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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

                            
444 
FXUS64 KHUN 220511
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
1211 AM CDT Fri Jun 22 2018

.UPDATE...
For 06Z TAFS.

&&

.NEAR TERM...(Rest of tonight)
Issued at 1002 PM CDT Thu Jun 21 2018

A loosely-organized arc of convection continues to translate east- 
southeastward across the region, driven by outflow from storms that 
developed earlier this afternoon across the mid-South region. High 
resolution model guidance suggests that the leading edge of the line
of storms will remain intact as it advances further across the 
region this evening, and this seems entirely plausible based on the 
degree of synoptic scale ascent evident by 40 m 12-hr height falls 
and enhanced by increasingly diffluent flow aloft. Although 
individual storm cells will continue to weaken as they encounter an 
increasingly stable boundary layer across northeast AL, the threat 
for frequent cloud to ground lightning, locally heavy rainfall and 
brief wind gusts in the 40-50 MPH range will persist with the 
strongest storms.

To the west of our CWFA, thunderstorms will continue to redevelop 
both along the westward trailing outflow boundary, and to the north 
of this boundary as the low-level jet strengthens atop a vast cold 
pool in the wake of the initial convective system. Deep-layer 
steering flow from the west will spread this activity eastward into 
the region late this evening and early Friday morning, and POPs have 
been adjusted upward region-wide to account for these factors. With 
time, we do anticipate the greatest coverage of storms to gradually 
shift south of the TN River, but this may not occur until closer to 
sunrise. If sufficient echo training develops later this evening, 
then a flash flood threat may materialize across portions of the 
region.

.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 06Z TAFS through 06Z Friday night)
Issued at 1211 AM CDT Fri Jun 22 2018

Rain and embedded tsra will persist at the terminals thru 08Z, with 
brief periods of MVFR cigs/vsby in the heavier pcpn. However, as the 
most concentrated region of shra/tsra shifts south/east of the 
terminals, an IFR stratus deck is expected to develop rapidly in its 
wake. Cloud bases will likely remain in the 800-1500 FT range thru 
14Z, before quickly lifting/scattering. Sct-nmrs tsra will redevelop
by 16-18z and have included -TSRA at both locations. Storms should 
diminish by 21/22z with showers ending by 01z. With clearing skies 
some light fog will be psbl at both airports by end of the valid TAF 
period.

&&

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

$$

NEAR TERM...70/DD
SHORT TERM...AMP.24
LONG TERM...12
AVIATION...70/DD


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