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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

                            
453 
FXUS64 KHUN 220854
AFDHUN

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

.NEAR TERM...(Today)
Issued at 354 AM CDT Fri Jun 22 2018

Light rain lingers over the area early this morning behind a decaying
line of storms/MCS that is now situated west to east near 
Birmingham. The upper low is centered over St. Louis this morning 
with a broad surface low encompassing much of the OH river valley. A 
vort max around the base of the upper low helped to propel the line 
of storms into the area and is helping to push it south. Water vapor 
is showing drier air moving in behind this initial wave, which is 
also evidenced on forecast soundings. So we should be dry through 12z
behind this rainfall.

The upper low continues to move almost due east through 12z with the
strong syntopic lift moving into NW AL between 12z-15z. At the
surface, a frontal boundary will be snaked from the IN/OH boundary 
south through KY/TN and then southeast through MS/AR and be right at 
the NW AL border. Guidance is consistent in showing development along
this front by 9-12z this morning in central AR. Guidance isn't sure 
if this development will move east and drop into our southwestern 
areas and then south through Cullman. OR...convection just blows up 
all along the front around 5z in NW AL, which seems most likely. So 
that line should move quickly to the SE and probably be out of the 
area by 18-19z. Even with that line, guidance is quickly recovering 
the atmosphere, since it is peak heating, and developing additional 
storms behind it. These move from NW Al around 21/22z through the SE 
areas by 2/3z tonight. (To note- each HRRR run is different in 
regards to these two rounds)

In terms of the hazards/strengths of the storms, not sure how strong
the morning round will be given how dry the atmosphere will be. 
These will probably just help saturate it for later. The core of the 
LLJ and upper jet streak start to move over at 12z but it will be a 
few hours later before the entire area is under the jet. It also 
depends on when the first line develops. If it's closer to 15z in the
NW, instability/shear will already be increasing and a strong to 
severe threat would occur across the region, especially in the SE. If
it's closer to 12z, the highest strong to severe threat would be in 
the SE as it gets there later in the morning. However, the second 
batch is what I would be most concerned about. These look to develop 
in a very supportive environment with sufficient lift, CAPE above 
2500-3000 J/KG, 0-6KM shear around 40kts and even 0-1km shear around 
20-25kts. Large hail would be the greatest threat with thick cape in 
the hail growth zone as well as reasonable WBZ/FZL heights. Damaging 
wind gusts would also be a threat just due to the amount of wind in 
the column and the shear although there isn't much of a mid level dry
slot. In terms of tornadoes, a brief, isolated, weak tornado would 
be possible with the highest chances with anything that develops 
after 21z ish, and mostly in the west. This is when helicity would be
maximized and LCL's would be lower (GFS maximizes everything 
slightly earlier). Even so, it's on the low end of climatological 
normals for tornadic events. Given how strong these storms could be 
overall, it wouldn't take much more than an interaction with an 
outflow boundary to produce a brief tornado. Confidence is low on 
this. 

The uncertainty with all of this is what happens with the morning 
convection. I have seen one too many times where models try to
redevelop the afternoon batch but we become too stable from the 
morning batch. However, today, hires is in abnormally good agreement 
on that second batch. 

Quick mixing today and some breaks in the clouds should put highs in
the lower to middle 80s.


.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 
90s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1211 AM CDT Fri Jun 22 2018

Rain and embedded tsra will persist at the terminals thru 08Z, with 
brief periods of MVFR cigs/vsby in the heavier pcpn. However, as the 
most concentrated region of shra/tsra shifts south/east of the 
terminals, an IFR stratus deck is expected to develop rapidly in its 
wake. Cloud bases will likely remain in the 800-1500 FT range thru 
14Z, before quickly lifting/scattering. Sct-nmrs tsra will redevelop
by 16-18z and have included -TSRA at both locations. Storms should 
diminish by 21/22z with showers ending by 01z. With clearing skies 
some light fog will be psbl at both airports by end of the valid TAF 
period.


&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

NEAR TERM...LN
SHORT TERM...LN
LONG TERM...KTW
AVIATION...70/DD


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