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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 121445

Area Forecast Discussion
National Weather Service Huntsville AL
945 AM CDT Sun Aug 12 2018

.NEAR TERM...(Rest of Today)
Issued at 945 AM CDT Sun Aug 12 2018

Mid/high level cloud cover continues to spread ewd into the cntrl TN
Valley this late Sun morning along the srn base of an exiting upper
trough pattern over the OH Valley/mid Atlantic states. Area radars
have shown a little bit of light rainfall trying to develop over 
west cntrl MS, although this activity is having a little bit of 
trouble developing more to the east. Otherwise, the local area
remains sandwiched between two weak frontal boundaries. The first is
layered west to east from the Midwest states into the mid Atlantic 
while the second is located from NW MS into cntrl GA. Intermediate 
model guidance doesn't move the nrn boundary much while the srn front
drifts a bit swd thru the day. In any case, there's not a real clear
cut focus mech in place for potential showers/tstms later today, xcpt
for perhaps along the wrn fringe of the srn boundary. This may at
least warrant iso showers/tstms across much of north AL later today,
or especially locations along/west of I-65. Outside of any rainfall,
it should be a seasonably warm afternoon, as highs climb well into
the lower 90s for most areas.

.SHORT TERM...(Tonight through Tuesday night)
Issued at 504 AM CDT Sun Aug 12 2018

The cutoff upper low is progged to remain nearly stationary across 
eastern OH tonight, with a secondary vort max rotating around this 
feature forecast to dig southeastward through middle TN and into the 
southern Appalachians. At the surface, the trough axis is expected 
to become oriented in a northwest-southeast fashion and eventually 
stall across the far southwestern corner of the CWFA. Although we 
will maintain a slight chance POP in this region throughout the night
due to the proximity of the low-level boundary, a much drier airmass
is expected to spread across the remainder of the region, with PWAT 
values falling to near or even slightly below 1 inch by 12Z Monday. 
Environmental conditions would normally favor the development of fog 
in this regime, but due to the magnitude of dry air advection, we 
have only included a patchy coverage at this point.

Short term models suggest that the cutoff low to our northeast will 
begin to shift eastward across the central Appalachians and into the
northern mid-Atlantic Monday/Monday night, as a high-amplitude mid- 
level ridge translates eastward across the region. We will maintain a
very low POP across the southwestern portion of the forecast area to
account for uncertainty regarding the precise location of the 
stalled surface wind shift axis. However, the majority of the region 
will remain dry and very warm with highs in the u80s-l90s. 
Fortunately, it appears that a deep mixing layer will develop on 
Monday afternoon, with falling dewpoints keeping Heat Index values in

The mid-level ridge axis will cross the region on Tuesday, with flow
aloft expected to back to west-southwest and increase on Tuesday 
night downstream from another trough lifting northeastward through 
the central Plains. Deep moisture will be advected northeastward into
the region as this occurs, with PWATS rapidly increasing to 1.5-1.7 
inches by Tuesday afternoon. We have indicated a low chance POP 
mainly for the northwestern half of the region on Tuesday/Tuesday 
night as a result of the moisture advection and related increase in 
instability. Highs will remain in the u80s-l90s on Tuesday, with mild
lows in the l-m 60s early in the week expected to warm into the 
u60s-l70s by Wednesday morning.

.LONG TERM...(Wednesday through Saturday)
Issued at 504 AM CDT Sun Aug 12 2018

Guidance overall continues to hint towards delaying rainfall over 
Alabama a bit longer ahead of the approaching longwave trough axis 
over the western Great Lakes region southwest into Missouri on 
Wednesday. Mainly due to the strong upper level ridge in place over 
Alabama/Georgia into the northeastern CONUS. More persistent and 
thicker cloud cover may hold off until the afternoon hours. With 925 
mb temperatures forecast to be 25 to 27 degrees (ECMWF as high as 27 
to 29 degrees) and ample morning to early afternoon heating near and 
south of the Tennessee River, think that highs between 93 and 95 
degrees looks possible in those areas. It may be even warmer if the 
ECMWF is right and the ridge holds the longwave trough axis further 
west into Wednesday afternoon across the entire area (maybe looking 
at highs in the lower to upper 90s areawide if it is right). Luckily 
it will not be extremely humid with dewpoints in the mid to upper 60s
primarily. However, heat index values will still reach the 95 to 100
degree range most likely. It should be a bit cooler in southern 
middle Tennessee and near Lauderdale county in Alabama due to cloud 
cover, most likely upper 80s to around 91 degrees.

Synoptic models seem to keep this trend into Wednesday night (the 
exception being the GFS, which brings higher pops into extreme 
northwestern Alabama by then). Given the strong ridge that the 
longwave trough has to break down, trending slower and closer to the 
ECMWF with pops. Kept most of the area free of shower or thunderstorm
activity Wednesday night - the exception being northwestern Alabama 
with 20-30 percent pop. Thursday morning should be pretty warm with 
moisture pooling over the area. Kept lower 70s in the forecast, but 
would not be surprised if this ends up being closer to 71 to 76 
degrees in many locations in northern Alabama and southern middle 

Most synoptic models finally break down the ridge over 
Alabama/Georgia Thursday morning as the longwave trough axis moves 
into eastern Arkansas/extreme southeastern Missouri. 500 mb winds and
forcing picks up as a result. Bulk shear values will remain very 
low. However being on the right entrance region of a weak low level 
jet ahead of the longwave trough may help to provide better lift over
the region. Thus have 30 to 60 pop (SE to NW) across the area. High 
temperatures should be a bit lower due to more prevalent cloud cover 
and rain chances, primarily in the upper 80s to around 90 degrees. 
PWATS are very high, around 1.9 inches, so heavy rainfall will be 
possible in stronger thunderstorm development. Some wind gusts to 
around 45 mph may occur with the strongest storms.

As this boundary stalls somewhere across northern Alabama or 
slightly further north through Friday before sinking further south 
into central Alabama by late Friday night, high chance to more 
numerous showers and thunderstorms seem reasonable during this 
timeframe. PWATS will remain around 1.8 or 1.9 inches, so heavy 
rainfall in stronger storms and any training activity will remain 
possible. Cooler daytime highs (85 to 90 degrees) and quite humid 
conditions are expected. Lower instability values and a more 
saturated atmospheric profile should keep wind gusts a bit weaker 
with the strongest storms on Friday/Friday night.

Models have quite different solutions synoptically Saturday through 
Sunday. At this point given model differences kept to blended 
guidance, keeping high chance to more numerous showers and 
thunderstorms and overall lower temperatures in the forecast through 
Sunday night. A bit more shear shows up in some models either Sunday 
or Monday with disturbances moving through the area. This could be a 
period of stronger thunderstorm activity, but pretty far out there in
the forecast right now.


.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 717 AM CDT Sun Aug 12 2018

VFR conditions will persist at the HSV/MSL terminals this forecast 
period. However, bkn-ovc altostratus/cirrostratus clouds generated by
an upper air disturbance over the southern Plains will provide cigs 
AOA FL120 for much of the day as they continue to overspread the 
region from the west. A few light shra or pockets of lgt rain may 
occur in this regime later today, especially to the southwest of the 
terminals, but threat for impacts is too uncertain to include in the 
TAFs at this point. Sfc winds will veer to NNW late this aftn as a 
sfc trough shifts southward, and this should coincide with a more 
pronounced clearing trend. SKC conds are anticipated overnight, and 
may allow for the formation of patchy river valley fog as flow veers 
to the NNE and subsides. However, anticipated coverage of fog does 
not warrant inclusion of vsby reductions in the TAFs attm.





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