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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 211450

Area Forecast Discussion
National Weather Service Huntsville AL
950 AM CDT Fri Sep 21 2018

.NEAR TERM...(Rest of Today)
Issued at 950 AM CDT Fri Sep 21 2018

An area of moderate to heavy showers continue to develop from
northeast of Lester in Limestone county, AL into the Fayetteville, TN 
area. Further south of this activity isolated to widely scattered 
showers are starting to develop at this time. All of this activity is
developing along a convergence axis that is being strengthened a bit
by an area of low pressure moving west into the eastern Gulf of 
Mexico that extends northward into the area. Most guidance moves this
area of low pressure only slightly west during the day. Models 
differ slightly on how where this convergence boundary moves during 
the afternoon today. However, the new 12Z NAM seem more scattered to
more numerous convection near and east of I-65 as this boundary 
shifts slightly east during the day. Increased cloud cover and pops
the remainder of the morning to reflect this. During the mid 
afternoon hours, it hints at more concentrated activity along this 

Based on satellite, plenty of sunshine/heating is occurring near and
east of I-65. Based on area morning soundings and SPC analysis, it 
looks as though thunderstorms will be very hard to come by this 
morning due to lower instability parameters and a cap aloft. However,
as we get more daytime heating, at least isolated thunderstorms look
possible this afternoon.

In addition, models hint at some scattered activity ahead of the 
approaching cold front well to our northwest pushing into 
northwestern Alabama. Kept 40 pop in late this afternoon to account 
for this. Overall, lowered high temperatures near and east of I-65 
into the mid to upper 80s in most locations due to more cloud
cover/higher pops. Further west, think more heating in the afternoon
due to lower pops/less cloud cover will allow highs to climb into 
the lower 90s, despite some cloud cover this morning.

.SHORT TERM...(Tonight through Sunday)
Issued at 202 AM CDT Fri Sep 21 2018

The upper ridge position will remain anchored across north GA into
central and northeast AL through tonight into Saturday. Tonight, clusters
of showers and thunderstorms will continue within the moist south- 
southwest flow at 850 mb across middle TN southwest into northern MS,
clipping far northwest AL. As the shallow surface boundary shifts
southeast into middle TN, far northwest AL into northern MS, there 
will indeed be a sharp gradient in the potential for showers and
thunderstorms. Locally heavy rainfall may clip far northwest AL
during this period if the boundary and convergence does indeed slip
southeast into this area. This may be somewhat short-lived as the
models suggest the front and convergence will lift back northwest on
Sunday. This will set up a good heavy rain pattern for areas from 
the Red River valley through AR, northwest TN and KY as numerous
shortwave impulses propagate out of the southern Plains into the H
valley.. For our areas, some or most of our central and southeast 
counties may remain dry both Saturday night into Sunday as the 
influence of the ridge and somewhat drier low to mid level flow will 
take place.

.LONG TERM...(Sunday night through Thursday)
Issued at 202 AM CDT Fri Sep 21 2018

A somewhat active pattern looks to continue through much of the next
work week, as an upper trough begins to move eastward and into the
Plains. This will allow for southwest flow in the mid to upper levels
to strengthen, with a steady stream of moisture advecting northward
into the TN Valley. Additionally, weaknesses within the upper flow
pattern will generate at least some lift over the area, and keep the
chance for showers and thunderstorms in the forecast for Monday and 
Tuesday. Meanwhile, the upper trough is expected to weaken a bit as 
it moves into the Mississippi Valley, though a cold front will 
continue to push eastward with the parent low over the northern Ohio 

Model discrepancies remain fairly high with regard to the system, as
the latest forecast runs are a bit different than those yesterday.
The weakening upper trough will be slower to move eastward Tuesday, 
as the strong subtropical ridge over the Atlantic holds. The surface 
low will continue its momentum eastward late Tuesday night, causing 
the cold front to become more east to west oriented as the southern 
extent of the front loses the upper forcing with flow aloft becoming
more zonal over the southern CONUS. While the GFS continues to show 
an initial shortwave moving through and generating widespread precip 
on Wednesday, both the GFS and the ECMWF stall the front just north 
of the area. Models inconsistencies further increase after the end of
the period, as the GFS shows a stronger closed low pushing eastward 
through the Great Lakes which would finally bring the front through 
the area on Friday. Meanwhile, the ECMWF has the upper low much 
further north, with zonal flow persisting over much of the eastern 
CONUS and the stalled front remaining north of the area. Given so 
much uncertainty, and the likelihood that model solutions will 
continue to differ for at least the next day or so, will follow 
closely to the blended guidance. This will make the highest chance 
for showers and thunderstorms on Wednesday, with the initial 
shortwave, with continued rain chances through the end of the week. 

Temperatures through the period will be closer to seasonal norms, as
the upper ridge moves east and upper height falls allow for some
cooling. Additionally, cloud cover will be fairly high and will
combine with at least scattered thunderstorms to further help temps
remain in the lower to mid 80s on Monday and Tuesday. The rest of 
the week will be highly dependent on just how quickly that front 
moves through, though an even greater coverage of cloud cover and 
precip will maintain highs in the upper 70s to lower 80s through the
end of the period. Overnight lows will generally remain in the mid to
upper 60s through the period. 


.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 551 AM CDT Fri Sep 21 2018

Isolated to scattered showers this morning will continue this
morning. Have included VCSH only through 15Z given the location 
uncertainty. With daytime heating, a few thunderstorms will also be
possible. Outside of the showers and thunderstorms, VFR flight
weather conditions are expected with high based cumulus at 050-060agl
and pockets of ceilings at or above 100agl. The probability of
showers or thunderstorms will persist into tonight, but is too low to
include at KMSL or KHSV at this time.





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