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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 121005

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

.NEAR TERM...(Today)
Issued at 504 AM CDT Sun Aug 12 2018

Near term guidance suggests that a 500-mb trough currently centered 
across the eastern Great Lakes will become increasingly cutoff from 
the mid-latitude westerlies this morning, with the low predicted to 
drop southward into eastern OH over the course of the day. A broad 
mid-level trough axis and accompanying surface trough/wind shift axis
extending southwestward from this feature have provided sufficient 
lift to maintain a small cluster of showers/thunderstorms tracking 
east-southeastward across southern TN early this morning. POPs 
through 12Z have been increased across our southern TN zones to 
account for this feature, and patchy fog has been removed from the 
grids as a rapid increase in mid/high-level clouds from the west 
should reduce this threat going into the hours around sunrise.

As the synoptic scale features responsible for the lift mentioned 
above shift southward across the local area later today, we 
anticipate development of at least isolated-widely scattered 
convection within the seasonably moist airmass featuring PWAT values 
in the 1.6-1.7 inch range. This activity will be most concentrated to
the south of the TN River, where the surface trough will likely 
reside around the time of peak destabilization early this afternoon. 
Although dewpoints will begin to gradually fall to the north of the 
trough axis, neutral thermal advection should support highs in the 
u80s-l90s once again, assuming that mid/high-level clouds dissipate 
by early afternoon.

.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 06Z TAFS through 06Z Sunday night)
Issued at 1241 AM CDT Sun Aug 12 2018

A weak boundary positioned west to east across middle Tennessee, was
serving as a focus for isolated/scattered convection. This 
convection should slowly dissipate during the course of the 
overnight, as it slowly moves to the SE. That said, an isolated 
shower/storm could affect KMSL after midnight, as a cluster of 
convection near KMSL moves ESE. Also at KMSL from last night trends,
did maintain MVFR fog around daybreak Sunday. Light/variable winds 
overnight should become NW around 5kt after daybreak Sun. VFR weather
otherwise should continue for the remainder of the TAF.





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