Area Forecast Discussion
Issued by NWS Huntsville, AL

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122
FXUS64 KHUN 220558
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
1258 AM CDT Sun Apr 22 2018

.UPDATE...
For 06Z TAFS.

&&

.NEAR TERM...(Rest of tonight)
Issued at 925 PM CDT Sat Apr 21 2018

The main surface low (storm system) continues to remain over south
central Texas this evening. One main area of convergence extends
north from it in the form of an inverted trough axis through central
Arkansas. This is where the majority of deeper moisture both aloft
and near the surface are confined. Therefore most of the
precipitation is occurring there. Further east, especially over
Alabama very dry air is still in place around 700 mb down to the
surface. Some virga is affecting western Mississippi, but at this
time even there it is too dry near the surface for any precipitation.

Models do show moisture convergence increasing significantly after
midnight over the area, especially towards daybreak on Sunday. For
this update, based on dry air in place and abundant cloud mid/high
cloud cover in place, delayed scattered precipitation chances until
between 2 and 4 AM. Between 5 and 7 AM, it does look like very
strong convergence and deeper moisture will coincide to produce more
widespread showers across northern Alabama and into southern middle
Tennessee. At this time, it looks like this should be rain, but some
of it could be moderate in intensity. Overall raised low temperatures
south of the TN/AL border, due to cloud cover and increasing winds
expected after midnight, as a 35 to 40 knot 925 mb jet develops over
the area. Luckily, the area should be decoupled from the brunt of
this jet, but winds around 10 mph in most locations are expected, but
higher elevations above 1000 feet could see winds between 10 and 20
mph.

.SHORT TERM...(Sunday through Monday night)
Issued at 300 PM CDT Sat Apr 21 2018

The cut-off/closed low will be located over central AR by Sunday
morning with an axis of moderate to heavy rain located over central
MS. Both of these features move west through the day and into the TN
Valley. As it stands now, forecast soundings indicate very limited to
no instability ahead of the system and have tried to pull back the
mention of thunder. It`s not until the afternoon hours when we start
to see the lapse rates aloft steepen do we see any increase in
instability and all of it will be elevated. So, have held onto the
lower end chances of thunderstorms embedded in the heavier precip
axis. PWATs do jump up to about 1.3-1.5 inches during the afternoon
hours and due to the slow movement of the low it`s possible that
storms could train over an area and the current rainfall totals might
be underdone. Due to the dry period leading into the event and things
are starting to grow, think that we can handle a good amount of rain.
For reference, flash flood guidance is upwards of 4 inches before we
need to start becoming concerned. So, for this event it`s likely
that the main impact from all of the rain will be river/stream
flooding and in coordination with the RFC`s this would only put river
levels to near minor flood stage. With that in mind have issued an
ESF discussing the potential for flooding along
rivers/streams/creeks. With constant rain and clouds tomorrow temps
will be on the cooler side and highs should only warm to the low 60s.

The core of the upper low moves over the area late Sunday evening
pushing the heavy rain/thunderstorm axis to the east. However, with
the continued steep lapse rates aloft and lift tied to the vort max
have continued higher end PoPs through much of Monday with a slight
chance of thunderstorms embedded in any precip. We start to lose the
deeper moisture with some drier air starting to work in aloft and
rainfall totals Monday won`t be nearly as high as Sunday. By Monday
night the system starts to finally lift off to the E/NE but we could
see a few wrap around showers on the backside of the system and have
kept PoPs in the forecast. Highs on Monday will be a few degrees
warmer with values in the mid to upper 60s. Lows both Sunday/Monday
night will be in the low to mid 50s, which is near normal for this
time of year.

.LONG TERM...(Tuesday through Friday)
Issued at 300 PM CDT Sat Apr 21 2018

Look for an unsettled pattern through the extended. The system that
is expected to bring a fair amount of rainfall Sunday-Monday will
still be lingering in the OH/TN Valley on Tuesday. This will keep
scattered showers lingering into Tuesday, but light QPF values. As a
shortwave in the Northern Plains sinks southward it will start to
lift the lingering trough northeastward and the associated sfc low
will start the trek up the east coast. Showers will begin to taper
off from west to east Tuesday evening with the progression of the
system. Afternoon highs will be in the upper 60s, cooling into the
lower 50s overnight.

On Wednesday, the system moving out of the Northern Plains will drag
a cold front into the area bringing showers back into the forecast.
The system has weakened quite a bit by this time and has phased with
what was left of the old lingering SE system and a stronger northern
stream shortwave that moved down from Canada and into the Great Lakes
region. With little moisture recovery, a weak front and model
discrepancy on the timing/strength of the phasing, kept with lower
POPs Wed and Thurs. Temps will be a rinse and repeat with highs in
the upper 60s and lows in the upper 40s. Once again, only a small
break of dry weather for the TN Valley, the next system arrives on
Friday as a cold front brings another round of showers and slightly
cooler low temps.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1258 AM CDT Sun Apr 22 2018

VFR conditions at the start of the TAF will gradually deteriorate in
the overnight, as a system to our west nears. CIGs should be reduced
into the MVFR range ~2kft AGL around 09Z over NW Alabama, and east of
the I-65 corridor towards daybreak. Along with lowering CIG values,
VIS should also be reduced into the MVFR range as showers begin
impacting the terminals, with IFR or lower reductions in the heavier
showers. Along with showers, an isolated thunderstorm is possible,
mainly embedded within the heaviest showers. Strong winds could
become an issue by the afternoon and early evening, as a SE flow in
the 15-25kt range is expected to develop. Winds could be stronger,
with low-level vertical wind shear also possible.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

NEAR TERM...KTW
SHORT TERM...Stumpf
LONG TERM...JMS
AVIATION...RSB


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