DISCUSSION NUMBER  24
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122016
500 AM AST TUE SEP 20 2016

Karl's satellite appearance late yesterday gave the impression that
it had increased some in organization, with the low-level center
partially underneath a large mass of deep convection.  Since that
time, the low-level center appears to have outrun the convection,
becoming exposed well to the west of the remnant convective mass
whose cloud top temperatures have warmed significantly.  The cloud
pattern is typical of a tropical cyclone undergoing significant
westerly shear.  Dvorak classifications have decreased to T2.5 from
TAFB and SAB, and on this basis, the initial intensity estimate is
lowered to 35 kt.

The initial motion is a faster and a more westerly 290/15.  Karl
should continue to be steered generally west-northwestward around
the southwestern periphery of well-defined low- to mid-level
subtropical ridge over the central Atlantic during the next couple
of days.  The cyclone should move into weakness in the ridge along
65W in 48 to 72 hours, which should result in a gradual turn toward
the north with a considerable decrease in forward speed by day 4.
Around 96 hours, Karl should become susceptible to the mid-latitude
westerly flow after it crosses 30N as it begins to undergo a sharp
recurvature.  The models vary tremendously on how quickly Karl will
recurve by day 5, with the GFS-based guidance whisking the cyclone
rapidly northeastward in response to a deep-layer trough moving
through the northeastern United States and Atlantic Canada. The
ECMWF and the majority of its ensemble members show Karl lagging way
behind before being ejected northeastward by a slowly amplifying
trough.  The new track forecast is shifted to the left and
is slightly faster through day 3, largely because of the initial
re-positioning of the cyclone.  After that time, a compromise
between the much faster GFS-based guidance and the ECMWF results in
a slower and somewhat more eastward forecast track.

Karl can be seen approaching an upper-tropospheric cold low in water
vapor imagery.  The unfavorable juxtaposition of Karl and this
feature should not result in much intensification for about the next
36 hours, since the flow over Karl is convergent and there is some
deep-layer westerly shear.  This is the rationale for keeping the
short-term intensity forecast low.  By 48 hours and beyond, the
shear is forecast to have finally decreased while the cyclone
reaches near 30 deg C waters, and intensification is likely.  With
the flow becoming increasingly divergent over Karl around the time
it recurves in 3 to 4 days, the intensification could occur more
rapidly than this forecast indicates, and there is some chance that
Karl could become a major hurricane.  Late in the period, increasing
southwesterly shear could become a mitigating factor and the
intensity is shown to be leveling off by day 5.  The new NHC
intensity forecast is a little lower than the multi-model consensus
initially and about the same or a little lower at days 3-5.

FORECAST POSITIONS AND MAX WINDS

INIT  20/0900Z 20.2N  52.8W   35 KT  40 MPH
 12H  20/1800Z 20.9N  54.6W   35 KT  40 MPH
 24H  21/0600Z 21.7N  57.0W   35 KT  40 MPH
 36H  21/1800Z 23.0N  59.2W   45 KT  50 MPH
 48H  22/0600Z 24.7N  61.5W   55 KT  65 MPH
 72H  23/0600Z 27.4N  64.7W   70 KT  80 MPH
 96H  24/0600Z 30.3N  64.5W   90 KT 105 MPH
120H  25/0600Z 34.0N  57.9W   90 KT 105 MPH

$$
Forecaster Kimberlain
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