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Potential Tropical Cyclone THREE (Text)


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Potential Tropical Cyclone Three Discussion Number   1
NWS National Hurricane Center Miami FL       AL032017
400 PM CDT Mon Jun 19 2017

A pair of ASCAT passes between 15 and 16Z showed that the
disturbance over the Gulf of Mexico has an area of tropical-storm-
force winds within a large area of deep convection 100 to 150 n mi
northeast of a broad low center. The ASCAT data and visible
satellite imagery show that the center is not well defined, and in
fact multiple low-level swirls are evident in the latest imagery. An
Air Force Reserve Hurricane Hunter aircraft is currently
investigating the system and will provide more data on its wind
structure.

The NWS now has the option to issue advisories, watches, and
warnings for disturbances that are not yet a tropical cyclone, but
which pose the threat of bringing tropical storm or hurricane
conditions to land areas within 48 hours. Under previous policy
this was not possible.  These systems are known as Potential
Tropical Cyclones in advisory products and are numbered from the
same list as depressions. Because of the threat to the central
Gulf coast, advisories have been initiated on Potential Tropical
Cyclone Three and the appropriate watches/warnings have been issued.
Users should be aware that forecast uncertainty for disturbances is
generally larger than for tropical cyclones, especially beyond 48-72
hours.

The initial intensity is set to 35 kt based on the ASCAT data. The
initial pressure is set to 1002 mb based on extrapolated data from
the aircraft. Only modest strengthening is expected given the
moderate to strong southwesterly shear of around 30 kt expected to
persist over the system during the next couple of days. The NHC
intensity forecast is close to the latest intensity consensus aid.

Given the broad nature of the low, the initial motion estimate is a
rather uncertain 350/08. During the first 12 to 24 hours the system
will move northward to north-northwestward due to the a weakness in
the subtropical ridge over the northern Gulf and then the system
should slow and turn more northwestward as it interacts with an
upper-level low currently situated off the Texas coast.  The details
of this interaction will have significant implications on the track
of the system, and there remains a fair amount of model spread.  The
HWRF is on the right side of the guidance envelope, with a track
toward southeastern Louisiana while the UKMET and ECMWF show a
solution toward southwest Louisiana or extreme eastern Texas. For
now, the NHC forecast lies in the middle of the guidance envelope
and a little left of the TCVA consensus aid. Given the disorganized
nature of the circulation and the fact that the wind and rain
hazards extend well north and east of the center, users are
encouraged to not focus on the details of the track forecast.

The primary hazard from this disturbance is expected to be heavy
rainfall over portions of the central and eastern Gulf Coast.

FORECAST POSITIONS AND MAX WINDS

INIT  19/2100Z 24.7N  88.7W   35 KT  40 MPH...POTENTIAL TROP CYCLONE
 12H  20/0600Z 26.0N  89.5W   35 KT  40 MPH
 24H  20/1800Z 27.2N  90.6W   40 KT  45 MPH...TROPICAL CYCLONE
 36H  21/0600Z 27.9N  91.4W   40 KT  45 MPH
 48H  21/1800Z 28.7N  92.3W   40 KT  45 MPH
 72H  22/1800Z 32.0N  93.5W   30 KT  35 MPH...INLAND
 96H  23/1800Z 35.5N  90.5W   25 KT  30 MPH...POST-TROP/INLAND
120H  24/1800Z...DISSIPATED

$$
Forecaster Brennan

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Page last modified: Friday, 20-Oct-2017 12:09:06 UTC