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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 160548 RRA

Area Forecast Discussion
National Weather Service Huntsville AL
1246 AM CDT Mon Jul 16 2018

For 06Z TAFS.


.NEAR TERM...(Rest of tonight)
Issued at 1009 PM CDT Sun Jul 15 2018

Lingering pockets of light-moderate rain persist across the TN 
Valley this evening, as the remnants of deeper afternoon convection 
continue to dissipate. However, we are closely monitoring the 
progress of a small but decaying MCS, as it shifts east- 
southeastward across southwestern TN/northern MS. In spite of the 
recent weakening trend, it appears as if the stratiform rain shield 
will advance as far southeastward as northwest AL, and POPs were 
increased slightly across these zones to account for this.

However, of more concern is the potential impact from a large 
outflow boundary associated with this feature, which could initiate 
development of new convection around the time of a subtle increase in
the low-level jet closer to sunrise. Although the coverage of 
thunderstorms in this scenario would likely be highest beginning 
shortly after 12Z, we have shown a gradual increase in POPs 
(especially for the northwestern half of the CWFA) in the 09-12Z 
period. Forecast soundings depict a thermodynamic environment 
supportive of frequent cloud to ground lightning and locally heavy 
rainfall as the primary nocturnal storm threats, although a few of 
the stronger cells could produce brief wind gusts in the 30-40 MPH 

Other gridded forecast elements are on track at this point, with 
only minor adjustments made to sky cover grids to indicate a return 
of lower stratus clouds once again late tonight. Conditions will 
remain warm/muggy for the entire region, with lows ranging from the 
lower 70s in northeast AL to the mid 70s elsewhere.

.SHORT TERM...(Monday through Tuesday Night)
Issued at 306 PM CDT Sun Jul 15 2018

As an upper-level trough swings across southern Canada, it will
continue to break down the ridge that was anchored across the 
Southeast. This will place us in a northwesterly flow pattern,
allowing a series of disturbances to dive down into the region. At
the sfc, a low moving out of the Great Lakes will pull a cold front 
into the TN Valley by Tuesday morning. With moist and unstable 
conditions, scattered to numerous showers and storms are possible 
Monday and Tuesday. Heavy rainfall, gusty winds and frequent 
lightning will again be the main threats with PWATS still ~2" and 
CAPE 1000-2000 J/kg. Steering flow will slightly improve, allowing 
for storms to have a lower chance of producing flooding. The increase
in storm coverage and cloud cover will limit heating, and thus temps
should only reach around 90 degrees. This will keep heat index 
values below heat advisory criteria over the next couple days. As the
front pushes through late Tuesday and drier air filters in, storm 
chances will decrease allowing for a drier air mass to work in by 
Tuesday night.

.LONG TERM...(Wednesday through Saturday) 
Issued at 306 PM CDT Sun Jul 15 2018

Large area of high pressure is xpcted to be in place across the 
Great Lakes/OH Valley states to start off this part of the forecast 
package, with a cold front located just south of the cntrl TN Valley.
Some slightly drier air between H5-H85 should also be in place over 
the area thx to the passage of the front earlier, which should offset
much in the way of rain chances except for perhaps the far SW corner
of the forecast area where slightly better convergence near the 
front may allow for a few showers/tstms thru Wed. Temps will still be
seasonably warm with highs in the upper 80s/near 90F, although the 
drier air will keep afternoon heat indices more in the lower/mid 90s.

Rain chances begin to gradually increase Thu/Fri, as the front to the
south rebounds to the north as a warm front. With the warm front
moving nwd, a low pressure system out of the mid Plains states also
looks to translate ewd into the Midwest/OH Valley areas, which should
help with the influx of moisture into the region. The latter half of
the global models are also suggesting an upper shrtwv/trough carving
thru the Great Lakes, placing the region in more of a nwly flow 
regime. All of this should result in sct showers/tstms developing 
across the mid TN Valley by Fri and persisting into the weekend. Even
with the increase in rainfall, highs still look to be fairly 
seasonal for this time of the yr, with temps again maxing out near 
the 90F mark for most locations, while overnight lows trend predom in
the upper 60s to lower 70s.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1246 AM CDT Sun Jul 16 2018

A weak boundary to the north of the terminals will slowly push south
into northern Alabama around daybreak. At this time, included a 
tempo (06Z to 10Z) group for vsbys dropping to around 5SM due to fog
at KHSV (due to light winds and some breaks in cloud cover expected 
over the next few hours). Also, included a period of IFR cigs in this
tempo group, due to recent broken deck below 1000 feet showing up in
current observations. This may continue periodically until around 
10Z at KHSV. Further west, thicker/more persistent higher cloud 
cover around 5000 feet will likely keep even patchy fog from forming 
at KMSL. Some periodic VFR cigs around 5000 feet could affect KMSL 
through 08Z at times, but cigs should become more persistent after 
08Z. There is some uncertainty whether a stratus deck will form at 
KMSL, due to the more persistent 5000 foot cigs moving in. Further 
east at KHSV, this persistent area of 5000 foot cigs should hold off 
until around 10Z, but expect at return to predominant 5000 foot cigs 
by then. Only included a prob30 group at KHSV between 15Z and 20Z due
to lower confidence of precipitation activity. Sfc winds will remain
arnd 5 kts or less, and assume a more uniform SSW component later 
this evening before veering to WSW by end of the TAF period. VFR and 
drier conditions should return after 18Z at both terminals. 





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