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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 221624

Area Forecast Discussion
National Weather Service Huntsville AL
1124 AM CDT Fri Jun 22 2018

For 18Z TAFS.


.NEAR TERM...(Rest of Today)
Issued at 841 AM CDT Fri Jun 22 2018

Visible imagery indicates stratus was beginning to show signs of
thinning out over the northern tier of AL counties into southern 
middle TN. The next vorticity impulse and upper jet max over AR 
arrives by midday into the afternoon over the TN valley. The main 
question will be thermodynamics and focusing mechanisms. We do expect
the low clouds to continue to dissipate. Very high theta-E air over 
the deep south into southern AR/LA (dew points in the m-u70s) is 
currently feeding strong convection over western MS and southern AR. 
This main MCS will likely shift east and southeast into central AL 
along the outflow boundary from overnight storms and not directly 
affect our area. The HRRR runs have been showing some rogue left 
moving storms from this MCS moving into northwest sections of our 
forecast area around midday. By that time, the HRRR is forecasting 
SBCAPE values of 1500-2000 j/kg as some of that higher theta-e air 
advects northeast from MS along with 6km bulk shear values of 
40-45kt. A few of these thunderstorms could produce large hail. Then,
a second round of thunderstorms is progged to develop along the 
weakening cold front this afternoon into early evening. Timing of 
this will be from 21Z until approx. 02Z exiting our southeast 
counties. The chief hazards from these storms will be large hail and 
damaging winds, although right moving storms could have a small 
threat of a tornado as well given 1km SRH values of 150-250 m2/s2.

.SHORT TERM...(Tonight through Sunday)
Issued at 354 AM CDT Fri Jun 22 2018

As mentioned above, the second batch of storms should be slowly
shifting southeast and coming to an end after 2/3z. However, lift
along the backside of the departing trough, lingering elevated 
instability combined with shear values still around 30kts and PW 
values near 2 inches, will keep scattered to isolated showers and
thunderstorms around all night. Low temps right now are in the upper
60s to near 70 but may be slightly cooler if additional rain falls. 

Saturday is a bit tricky with GFS/ECMWF/NAM all consistently bringing
a batch of heavy rain/storms west to east across the area by 18z.
However, hires guidance (NSSL WRF, NMMB) has nothing except for a
decaying MCS coming in around 21/22z. The upper trough will be much
weaker and well into the NE at this time and a mid level ridge starts
to build in from the SE. This looks to push the cold front from
Friday back northward. This, combined with an upper vort max, will
develop the showers/storms along it after 15-18z. Really wanted to go
likely pops but in coordination with neighbors and because of
uncertainty, stuck with a 50%. Although I would say chances are much
higher than that for some thunderstorms at least by the afternoon.
Given the soundings for Saturday, would think anything that does
develop has the potential to be strong to severe. Hail would again be
the main threat with potentially even more instability than Friday
(temps will be warmer) and very similar shear values. The one thing
lacking Saturday is the broad scale lift and the LLJ. PW values over
2 inches will mean storms will produce very heavy rainfall. If we see
the continued rain on Friday, Saturday could have some flash flooding
potential but that depends on rainfall coverage until then. 

Most of Saturday night should be dry as the front pushes farther
north into the southern middle TN counties. Another strong upper wave
develops in the westerly flow aloft and will develop showers/storms
along the frontal boundary Sunday morning. The GFS keeps this all in
TN and is dry after 18z. NAM is similar, but keeps everything north 
out of our area. ECMWF keeps it raining in the TN counties all day. 
Will back off on POPs in the south for Sunday and keep high chance 
pops up north. Given ongoing rain/clouds, storms may not be *as* 
strong Sunday but the potential for a strong storm or two in TN still
exists. Sunday will see the warmest temps with highs near 90. heat 
index values will be in the mid 90s to near 100. 

.LONG TERM...(Sunday night through Thursday)
Issued at 354 AM CDT Fri Jun 22 2018

Most guidance continues to build an upper level ridge north into
southern Tennessee on Sunday night. This may keep more widespread shower
and thunderstorm activity north of the area and closer to Nashville 
and eastern Tennessee Sunday night. GFS is not as amplified with the
ridge due to some dampening of it by some upper level energy and 
deeper moisture advecting southeast into its northern periphery. This
would mean higher shower and thunderstorm chances in northern Alabama
and southern middle Tennessee, if this forecast option is correct. 
For now though only kept 20 to 30 percent chance of precipitation in 
the forecast in Alabama. Further north in our counties in southern 
middle Tennessee, went with 40 percent chance. Overall another warm 
night expected with lows in the upper 60s in northeastern Alabama to 
lower 70s further west. Little shear is shown in guidance, but it
will remain fairly unstable with SBCAPE values over 3000 J/KG. Thus,
activity will be the typical summertime pulse type convection. With 
some dry air aloft and very moist low level moisture, some stronger 
storms capable of gusty winds around 50 mph, heavy rainfall, frequent
lightning, and small hail will remain possible. 

By Monday, most models amplify the upper ridge quite significantly,
pushing the northern edge of it into southern Indiana and Illinois.
However, they still push varying amounts of upper level energy
south or southeastward into the strengthening ridge. Most guidance
show at least some widely scattered showers or storms developing
during the day on Monday. GFS again is the most ambitious with more 
widespread QPF depicting stronger energy aloft making it into the 
upper ridge. At this point, going slightly above blend with 40 to 50 
pop. Highs could be a tad cooler than Sunday, topping out in the 85 
to 90 degree range mainly. The same threats remain in place on Monday
as well, with the environmental conditions changing little. 

This activity should quickly wane with the loss of daytime heating,
although GFS holds onto QPF through the entire night (due mainly to
continued stronger upper level forcing shown in model). At this 
point, with such a strong upper ridge and little shear, leaning 
toward the lower pop (around 20 percent at best). Lows will not
change much, but could be a tad cooler overall (mainly due to daytime
mixing of dry air aloft lowering dewpoints). However, this may be a
bit dependent on how quickly cloud cover scatters out or dissipates
Monday night. Think upper 60s in the east and around 71 degrees in
the west looks reasonable though. 

Despite a fairly amplified ridge over the area, models continue to
show additional upper level energy sinking into the northeastern edge
of the ridge on Thursday. Enough moisture is shown that isolated to 
widely scattered showers and storms could remain possible. A slight 
warming trend looks reasonable, despite scattered convection, as 925 
mb temperatures warm due to the strengthening upper ridge. Again, not
much change in thunderstorm threats are expected. 

By Thursday night into Friday, models are in even bigger disagreement
on how strong the upper level ridge is and how far east the center of
that ridge moves. The ECMWF shows the strong/drier ridge extending
east into northern Alabama. However, GFS shows it much further west
and a much stronger longwave trough axis over the area. At this 
point, keeping with the blend and a compromise between the two 
solutions keeping scattered showers and thunderstorms in the 
forecast. High temperatures are most uncertain during this timeframe.
However, it should remain warm with highs in the upper 80s to lower 


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1124 AM CDT Fri Jun 22 2018

Scattered showers and thunderstorms will affect north AL and south TN
this afternoon and possibly into this evening. Cells should be within
the vicinity of both KMSL (thru 19Z) and KHSV (thru 22Z). Then the
probability is a bit more uncertain into early this evening. VFR
conditions are otherwise noted and expected until lower clouds
develop late tonight (by ~09Z). Ceilings of ~025agl (MVFR) are 
anticipated until 16-17Z when they should scatter out and lift.





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