Area Forecast Discussion
Issued by NWS Huntsville, AL

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000
FXUS64 KHUN 301110
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
610 AM CDT Sun Apr 30 2017

.UPDATE...
For 12Z TAFS.

&&

.NEAR TERM...(Today and Tonight)
Issued at 348 AM CDT Sun Apr 30 2017

Current observations at 2am show temps still in the lower to middle
70s with dew points in the middle 60s. The pressure gradient is
already tightening between the departing surface high to the east
and the approaching and strengthening, surface low currently located
over SW MO. Winds are already gusting in the west around 20-25kt.
Winds will continue to increase today as the gradient tightens and
the surface low strengthens and moves off to the northeast. This
combined with a 40-50kt LLJ that will be strengthening this
afternoon puts sustained winds around 20kt this afternoon and gusts
around 30kt and will therefore keep the Wind Advisory as is,
starting at 10AM and going to 10PM.

The real forecast issue of the day will be the thunderstorms- when,
where and how strong. Confidence is not overly high in the
forecasted solution. The guidance, including hires/cam guidance, is
all over the place, to put it lightly. There are a variety of things
that could happen today and this will discuss the most likely
scenario, touching briefly at the end with a stronger solution.

Current radar shows two messy areas of convection, one in NE AR and
then another in central LA. These areas looked to be aided by the
developing southerly jet and a weak short wave/upper convergence
ahead of the main lift/surface cold front. The main upper low is
still over the TX Panhandle and that will deepen and move into
western KS by this evening. There is general agreement that a line
forms within these areas of convection and moves east through the
morning hours. When the southerly LLJ kicks in by 15-18z, my thought
is that the easterly progression of the line will slow slightly. The
HRRR does seem to weaken the LLJ after 15z as compared to the other
hires models. The other guidance "squishes" the jet E/W, making it a
much more narrow axis but at least holds it strong and moves the jet
east after 00z.

As it moves east, the line will outrun the stronger lift. Locally,
even with a broken low cloud deck and even a cirrus layer from
ongoing convection, we should be able to mix and warm (into the mid
80s) and muster MUCAPE of 1500-2000 J/KG. However, we will be capped
in the mid levels. GFS/ECMWF/NAM seem to want to warm quick enough
to break the cap but the RAP, even though it is forecasting
sufficient SBCAPE, keeps the mid level cap in place. There is quite
a bit of mid level dry air in place and forecast to advect in as the
mid level ridge is slow to retreat this evening. So as the line
previously mentioned moves east, not only will it start to lose it`s
lift but also encounter a more capped environment. So most likely
scenario is that this line makes it to NW AL around 18-20z and I
wouldn`t rule out a strong to severe storm with it. If the line can
tap into surface based instability, there will be enough low level
wind shear/helicity to support a tornado or two although the highest
threat will be damaging winds. As it moves east, the strength should
diminish and thinking it will turn into scattered showers and
thunderstorms.

The next question will be what happens along the actual cold front.
This line of storms looks to follow pretty quickly behind the
previous line and get to NW AL by 00z. There isn`t a clear idea of
how strong this line will be. The first line could stabilize us too
much and we end up with just a line of showers and embedded storms.
Or, the line could throw out outflow boundaries, leave lingering
moisture and we could get stronger, rotating storms embedded in the
line, especially given the high shear values. I think the first
option seems more likely only because I think we will really lose
the surface based instability after sunset.

Touching briefly on the alternative solutions. 1. This first line
could slow down quite a bit and eventually merge with the cold front
line. This is what some of the guidance is and has been hinting.
This would bring a line of strong to severe storms through NW AL
around 21-00z and track it east. I can`t rule this solution
completely out. We would be losing instability later in the day but
it provides a sweet spot between 21-00z in NW AL where a tornado or
two would be possible. 2. The second line ends up much stronger than
forecast and we end up with another line of severe storms with
damaging winds being the highest threat with a tornado or two.
Again, can`t rule it out but think this is the least likely
solution.

Also, latest QPF values have come in a bit higher no matter the
solution. One strong line with heavy rain which will be slow moving
or two lines of storms will produce significant rainfall given the
high PW values. This could produce rainfall amounts of 2-3 inches
across the area. Given the already saturated ground, isolated
instances of flash flooding and river flooding are possible. No
flood watches are being issued at this time given the high
uncertainty on how the systems today evolve.

.SHORT TERM...(Monday)
Issued at 348 AM CDT Sun Apr 30 2017

By 12z Monday, the line should be east of Huntsville and mostly
exiting NE AL. ECMWF/NAM lingers the line westward back a little and
pops were done to account for this. There could be some lingering
clouds in the north where the upper low dips to the south but
overall skies should clear out by late afternoon. We will see a
brief break in the hot weather with highs on Monday in the lower
70s.

.LONG TERM...(Monday night through Saturday)
Issued at 348 AM CDT Sun Apr 30 2017

Little change xpcted initially going into the latter part of the
forecast, with the previous cold front responsible for much of the
active wx ending the weekend period now well ewd into the Atlantic
Basin. High pressure originating out of the srn Plains will quickly
make its way ewd across the SE region thru Tue in the wake of the
frontal passage. Other than perhaps a brief shot of cooler/drier air
to start of the new work week, little change in overall temps is
xpcted with the earlier frontal passage thru mid week, as the low
level wind field veers back to the se/s and the upper flow pattern
turns more toward the wsw. Afternoon temps into Wed look to return
close to 80F, as another upper trough pattern drops well into the
srn Plains states.

The latest model runs unfortunately remain fairly inconsistent going
into the second half of the work week, with respect to the progression
of this upper trough axis over the Plains states and the developing
sfc wave. The 00Z GFS/ECMWF have trended back toward a faster and
more open upper trough axis, as it translates ewd across the SE
states thru Fri. The associated sfc low continues to lift newd into
the Lower MS/TN Valley regions Wed/Thu, with sct/num showers/tstms
developing along/ahead of the advancing sfc wave and attendant cold
front. Rainfall/showers may then linger Fri/Sat as the upper trough
pattern exits the region to the e.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 610 AM CDT Sun Apr 30 2017

Tricky forecast today especially in terms of potential thunderstorms.
To start, wind will continue to increase today with gusts around 30kt
likely by the later afternoon which will decrease slightly as a cold
front moves through after 00z, west to east. A line of thunderstorms
is possible late this afternoon and have included a VCTS to account
for this possibility. Then another line is going to move through
with the cold front this evening and continued the previous TAF`s
TEMPO group. CIGS should rise to VFR this afternoon but plunge back
down to MVFR with the frontal passage. CIGS should rise and clear
quickly at the end of the TAF period.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Wind Advisory from 10 AM this morning to 10 PM CDT this evening for
     ALZ001>010-016.

TN...Wind Advisory from 10 AM this morning to 10 PM CDT this evening for
     TNZ076-096-097.

&&

$$

NEAR TERM...LN
SHORT TERM...LN
LONG TERM...09
AVIATION...LN


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