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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 211705 AAC

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
1205 PM CDT Thu Jun 21 2018

For 18Z TAFS.


.NEAR TERM...(Rest of Today)
Issued at 857 PM CDT Thu Jun 21 2018

An upper-level shortwave and associated LLJ is pushing east into the
Tennessee Valley this morning. A plume of very moist, tropical
moisture from the Mexican Rivera and South Texas continues to advect
into the region along the northern edge of a broad subtropical ridge
displaced to the SE over the Florida Panhandle. The combination of 
this lift and very deep moisture (PWATS as high as 2.2 inches!) has 
generated broad area of moderate to locally heavy rain showers and 
scattered thunderstorms across Northern and Central Alabama. This 
activity will remain tied to the LLJ and will continue to overspread 
the region through 18z, before tapering off from west to east, 
thereafter. The best instability currently is displaced to the south
of the region, generally, so have adjusted Wx grids to favor rain
showers as the predominant precip mode, with scattered storms
embedded within this activity.  

Models continue to hint at the development of some instability (CAPE
values as high as 1500-2000 J/Kg) late this afternoon and evening. 
That will be a big if, should the cooler, cloudy regime be the rule 
through most of the day. However, if some breaks can develop late in 
the day, there may be a brief window for a couple of strong storms, 
should something close to the aforementioned thermodynamic 
environment be realized. The dense cloud cover and rain-cooled air 
will limit temperatures and readings will likely run around 5 to 
perhaps 10 degrees below normal, peaking in the low to mid 80s across
much of the region. Regardless, with dewpoints in the low to mid 
70s, it will feel very humid outside today and it won't take much to 
generate shower and/or thunderstorm activity. 

.SHORT TERM...(Tonight through Saturday Night)
Issued at 330 AM CDT Thu Jun 21 2018

Hires guidance gives us possibly a small break from 22-00z today
before a MCS/line of storms starts to develop near Memphis around
that time. Not completely sure on what will happen once it gets here
as it all depends on how unstable we are. If we get pretty worked
over today, the line probably will have a hard time holding together,
and if it does, it won't be strong. Having said that, the NAM holds
onto instability near 1000 J/KG of CAPE all night with the GFS/RAP
bottoming out to near zero after 06z. Shear stays strong (40kts of
0-6km shear out of the west) through the night so the line/MCS will 
be sustained by that and the base of the upper trough pushing into 
the region. For now, will go with a 50 POP overnight to account for 
uncertainties regarding this line and the fact that guidance is 
showing scattered showers/storms all night. It also looks like we get
dry slotted of sorts tonight with drying occurring at the 700-500mb 
level and farther aloft, even a little at the 850-700mb level. This 
drops PW values to 1.5".

What happens Friday is anyone's guess at this point as model
consistency is slim to none. To start, the upper low will be over 
the IN/IL border with a weak surface low beneath it. The cold front 
could be near the AR/MS border by 12z Friday and into AL by 00z 
Saturday. This front, combined with strong lift along the base of the
trough, and another 30-40kt LLJ will provide sufficient support for 
thunderstorms, even strong to severe thunderstorm development. Even 
with the overnight drying and only a slight increase in PW values 
Friday afternoon, we will saturate a bit more, with afternoon dew 
points in the upper 60s. The dry air early in the day may also help 
us mix out the clouds and destabilize us. The less aggressive GFS 
still has CAPE around 1800 J/KG at its max Fri afternoon with 0-6km 
shear around 40 kts and 0-1km shear at 15-20kts. Decent lapse rates 
and WBZ value around 10kft could again result in marginally severe 
hail. But with the LLJ/shear that we have and some mid level drying, 
would expect damaging winds to be the greatest threat. SPC in their 
slight risk mentions a non zero threat of a weak tornado. We can't 
completely rule that out with helicity around 150 m2/s2 and the 
shear/instability/lift combo. Although LCL's are a bit high if we see
the true heating/afternoon mixing the models are suggesting. (The 
NAM is highest on ALL these parameters so chances increase if the NAM
is realized). 

There are always caveats! The dry air, for one, could make us too dry
to initiate many thunderstorms. Also, if the overnight convection is
more widespread, we may not clear out/mix out enough to realize any 
of the parameters mentioned above. The NSSL WRF and 3KM NAM for 
example, are almost bone dry for us on Friday, just a few isolated 
storms. WPC seems to be going towards a drier solution as well with 
their QPF forecast. Would think given the synoptic set up, we will 
get strong to severe storms Friday, even if they are slightly more 
isolated in nature. Highs should be able to reach into the mid 80s 
with lows in the upper 60s/lower 70s.

