Special Weather Statement, NWS Huntsville, AL

Area Forecast Discussion
National Weather Service Huntsville AL
248 PM CDT Sat Oct 7 2017

.NEAR TERM...(Tonight)
Issued at 245 PM CDT Sat Oct 7 2017

Conditions have remained quiet across the TN Valley so far today, as
an upper trough moves into the Upper Midwest and Hurricane Nate
moves into the north central Gulf of Mexico. Several bands of light
rain can be seen on area radars; one to our west along a convergence
axis associated with the trough and another to our south moving
northward. Given this activity, the area should begin to see light
to moderate rain push from the west and the south through the late
afternoon hours. Extensive cloud cover has limited the
destabilization across the area, however some marginal instability
may allow for an isolated thunderstorms through the early evening
hours.

The coverage of showers will increase during the evening and
overnight hours, as Hurricane Nate continues to move north and makes
landfall along the central Gulf coast. As stated in previous
discussion, PW values rise to near 2.25 inches which exceeds well
above the daily max for early October. The abundance in moisture,
along with continued bands of convergence will make for heavy rain
producers in any of the showers/storms that develop. What limited
instability we currently have will decrease further overnight,
however there will be just enough elevated CAPE to keep isolated
thunderstorms in the forecast. Despite the heavy rainfall potential,
storms should be relatively quick moving so not anticipating a flood
or flash flood threat during the overnight period. Temperatures will
be rather mild given the overcast skies and moisture, with overnight
lows in the 70s.

.SHORT TERM...(Sunday through Monday)
Issued at 245 PM CDT Sat Oct 7 2017

Conditions will deteriorate further on Sunday, as Nate weakens to a
tropical storm and moves northward into Central Alabama. Model
guidance is still somewhat split on the exact track and speed of
Nate, however based on the latest guidance from NHC, the GFS is
slightly more favorable. Given the latest forecast and the quick
forward speed of the system, tropical storm force winds may reach
the southeastern portion of the forecast area by mid morning Sunday.
However, there is still some uncertainty on just how strong the
winds will be and if our sustained winds will reach above 38 mph.
Again, the highest probability for this would be in northeast
Alabama, generally along and east of Interstate 65. Wind gusts will
approach the 50 to 55 mph range. In addition to the strong winds,
heavy rainfall will continue in waves through the day. Rainfall
totals of up to 4 inches are possible across our eastern counties,
with lesser amounts to the west. Given the high amount of rainfall
falling around the same time as the highest winds, it will not take
much to bring weaker trees and tree limbs down. Thus, a wind
advisory will be issued for most of the area, with the exception of
our three most western counties.

The tornado threat at this point appears to be minimal, however this
(and really most of the forecast) depends on exactly where the
center of Nate tracks. With the TN Valley right on the northern edge
of the system, conditions do not look favorable as we won't be able
to tap into much instability. But again, if there's even the
slightest westward shift, the threat would increase. Even still,
given the tropical airmass in place, any tornadoes that were to
develop would be rather weak.

Tropical storm Nate will move out of the area on Sunday evening,
with conditions rapidly improving after sunset. Lingering wrap
around moisture will keep light rain across the area through the
overnight hours on Sunday, though little in the way of accumulations
are expected. The TN Valley will then be on the western periphery of
an upper ridge, keeping southwest flow across the area through much
of the vertical column. The influx of moisture will continue in the
TN Valley, with little to no airmass change. Meanwhile, several weak
disturbances are expected to move around the ridge and enhance the
coverage of showers and thunderstorms during the day Monday. This
activity will bring another round to half an inch to an inch of
rain. But this should not pose an additional flash flood threat for
the area.

.LONG TERM...(Monday night through Friday)
Issued at 245 PM CDT Sat Oct 7 2017

The subtropical ridge will become established across the
southeastern CONUS, while a weak upper trough will swing through the
northern Plains and into the Great lakes region. A weak frontal
boundary will approach the area on Tuesday and may reach this far
south and east by Wednesday. This, along with the moisture in place,
will keep the chance for isolated to scattered thunderstorms in the
forecast for Tuesday and Wednesday. The trough/front may not quite
make it into the area as the subtropical ridge keeps its hold just
to our east. The upper ridge continues to strengthen over the area
on Thursday, with rain chances diminishing and coming to an end by
the end of the work week.

Temperatures through the long term period will be on the hot side,
given the southerly flow and upper ridge. Highs each day will climb
into the lower to mid 80s once again, with overnight lows in the 60s.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1245 PM CDT Sat Oct 7 2017

VFR conditions with the latest obs at KMSL and KHSV. However,
ceilings at KHSV have been fluctuating between upper-end MVFR and VFR
recently. Conditions are likely to begin deteriorating later this
afternoon as SHRA activity is expected to expand in coverage across
the region, per the suite of hi-res guidance. Regional radars have
shown some localized banding of precipitation along low-lvl
convergence axes, but not too much in our area so far. Confidence
remains moderate to high in the expanded coverage of SHRA, but they
will tend to be relatively small and quick-moving. Vis impacts thus
will be limited, but ceilings are likely to gradually lower through
the day with the influx of maritime tropical air into the region.
SHRA activity is likely to continue overnight with ceilings gradually
lowering to low-end MVFR/Alt Mins by daybreak. However, confidence
is rather low in ceiling heights and IFR ceilings are possible.
Stronger winds will not likely begin at KHSV until after 18Z period,
with the remnants of Nate could bring sustained winds ~25-30 kts to
the site around 18-21Z.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Flash Flood Watch from Sunday morning through Monday morning for
     ALZ008>010.

     Tropical Storm Watch for ALZ016.

TN...NONE.
&&

NEAR TERM...73
SHORT TERM...73
LONG TERM...73
AVIATION...KDW

For more information please visit our website
at weather.gov/huntsville.

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