Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 270012
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
712 PM CDT Wed Apr 26 2017

.UPDATE...
For 00Z TAFS.

&&

.NEAR TERM...(Tonight)
Issued at 335 PM CDT Wed Apr 26 2017

The forecast for tonight remains a little uncertain due to a few
different factors and latest model suites. Soundings from around
the area show warm temps between 850-700mb with values of 15-20C.
These are coincident with a very dry layer from this region up
through 200mb. The CU field that is out there has been a limited due
to this drier air but is the result of a shallow layer of moisture
right at the surface.

A shortwave currently located over TX/OK is forecast to lift NE up
through the Mid MS Valley and into the OH Valley tonight. A cold
front associated with the system has been and will continue to be the
focus for convection as it moves east with the low. Models show a
strong SW orientated LLJ/Mid-level jet (850 and 500mb) moving across
western TN this evening. Winds over the TN Valley though are expected
to remain out of the S/SE ahead of this feature. This should limit
the deeper moisture from streaming into the area and it`s not until
the line of storms is moving through the area do we see saturation of
the mid-level dry layer. The question for tonight though is the warm
layer and whether or not we can cool the layer. Based on projected
temp/moisture advection into the area tonight, do not see anything
that would support rapid cooling of the layer. We could see evapo-
cooling as the line arrives and forecast soundings are trying to
indicate this. But with it occurring coincident with the arrival of
the line, we would not have surface based instability. Thus storms
will likely be elevated as they enter the area. The concern though is
the strong shear profile that shows good turning of the winds with
height and if anything could be surface based it could tap into this
and the severe threat would be higher. But the confidence in surface
based instability is very low.

We are expecting the line of thunderstorms that is developing over
AR/LA to continue moving east towards the TN Valley tonight. Models
have been consistent in showing the line arriving close to midnight
tonight in NW AL. However, based on previous events, models tend to
be a little on the slow side in the arrival of lines into the area.
Think that with the strong winds aloft, the timing would end up a bit
faster than what is advertised. Will have to watch closely the
progression of the storms over TX/S AR as these would be the storms
that arrive into the area.

One interesting thing we`ve been discussing is the potential for a
wake low to develop tonight. Hires guidance has been consistent in
showing a few weak low pressure systems developing behind the line.
Soundings continue to indicate that we will hold onto the significant
dry layer and inversion from 850-700mb up until storms arrive
tonight as mentioned above. This layer could be one of the reasons
why we are seeing the weakening in models. The stronger jet behind
the line could be ducted downwards and we could see a period of
strong winds behind the line. One prog that we can look at to gauge
the chance for winds to be ducted downward is the ducting function
and values would support this. This will be something that we will
have to watch closely. If one were to develop we should see some
initial indications over N MS.

We will be keeping a close eye on how things evolve tonight.

.SHORT TERM...(Thursday through Friday night)
Issued at 335 PM CDT Wed Apr 26 2017

The front will be well to the east of the area by Thursday morning
and with some slight CAA, temps will be on the cooler side for
Thursday with highs in the lower 70s. This quickly changes as winds
shift back to the south Thursday night into Friday. A warm front is
expected to be advected northward during the day Friday with low to
mid 60 dewpoints surging northward. Models have backed off a bit on
the QPF for Friday but they haven`t shown any major sources of lift
during the day. But with warm temps and mid 60s dewpoints instability
is forecast to be very high with values of 2,000-3,000 J/kg. With
shear of only 20-30kts on Friday, storms will likely be of the summer
time variety and trigger off any differential heating or other
lingering boundaries. Hodographs would support a brief strong to
severe storm with damaging winds and hail the main threats.

By Friday night, we lose the daytime heating but with these high
dewpoints there is potential for fog development. Something we need
to keep an eye on.

.LONG TERM...(Saturday through Tuesday)
Issued at 335 PM CDT Wed Apr 26 2017

The surface high off the southeastern coast remains fairly stout on
Saturday. This should keep most precipitation further north along
the fore-mentioned frontal boundary.  However, with very unstable
conditions(SBCAPE 2000-3000 J/KG) being forecast by models, the area
could see isolated strong to marginally severe storms developing on
Saturday. This is mainly due to very weak and shallow low level
convergence that might be in place south of the frontal boundary.
DCAPE values are forecast by models around 900 J/KG. Main threats
with any marginally severe storms would be large hail and damaging
winds. However, with no shear these would be short-lived pulse type
thunderstorms. Very warm 925 mb temperatures and abundant sunshine
should allow highs to climb into the upper 80s to around 90 degrees.
Isolated strong thunderstorms cannot be ruled out Saturday night.

Saturday night, models develop a surface low over Arkansas and
eastern Oklahoma. The surface high off the southeastern coast does
slide slightly further east but it does become stronger. By Sunday
morning, the surface low deepens and moves north into Missouri and
Iowa. This will likely amplify the syntopic pattern in place ahead of
the approaching storm system. Thus, the slower timing of
precipitation onset shown by ECMWF is likely more on track (GFS is a
bit faster). Models differ concerning how strong a pre-frontal trough
axis is that develops ahead of the front. The GFS shows a stronger
pre-frontal trough axis, while the ECMWF is much weaker with this
energy. Therefore included a 20 to 40 percent chance of showers and
thunderstorms given the unstable conditions (SBCAPE 1000-2000 J/KG)
models have forecast to be in place. Given the instability, cannot
rule out a strong to severe thunderstorm earlier in the day on
Sunday. However, a strong low-level cap may keep this activity more
on the isolated side in the morning.

As better forcing pushes east into the area Sunday afternoon and
evening, low level helicity, bulk shear, and forcing should
increase. Although instability should decrease some, there still
should be between 500 and 1500 J/KG of SBCAPE available for storms
to tap into. A solid line of showers and storms should develop as a
result and move from eastern Mississippi into the area Sunday
afternoon into Sunday evening. Again all modes of severe weather look
possible. The details of the event remain highly uncertain.
Especially if the timing of this line is further delayed and moves
through overnight, when less instability might be available. Although
a few showers might linger after midnight, thunderstorms should be
gone by then.

Cooler temperatures along with much drier air will move into the the
Tennessee Valley behind the front on Monday. Highs around 70 degrees
are forecast by most models, which would be several degrees below
normal. A slow warmup is expected into the middle of next week, as
southwest flow returns and highs climb back into the upper 70s to
lower 80s.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 712 PM CDT Wed Apr 26 2017

A rapidly deepening sfc low will shift nnewd from sern MO/nern AR
into the wrn Great Lakes overnight, with squall line along trailing
cold front as the main feature of interest this TAF period.
Initially, VFR conds will exist at both terminals this evening with a
gradual increase in ci and sse wind at 13G21 kts. Mid/high-lvl cigs
will develop by mid-evening as the squall line approaches, with a bkn
stratocu deck arnd 040 and VCTS introduced at 04Z/MSL and 06Z/HSV...
followed by a 2-hr tempo for MVFR cigs/vsby in tsra. Lgt rain and
reduced vsby will persist for a couple of hours in the wake of the
squall line, with sfc winds diminishing/veering to wsw and cigs
falling into the IFR category. IFR stratus will prevail after sunrise
given expectations of weak flow/limited mixing...with cigs not
expected to lift to MVFR until 14-15Z. Attm, we will indicate
scattering of the stratus deck by early aftn, but there are
indications that this may not occur until closer to 28/00Z.

&&

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

$$

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


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