The convection should mostly push to the south by Friday night along
outflow boundaries. The low will be moving out to the east as we
enter a northwest flow regime behind it. With lingering moisture and
instability, will carry chance pops through the night. Another
shortwave approaches on Saturday and it looks like we will either get
another MCS in from the NW in the afternoon or just a batch of
showers/storms. Although instability will be impressive, shear is
back around 20kts with DCAPE/WBZ being unimpressive. Will still see 
heavy rainfall with high PW values and a strong storm is possible. 
With less rain/clouds, highs should be in the mid to upper 80s. Much 
drier air moves in Saturday night and it could be mostly dry. To keep
in line with my neighbors, have kept an isolated to scattered POP 
in. Another wave/MCS will be moving into S. MO and move across TN 
through 12z so we may start to see some of that by 10-12z. 

.LONG TERM...(Sunday through Wednesday)
Issued at 330 AM CDT Thu Jun 21 2018

Models weaken the surface low as it moves east with slight upper
ridging building into the region by Sunday. Another longwave trough 
axis (and associated frontal boundary) swings east-southeast into 
northern Tennessee Sunday afternoon. Good forcing and moisture 
advection ahead of this feature should allow scattered to more 
numerous showers and thunderstorms to develop. The highest 
precipitation chances look to be near and north of the AL/TN border, 
due to zonal flow in place aloft. With decent 0-6 km shear values 
between 30 and 45 knots shown by most guidance and SBCAPE values over
3000 J/KG, could see some strong to severe thunderstorm activity,
mostly in the TN counties. The main threats look like damaging winds
and frequent lightning right now. Not sure if this stronger activity
and high precipitation chances will make it very far south into 
northern Alabama at this point. Thus, have a decent gradient in high 
temperatures from north to south with mid to upper 80s north of the 
TN/AL border and lower 90s further south. Very high PWATS (around 1.9
inches) will be in place, so heavy rainfall will also be a threat, 
especially if training occurs in southern middle Tennessee. 

It is hard to say exactly where this frontal boundary ends up Sunday
night. However, models hint it may still linger somewhere between 
northeastern Alabama and the Birmingham area by daybreak. Thus left 
mainly scattered shower and thunderstorm chances in the forecast 
through Sunday night. Expect a very warm night as moisture 
convergence should pool higher dewpoints towards daybreak through 
northern Alabama with lows only dropping into the 70 to 73 degree 
range in many locations. 

Yet another piece of energy swings southeast on the southwestern edge
of another upper low swinging from eastern Canada into Maine on 
Monday. Most models show at least some of this energy associated 
with a weaker frontal boundary swinging into the area Monday 
afternoon. At this point shear looks a good bit weaker. However, with
a very moist low levels, good instability, and good low level lapse 
rates, stronger microbursts look possible with pulse thunderstorm 
activity. Gusty winds of at least 50 mph look possible given DCAPE 
values and instability. Also, will need to watch for training storms 
again for any heavy rainfall potential with PWATS remaining around 
1.7 inches. Temperatures look to remain in the upper 80s to around 90
degrees with expected precipitation and cloud cover.

As a stronger upper level ridge builds into the Tennessee Valley
Monday night through Tuesday night, it looks like we may receive a 
break from scattered to more widespread shower and thunderstorm 
chances. Cannot rule out isolated to widely scattered activity
though. With a bit more instability and dry air aloft, stronger
storms producing gustier winds, small hail, and frequent lightning 
will remain possible though. High temperatures should climb a bit
into the low 90s in most locations, except in higher elevations of
northeastern Alabama and southern middle Tennessee. 

Precipitation chances could increase again by the end of the week, as
additional pieces of energy affect the area.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1205 PM CDT Thu Jun 21 2018

VFR conditions will be the predominant flight category at each
terminal this afternoon, with only light, isolated showers creating
VCSH at both sites. Ceilings/vis may drop this evening at HSV and MSL
from a cluster of storms progged to drop SE thru the region between
00-06z. Thereafter, dry conditions, but low MVFR ceilings around 
1-2kft are expected to prevail. Clouds will gradually lift to VFR 
levels after sunrise. More robust convection is expected after 18z on
Friday, beyond the scope of this TAF period (and was not mentioned).





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