[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 071455
TCDAT1

Hurricane Nate Discussion Number  13
NWS National Hurricane Center Miami FL       AL162017
1000 AM CDT Sat Oct 07 2017

Nate is showing improved organization in satellite imagery this
morning, with the formation of a symmetric central dense overcast
with cloud tops colder than -80C.  In addition, Air Force Reserve
and NOAA Hurricane Hunter aircraft have reported a central pressure
near 984 mb inside a 25 n mi wide eye that is starting to form under
the overcast.  The planes have reported 700-mb flight level winds of
up to 88 kt and surface wind estimates from the SFMR instrument
between 75-85 kt.  Based on these data, the initial intensity is
increased to a possibly conservative 80 kt.

Conditions appear favorable for Nate to continue strengthening for
the next 12 h or so before it encounters land and increasing shear.
The new intensity forecast thus follows the SHIPS model in showing
Nate become a Category 2 hurricane before landfall on the central
Gulf Coast.  After landfall, Nate should weaken rapidly as it moves
through the eastern United States.  The cyclone is now forecast to
become extratropical near the 72 hr point and dissipate completely
by 96 h.

The initial motion is toward the north-northwest at a very rapid 24
kt.  The hurricane is being steered by the flow between a large
cyclonic gyre over the southwestern Gulf of Mexico and a mid-level
ridge over the western Atlantic.  In addition, a mid-latitude trough
in the westerlies is moving eastward through the central United
States.  This pattern is expected to cause Nate to turn northward
during the next 12-18 h as it rounds the western end of the ridge,
followed by a turn toward the northeast as it enters the westerlies.
The new forecast track is nudged a little to the left of the
previous track, and it lies near the center of the tightly clustered
guidance.

KEY MESSAGES:

1. Nate is expected to bring life-threatening storm surge flooding
near and well east of where the center makes landfall, and a storm
surge warning is in effect from Morgan City, Louisiana, to the
Okaloosa/Walton county line in Florida.  Maximum flooding of 7 to 11
feet above ground level is expected in portions of southeastern
Louisiana and along the Mississippi coast.  Residents in these areas
should immediately heed any evacuation instructions given by local
officials.

2. Nate is forecast to reach Category 2 intensity before landfall.
A hurricane warning is in effect for portions of the northern Gulf
Coast from Louisiana to Alabama, with the strongest winds expected
to occur primarily to the east of the center.  Preparations to
protect life and property should be rushed to completion in these
areas, as tropical storm conditions will first arrive in the warning
area this afternoon.

3. Nate's fast forward speed after landfall will bring tropical-
storm-force winds well inland across portions of the southeastern
U.S.  Tropical storm watches and warnings are in effect for portions
of southeastern Mississippi, Alabama, and western Georgia.

4. Nate will bring heavy rainfall of 3 to 6 inches with isolated
totals of 10 inches east of the Mississippi River from the central
Gulf Coast into the Deep South, eastern Tennessee Valley, and
southern Appalachians through Monday, resulting in the potential for
flash flooding in these areas.

5. Moisture from Nate interacting with a frontal zone will also
bring 2 to 4 inches of rain with isolated totals of 6 inches across
the Ohio Valley and central Appalachians Sunday and Monday, which
will increase the risk for flash flooding across these locations.

FORECAST POSITIONS AND MAX WINDS

INIT  07/1500Z 26.6N  88.4W   80 KT  90 MPH
 12H  08/0000Z 29.1N  89.0W   90 KT 105 MPH
 24H  08/1200Z 32.1N  88.3W   60 KT  70 MPH...INLAND
 36H  09/0000Z 35.7N  85.5W   35 KT  40 MPH...INLAND
 48H  09/1200Z 39.2N  80.6W   30 KT  35 MPH...INLAND
 72H  10/1200Z 44.5N  67.5W   25 KT  30 MPH...POST-TROP/EXTRATROP
 96H  11/1200Z...DISSIPATED

$$
Forecaster Beven
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