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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 151057

Area Forecast Discussion
National Weather Service Huntsville AL
557 AM CDT Wed Aug 15 2018

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 420 AM CDT Wed Aug 15 2018

The storm system currently centered over western Kentucky and 
southern Illinois/Indiana is close by and producing scattered to more
numerous showers and thunderstorms in those areas. Further south some
isolated to widely scattered showers can be seen as far south as
Murfreesboro, TN and in extreme northwestern Mississippi. Most model
guidance moves the upper level forcing with this storm system
northeast with time today. The movement to the northeast should keep
precipitation north of the area, but some isolated showers or 
thunderstorms could come as close as McNairy or Maury counties in 
Tennessee. Some higher dewpoints are already pooling ahead of this 
system though over northern Alabama and southern middle Tennessee,
with dewpoints west of I-65 around 70 degrees. This and abundant
mid/high cloudiness should help keep lows a bit warmer west of I-65
in the 70 to 73 degree range. near and east of I-65, though drier air
is still in place and this should allow lows to drop into the 63 to
68 degree range in most of those locations, especially with more
prevalent and thicker cloud cover holding off until closer to
daybreak in those areas. Dewpoint depressions are currently zero or
almost zero east of I-65 in northern Alabama. Thus, keeping areas of
fog in the forecast, which could become dense (lowering visibilities
to around 1/4 of a mile at times through daybreak) in some areas of 
the Paint Rock Valley, near Lewis Smith Lake and in the river 
valleys of DeKalb county. 

Cloud cover associated with activity skirting the area through 
portions of the afternoon hours, will keep high temperatures a bit 
lower in southern middle Tennessee, mainly the upper 80s to lower 
90s. Futher south impacts and the extent of thicker cloud cover is 
more uncertain. Given the very warm 925 mb temperatures forecast, 
think highs could be a degree above guidance near and south of the 
Tennessee River. For now, frecasting highs to reach the 93 to 96 
degree range in most of these areas. The exception of course will be 
the higher elevations of northeastern Alabama, where highs will 
likely be in the lower 90s. Not expecting any precipitation at this 

As the main storm system moves only slightly east during the day
today, additional energy aloft develops on the southern edge of the
longwave trough axis associated with this system over Arkansas. What
happens with this energy will be the next puzzle of the forecast. 

.SHORT TERM...(Tonight through Friday)
Issued at 420 AM CDT Wed Aug 15 2018

As the main storm system weakens and meanders around the central 
Great Lakes region Wednesday night, this energy moves closer to 
northwestern Alabama. The main impact this will have is to continue 
to increase moisture advection and surface dewpoints over the area. 
This should keep lows warmer in the lower 70s at least. However, 
forcing still remains north and west of the region through daybreak 
on Thursday. 

During the day on Friday, moisture and instability will couple with
some weak lift ahead of the feature as it moves east towards Memphis
to produce 20-30 percent chances of of showers or thunderstorms. 
With several models showing SBCAPE values between 2000 and 3500 J/KG 
west of I-65 and in southern middle Tennessee, cannot rule out a 
marginally severe storm or two. However, most storms should just be 
strong and very pulsy in nature, given the weak bulk shear with this

Models forecast stronger upper level energy to move southeast from 
the western Great Lakes area courtesy of the main jet stream and into
the briefly weakening longwave trough axis over the central Great 
Lakes region Thursday night. This should amplify the longwave trough
axis and storm system over the central Great Lakes region. The end
result of this will be higher rain chances overnight (30-60 percent). 
This trend should continue into Friday and Friday night, as the
stronger forcing and slightly stronger shear closer to the main 
longwave trough axis associated with the storm system moves closer to
the region. Will have to watch this period for stronger thunderstorm
activity given higher shear, but a more saturated atmospheric column 
might minimize this threat. 

.LONG TERM...(Friday night through Tuesday)
Issued at 420 AM CDT Wed Aug 15 2018

A wet period already on-going at the end of the workweek will
continue into the weekend, as upgliding lower level moisture and 
approach of an upper level system bring clouds, and widespread 
showers and thunderstorms to the Tennessee Valley. The 305K region 
indicated continuing moisture upglide in the 900-750mb level, keeping 
clouds and convection. The GFS depicted a series of upper level 
systems traversing the region during that time frame, providing 
additional upper support. The ECMWF was similar but displaced a bit 
further to the north with its depiction of upper level support.
Model mass precipitation fields are suggestive that much of the area 
should receive measurable rainfall in the Saturday/Saturday night 
time frame. Some of the storms during Saturday could become strong, 
with gusty winds the main threat. Map view and soundings do indicate 
a rather unstable atmosphere with surface based CAPES greater than 
2000 both Sat/Sun. With the showers, high temperatures to end the 
week should be in the mid/upper 80s, then rise into the upper 80s on 
Sunday. Given that upper support should be displaced further to the 
north Sunday, decreased rain chances into the high "chance" range. 

Do not have very high confidence in the forecast after the weekend. A 
summary for first couple of day of a new work week will be unsettled 
weather, with scattered showers/thunderstorms in the period. More 
clouds than sun should keep highs into the mid/upper 80s, and lows in
the upper 60s to around 70. The models were hinting at a frontal 
boundary moving across the region in the middle of next week. If this
occurs, lower rain chances and slightly cooler conditions could be 
realized the latter half of next week. 


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 555 AM CDT Wed Aug 15 2018

VFR conditions are expected throughout the TAF period.





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