[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 070255
TCDAT1

Tropical Storm Nate Discussion Number  11
NWS National Hurricane Center Miami FL       AL162017
1000 PM CDT Fri Oct 06 2017

Recent data from NOAA and Air Force Reserve reconnaissance flights
indicate that Nate continues to strengthen and is near hurricane
strength.  The NOAA plane measured a maximum 700-mb flight-level
wind of 75 kt, but the SFMR instruments on both planes have only
measured surface winds as high as 55 kt.  It seems that the highest
winds haven't quite mixed down to the surface, but it's likely that
the system is at least producing surface winds of 60 kt, which will
be the initial intensity for this advisory.  The NOAA flight also
reported that an eyewall is forming to the east of Nate's center.

Nate has been accelerating toward the north-northwest between a
large cyclonic gyre centered over southern Mexico and a mid-level
high located over the southwestern Atlantic, and the initial motion
estimate is 340/19 kt.  This north-northwestward motion is expected
to continue for the next 24 hours, with Nate turning northward and
slowing down only slightly around the time it is forecast to cross
the northern Gulf coast between 24-36 hours.  After landfall, the
cyclone is expected to get swept up ahead of a large mid-latitude
trough, and accelerate northeastward over the eastern United
States.  At least up until landfall, the track guidance is in good
agreement, and the NHC forecast is relatively unchanged from the
previous one.  After landfall, the new forecast is just a tad slower
in order to give some credence to the slower solution provided by
the ECMWF model.

Atmospheric and oceanic conditions appear conducive for Nate to
continue strengthening up until the time it makes landfall along
the northern Gulf coast.  Nate is likely to become a hurricane
overnight, and the new NHC forecast has been raised to a peak
intensity of 75 kt at 24 hours based on guidance from the SHIPS and
LGEM models.  The intensity consensus and HCCA are lower than that
due to the HWRF solution, which shows absolutely no strengthening
before landfall.  Disregarding that solution, it seems prudent to
be above the consensus, close to the upper end of the guidance
envelope.  Rapid intensification still cannot be ruled out since
the indices are showing a 50/50 chance of a 25-kt increase over
the next 24 hours.  If Nate becomes a hurricane soon, then
additional increases in the official intensity forecast may be
necessary in subsequent advisories.

Aircraft data indicate that Nate is an asymmetric storm, with most
of the winds located on the eastern side of the circulation, and
this structure is likely to continue until landfall due to the
cyclone's fast forward speed.  Therefore, locations to the east of
where Nate makes landfall are expected to receive significantly
stronger winds than locations to the west of the center. Regardless,
there is still too much uncertainty to know exactly where landfall
will occur, and all locations within the hurricane warning area
should be preparing for hurricane-force winds.


KEY MESSAGES:

1. Life-threatening storm surge flooding is likely along portions
of the northern Gulf Coast, and a storm surge warning is now in
effect from Morgan City, Louisiana, to the Okaloosa/Walton county
line in Florida.  Residents in these areas should heed any
evacuation instructions given by local officials.

2. A hurricane warning is in effect for portions of the northern
Gulf Coast from Louisiana to Alabama, and preparations to protect
life and property should be rushed to completion in these
areas.

3. 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 the potential
for flash flooding in these areas.

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


FORECAST POSITIONS AND MAX WINDS

INIT  07/0300Z 22.3N  86.4W   60 KT  70 MPH
 12H  07/1200Z 25.1N  87.8W   70 KT  80 MPH
 24H  08/0000Z 28.5N  88.9W   75 KT  85 MPH
 36H  08/1200Z 31.4N  88.1W   65 KT  75 MPH...INLAND
 48H  09/0000Z 34.7N  85.8W   35 KT  40 MPH...INLAND
 72H  10/0000Z 40.5N  76.0W   30 KT  35 MPH...POST-TROP/REMNT LOW
 96H  11/0000Z 44.5N  63.5W   30 KT  35 MPH...POST-TROP/REMNT LOW
120H  12/0000Z...ABSORBED

$$
Forecaster Berg
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