[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 181454
TCDAT2

TROPICAL STORM KARL DISCUSSION NUMBER  17
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122016
1100 AM AST SUN SEP 18 2016

The overall appearance of Karl has changed little in satellite
imagery since the previous advisory.  However, recent water vapor
imagery suggest that the strong southwesterly shear that has been
plaguing the cyclone for the past few days is beginning to relax.
The initial wind speed is held at 35 kt based on a TAFB satellite
estimate of T2.5/35 kt.  Ship BATFR18, which has been skirting the
eastern portion of Karl's circulation the past 12 hours, has
reported winds as high as 32 kt.  That data has been helpful in
determining the extent of the 34-kt wind radius in the northeastern
quadrant.

Karl has trended westward and the initial motion estimate is now
270/11 kt.  A deep-layer subtropical ridge over the central Atlantic
to the north of Karl is expected to keep the cyclone moving in a
general westward direction for the next 48 hours or so, followed by
west-northwestward motion during the remainder of the forecast
period.  The new NHC track forecast is similar to the previous
advisory, and lies close to the various consensus model solutions.

Although Karl's convective pattern is currently somewhat disheveled
due to the hostile shear and dry mid-level conditions that the
cyclone has been encountering the past several days, the surface
wind field has remained remarkably robust, including a tight
inner-core.  The GFS and ECMWF models are forecasting the vertical
wind shear to decrease to less than 5 kt from 24-120 hours, which
favors a strengthening trend, especially since Karl will be moving
29-30C SSTs during that time.  However, only a marginally moist
mid-level environment is expected, a condition that could slow down
the intensification process.  The official intensity forecast
follows the trend of the previous advisory, showing gradual
strengthening throughout the forecast period, and remains on the
conservative side close to the intensity consensus model IVCN.

FORECAST POSITIONS AND MAX WINDS

INIT  18/1500Z 18.0N  43.4W   35 KT  40 MPH
 12H  19/0000Z 18.2N  45.5W   35 KT  40 MPH
 24H  19/1200Z 18.6N  48.2W   40 KT  45 MPH
 36H  20/0000Z 19.3N  50.9W   45 KT  50 MPH
 48H  20/1200Z 20.2N  53.3W   50 KT  60 MPH
 72H  21/1200Z 22.3N  58.4W   55 KT  65 MPH
 96H  22/1200Z 24.7N  63.2W   65 KT  75 MPH
120H  23/1200Z 27.3N  66.7W   75 KT  85 MPH

$$
Forecaster Stewart
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