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Potential Tropical Cyclone Eighteen Discussion Number   3
NWS National Hurricane Center Miami FL       AL182017
500 AM EDT Sat Oct 28 2017

Scatterometer data from several hours ago indicated that the
disturbance consists of a broad circulation with a 100-150 n mi wide
area of light winds (less than 10 kt) and an equally as large radius
of maximum winds.  The system still does not appear to have a
well-defined center of circulation, and in fact, the ASCAT data also
showed that the maximum winds are down to about 30 kt.  The
associated deep convection is rather disorganized but has been
increasing in coverage near the estimated center during the past
few hours.

While the exact center is difficult to locate, the disturbance's
circulation appears to be moving just east of due north, or 010/9
kt.  The disturbance is beginning to accelerate in the flow ahead
of a deep-layer trough located over the eastern three-quarters of
the United States, and this pattern is expected to cause the system
to turn northeastward and accelerate further during the next couple
of days.  Although the track models are in agreement on this general
scenario, the overall guidance envelope has shifted a little
westward, most likely as a result of a repositioning of the
disturbance's current location.  The new NHC forecast has therefore
been nudged westward as well, lying closest to HCCA and TVCA models
through 36 hours.

The disturbance is currently located in its best environment
shear-wise, but the system's broad and elongated structure is
likely delaying intensification.  Although the shear will be
increasing, it should remain low enough for the next 12-18 hours to
support some strengthening if the circulation can tighten up.  In
addition, upper-level divergence is expected to increase, which
should also support some strengthening.  Since the disturbance has
such a broad circulation, the NHC intensity forecast closely
follows the intensity trends of the GFS and is not too different
from the previous advisory.  The system is now expected to merge
with a cold front and be extratropical by 48 hours and then
dissipate by 72 hours.

Even though the track forecast has shifted a little closer to South
Florida and the Florida Keys, the strongest winds are expected to
be well to the east and southeast of the center over the Atlantic
waters and the Bahamas.  Therefore, tropical storm watches or
warnings do not appear necessary for Florida at this time.


FORECAST POSITIONS AND MAX WINDS

INIT  28/0900Z 19.6N  84.2W   30 KT  35 MPH...POTENTIAL TROP CYCLONE
 12H  28/1800Z 21.7N  82.9W   35 KT  40 MPH...TROPICAL CYCLONE
 24H  29/0600Z 24.9N  80.1W   40 KT  45 MPH
 36H  29/1800Z 28.7N  76.5W   45 KT  50 MPH
 48H  30/0600Z 34.2N  71.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  31/0600Z...DISSIPATED

$$
Forecaster Berg

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