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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 212008

Area Forecast Discussion
National Weather Service Huntsville AL
308 PM CDT Thu Jun 21 2018

.NEAR TERM...(Tonight)
Issued at 308 PM CDT Thu Jun 21 2018

An upper-low over Missouri will continue to deepen as it tracks into
the Ohio Valley this evening and overnight. Ahead of this feature a
shortwave trough/cold front as triggered a growing line of strong to
severe thunderstorms over the Mississippi Delta region of eastern
Arkansas. This line of storms will continue to push ESE/SE across
Western Tennessee and Northern Mississippi late this afternoon and
potentially into Northwest Alabama by 00-01z. Current observations
suggest the thermodynamic environment is less favorable across
Northern Alabama, with CAPE values around 1000-1500 J/kg (as opposed
to 2500-3000 J/kg further to the NW). Thus, am expecting this
activity to weaken by the time these storms make it into the
Tennessee Valley (especially as the atmosphere stabilizes with the
loss of solar heating). However, cannot rule out a localized strong
to perhaps marginally severe storm across NW Alabama late this 
evening -- mainly from gusty, damaging outflow winds. 

As alluded to, this activity will continue to weaken as it moves 
across the region between 03-06z, with generally scattered general 
thunderstorms prevailing through the remainder of the night. The high
RH values and PWATs will make for heavy downpours in any of the more
robust activity that remains. The cloud cover and precipitation will
keep low temperatures on the mild side, in the lower 70s. Due think 
that some persistent SW winds and these clouds will limit fog 
potential overnight despite the moist boundary layer conditions. 
However, some low ceilings from stratus are certainly possible, 
especially early Friday morning. 

.SHORT TERM...(Friday through Saturday)
Issued at 308 PM CDT Thu Jun 21 2018

Attention quickly turns to Friday as the upper-low swings across the
Ohio Valley, dragging a cold front into the Tennessee Valley and
Central Alabama. Clouds will continue to linger across the region
Friday morning, but a plume of drier air aloft, ahead of the actual
shortwave/cold front help to scour some them out -- resulting in some
partial clearing and opportunities for heating and destabilization 
from late morning through the afternoon hours. Model soundings 
suggest between 2000-3000 J/kg of CAPE is possible by 18-21z Friday. 
Steep low/mid level lapse rates and LI values around -4C to -7C were 
also progged per the NAM/GFS model soundings by the afternoon as 
well. This will set the stage for the possibility of robust 
convection from early Friday afternoon through Friday evening. 
Additionally, a LLJ will begin to develop and lift NE into Northern 
Alabama by 18z, helping to provided some added lift and 30-40 kts of 
0-3/0-6 km wind shear. Thus, organized convection should quickly 
initiate along/near the frontal boundary, as this additional forcing
kicks in. 

Given the aformentioned parameters in place, the threat for strong to
severe thunderstorms will exist across the region, mainly in the form
of damaging winds and large hail (thanks to the combination of CAPE
and Melting Levels around 13 kft). While isolated tornadoes will not
be likely, one cannot totally be ruled out given the added wind
shear with this system. Given that the profiles suggest this wind
shear will mostly be unidirectional, the tornado threat will mainly
be confined to areas where surface winds can locally back and/or and
mesoscale boundaries. The greatest window for strong/severe storms
will be around 3 to 9 PM CDT. 

Thereafter, this activity will wane by late Friday evening as the
boundary sags south. Still, do have a mention of scattered general
thunderstorms in the forecast through the overnight hours. With the
boundary nearby for Saturday and models showing the potential for
good destabilization again, think that scattered strong to severe
storms will be possible again on Saturday -- through coverage will be
lower due to the weaker shear and the lack of forcing as the upper
trough lifts NE into the Appalachian Mountains. 

.LONG TERM...(Saturday night through Wednesday)
Issued at 308 PM CDT Thu Jun 21 2018

The extended period begins Sunday morning with ongoing showers and
storms in the vicinity, thanks to an H5 disturbance moving W to E in
relatively zonal flow aloft. Additional showers and storms will
develop through the day along a stalled frontal boundary to the N,
sprawled W to E across TN. As such, PoPs were oriented higher to the
N and lower to the S, with the boundary expected to sag S overnight
into Monday. Storm threats Sunday/Sunday evening include 
gusty/damaging winds (with microbursts possible) thanks to MUCAPEs 
around 3000 J/kg and DCAPEs approaching 1500 J/kg. 

Showers and storms will affect the area off/on Monday as upper level
ridging begins to build in aloft, eventually ending convective 
chances by Monday night/Tuesday. This brief dry period will linger 
for much of midweek next week as the upper level ridge becomes 
entrenched over much of TX and the SW States. We'll be on the Ern 
periphery of the ridge aloft, with sfc high pressure anchored well 
N/E of our local CWFA over the SErn Great Lakes region. This will set
up Erly flow at the sfc, and NW flow aloft. Models are trying to 
spark convection beneath the ridge by Wednesday/Thursday, but am not 
going much above low-end chance PoPs for that time period just yet. 
It all depends on how far E the upper ridge extends, governing 
mid/upper level subsidence in place. At the sfc, there will be 
moisture slowly returning to the area to end the week as sfc high 
pressure off the E Coast provides SErly flow around the Wrn periphery
of the sfc high. That could easily spark isolated/scattered storms, 
but am not too confident in much more than that this far out. Overall
in the extended, daytime highs will top out in the upper 80s/lower 
90s, with morning lows remaining rather soupy, starting out in the 
69-74F range. 

By the weekend, the upper ridge begins to retrograde Wwrd and break
down over our area, putting the CWFA back into NW flow aloft. As
disturbances ride up/over the ridge, this should allow MCSs to march
SE into the area, providing showers and storms once again as we head
into the last weekend of June. Somewhat lower temperatures would be
expected during this time with the additional cloud cover likely in
place, but will wait and see on the finer details coming to light in
the coming days. 


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1205 PM CDT Thu Jun 21 2018

VFR conditions will be the predominant flight category at each
terminal this afternoon, with only light, isolated showers creating
VCSH at both sites. Ceilings/vis may drop this evening at HSV and MSL
from a cluster of storms progged to drop SE thru the region between
00-06z. Thereafter, dry conditions, but low MVFR ceilings around 
1-2kft are expected to prevail. Clouds will gradually lift to VFR 
levels after sunrise. More robust convection is expected after 18z on
Friday, beyond the scope of this TAF period (and was not mentioned).





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