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Tropical Storm Gordon Discussion Number   9
NWS National Hurricane Center Miami FL       AL072018
1000 AM CDT Tue Sep 04 2018

After an earlier disruption of the inner-core convection due to some
southerly wind shear, Gordon has since developed a band of deep
convection very near the well-defined center noted in both NOAA
GOES-16 high-resolution satellite imagery and coastal NOAA WSR-88D
Doppler radars.  In addition, there has been a significant increase
in lightning activity during the past couple of hours in the inner
core, and Doppler radar velocities have increased to more than 45 kt
at 25,000 ft in the band of convection near the center. NOAA and Air
Force Reserve reconnaissance aircraft further indicate that Gordon
has maintained a small radius of maximum winds (RMW) of 10-15 nmi,
and that SFMR surface winds of 53-55 kt exist in the northeastern
quadrant. These data support an intensity of 55 kt.

The initial motion estimate is now toward the northwest at a
slightly slower forward, or 305/13 kt. There is basically no change
to the previous NHC forecast track reasoning. Reconnaissance fixes
continue to fall along the previous forecast track, and the latest
model guidance remains tightly packed and on top of the previous
NHC forecast. As a result, there is high confidence that Gordon will
make landfall along the north-central Gulf of Mexico coast in about
18 hours. After landfall, steering currents are forecast to weaken,
resulting in Gordon slowing down considerably, which will enhance
the heavy rainfall potential. By Friday, an approaching cold front
and associated mid-/upper-level trough are expected to break down
the subtropical ridge, allowing Gordon to turn northward and
eventually turn northeastward on Saturday and merge with the cold
front. The new official track forecast is near the model consensus
and essentially on top of the previous NHC track.

The aforementioned inner-core changes that have been occurring could
be a harbinger that Gordon is finally starting to intensify. Intense
lightning activity inside the small RMW is an indication of strong
updrafts, which can enhance the strengthening process. Although the
GFS and ECMWF models continue to forecast northwesterly shear of
10-15 kt, the explicit upper-level wind fields in the GFS, ECMWF,
and UKMET models indicate that the 200-mb flow is forecast to become
southeasterly and diffluent across Gordon in the next 12 h or so,
which would act to enhance the development of inner-core convection.
As result, the official intensity forecast maintains continuity and
shows Gordon becoming a hurricane before landfall, which is above
the guidance through that time. Once Gordon moves inland, the small
circulation should spin down fairly quickly, and that portion of the
forecast closely follows the various consensus-model intensity
guidance. By 120 h, Gordon's circulation is expected to merge with a
cold front over the Mississippi Valley region.

Key Messages:

1. Gordon is expected to bring life-threatening storm surge and
hurricane conditions to portions of the central Gulf Coast where a
Storm Surge Warning and Hurricane Warning are in effect. Residents
in these areas should listen to advice from their local officials.

2. Heavy rainfall from Gordon will affect the western Florida
Panhandle, southwest Alabama, southern and central Mississippi,
much of eastern Louisiana, and southern Arkansas, where totals could
reach as high as 12 inches. This rainfall will cause flash flooding
in portions of these areas.

FORECAST POSITIONS AND MAX WINDS

INIT  04/1500Z 28.5N  86.8W   55 KT  65 MPH
 12H  05/0000Z 29.8N  88.5W   65 KT  75 MPH
 24H  05/1200Z 31.5N  90.2W   35 KT  40 MPH...INLAND
 36H  06/0000Z 32.8N  91.6W   25 KT  30 MPH...INLAND
 48H  06/1200Z 33.7N  92.7W   20 KT  25 MPH...INLAND
 72H  07/1200Z 35.4N  94.3W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  08/1200Z 37.7N  93.8W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  09/1200Z 40.9N  90.7W   20 KT  25 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart

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