Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 270253
AFDHUN

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

.NEAR TERM...(Rest of tonight)
Issued at 953 PM CDT Wed Apr 26 2017

Guidance remains in good agreement this evening in forecasting a
negatively-tilted shortwave trough to eject north-northeastward from
the Ozarks into the mid-MS valley region by 12Z. The cold front
attached to a deepening surface low associated with this feature will
continue to push eastward, with modest mid-level height falls
expected to sustain a squall line along the prefrontal trough/wind
shift axis. Extrapolation of this feature suggests that it will begin
to enter northwest AL between 04-05Z and should exit the region
between 12-14Z. Forecast soundings suggest that MLCAPE in the 500-750
J/kg range will exist across our region, which should be sufficient
to support thunderstorms based on magnitude of deep-layer ascent. As
the primary synoptic trough ejects northeastward, wind fields
throughout the lower-middle troposphere will strengthen, with the
main threats with the convection expected to be wind gusts in the
50-60 mph range and small hail. However, of more concern is nocturnal
strengthening of the low-level jet -- which all models suggest
should peak in the 45-50 knot range prior to arrival of the
convective line. This will provide low-level shear more than adequate
for the development of transient mesocyclones/brief tornadoes with
any supercellular storm structures embedded within the squall
line...and the HWO will be updated to reflect this. Other elements of
the near term forecast are on track at this point and require no
adjustments.

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


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