Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 232340
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
640 PM CDT Thu Mar 23 2017

.UPDATE...
For 00Z TAFS.

&&

.NEAR TERM...(Tonight)
Issued at 338 PM CDT Thu Mar 23 2017

A very pleasant afternoon is in progress across northern AL/southern
TN courtesy of a 500-mb ridge axis extending from central Mexico
northward into the Ozarks...which will continue to migrate eastward
in advance of a shortwave trough crossing the southwestern deserts.
As the trough deepens and spreads eastward into eastern CO/western KS
overnight, the downstream ridge will translate across the TN
valley... with mid/high-level cloud cover expected to clear the area
by late this afternoon.

At the surface, low-level southeasterly flow has increased rather
substantially today...and will continue to become more established as
a high spreads eastward off the mid-Atlantic coast. All guidance
suggests that a 5-7 mb pressure gradient will exist across the region
overnight...and this should maintain a moderate southeasterly flow
and well-mixed boundary layer, especially when combined with a
strengthening southwesterly low-level jet. Elevated warm advection
and moisture return should also support the development of widespread
low stratus clouds late this evening...which will likely persist for
the remainder of the night. Based on these factors, lows were raised
by a few degrees above blended guidance...despite the presence of a
lingering dry boundary layer.

.SHORT TERM...(Friday through Saturday night)
Issued at 338 PM CDT Thu Mar 23 2017

Flow aloft will back to the southwest and increase during the day
tomorrow as the southern High Plains shortwave trough advances slowly
eastward across western OK/KS. Temperatures will be dictated by how
quickly the morning stratus layer mixes out -- if it does at all.
Forecast soundings suggest that stratus may be very slow to
lift...with breaks possibly not occurring until mid-late afternoon.
Thus, in spite of a continuation of strong southeasterly low-level
flow we have forecasted highs in the u60s east and lower 70s west --
a little below MOS guidance in a few spots.

Models indicate that the trough will track eastward into the western
Ozarks Friday night...with deep-layer forcing for ascent increasing
most substantially during this period...based on magnitude of 12-hr
height falls aloft. Lift via low-level warm advection will also
increase during this period as the southwesterly low-level jet
increases to 40-50 knots...and this should support an increase in
showers/thunderstorms before sunrise across mainly northwest AL -- as
dewpoints begin to increase into the u50s/l60s.

Although guidance does not indicate that the upstream trough will
deepen significantly on Saturday, it does appear to assume a
negative-tilt as it pivots northeastward across MO -- and this should
act to further strengthen upward vertical motions across our
forecast area in light of additional factors mentioned above. Thus...
as the plume of greater low-level moisture spreads northeastward
through the day...widespread showers/thunderstorms should gradually
shift eastward to encompass nearly the entire CWFA by late afternoon.
This activity will be developing in a zone of strong low- level WAA
and should not be related to a discernible low-level boundary...but
it will pose a risk of severe weather based on MLCAPE in the 500-1000
J/kg range and deep-layer shear of 30-40 knots. Although the
distribution of CAPE suggests a threat for both marginally damaging
winds and large hail...0-3 km shear of 30-35 knots presents concern
for the development of low-level mesocyclones and brief tornadoes
with the deepest updrafts. This activity will likely progress
eastward during the afternoon hours...with another separate episode
of convection anticipated as a remnant dryline shifts eastward into
the region late Saturday afternoon. Subtle low- level streamline
confluence will exist in the vicinity of this boundary...supporting a
more linear convective feature as it spreads slowly eastward
Saturday night and early Sunday morning. Although both low-level and
deep-layer shear are forecast to weaken with time Saturday evening, a
risk of severe storms will exist with this feature as well.

.LONG TERM...(Sunday through Wednesday)
Issued at 338 PM CDT Thu Mar 23 2017

The strong nature of the surface high looks to keep the main front
associated with the surface low moving into the Great Lakes region to
our northwest through Sunday. However, the weakening/departing
convergence boundary responsible for the activity overnight will
continue to push southeast and into Georgia on Sunday. This should
produce a trailing line of isolated to scattered showers and
thunderstorms in northeastern Alabama into Cullman county. This
activity will be slow to move out of the area into Georgia and
central Alabama on Sunday. Based on most model guidance including the
GFS, this could last into the early afternoon hours. So mostly
cloudy conditions will likely hold on as well in these areas and into
Cullman county. Due to the expected more stable nature of the
atmosphere due to previous convection, not expecting any strong
storms as instability will be elevated and minimal. Some clearing may
occur west of I-65 and with very warm 925 temperatures near and just
ahead of the boundary, mixing should allow highs to climb into the
lower to mid 70s in most locations. Some 75 to 80 degree temperatures
look reasonable given 925 mb temperatures and possibly more sunshine
west of I-65. However, models do weaken the center of the ridge
aloft over the southeast and shift it further east into the Atlantic,
after this boundary pushes into Georgia.

Since the main front will still remain northwest of the area, not
much modification of the air mass is expected behind the departed
boundary. Models continue to quickly develop another upper level
disturbance moving east out of southwestern Arkansas towards the
Tennessee Valley and develop a new frontal boundary to the northwest
of the Tennessee Valley on Monday morning. With decent lower level
winds redeveloping ahead of the approaching system (925 mb winds ~ 30
knots), this should advect lower 60s degree dewpoints at least into
locations west of I-65. This along with highs in the mid 70s should
combine to produce strong surface based instability (CAPE values
1000-1500 J/KG) and elevated instability (MUCAPE values ~ 2000 J/KG).
Most of this looks right now to be in the hail growth area of the
atmospheric column. Wet-bulbs are also are forecast by models to be
between 7000 and 9000 feet much of the day. So given fairly strong
lift, as the surface and associated strong dynamics move across the
Tennessee Valley, expect a potential for strong to severe storms on
Monday. Main threat at this time looks to be large hail and damaging
winds, but this could change as we get closer to the event. Isolated
to scattered showers and storms could linger into Tuesday, as the
front is slow to push Alabama. These storms should just produce some
lighting and gusty winds though.

Somewhat cooler and drier air will move into northern Alabama
Tuesday night into Wednesday, as upper level ridging slowly builds
over much of the southeast. A few showers and the slightly cooler
temperatures could linger into Wednesday. Highs will return to the
upper 60s or lower 70s much of that period, with lows primarily in
the 50s.

A more unsettled period is expected by later in the week as another
strong system pushes from Texas towards the Tennessee Valley. Given
the strong ridge setting up aloft in most models Tuesday through the
end of the week, delayed pop from quicker ECMWF solution.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 640 PM CDT Thu Mar 23 2017

VFR conditions are forecast through late evening. Then as the low-
level jet strengthens a low stratus deck is forecast to move into
the area with cigs lowering to MVFR. Have continued the LLWS as
speeds off the surface increase to around 35-40kts with the wind
direction still out of the S/SE. After sunrise the stronger winds
aloft mix down to the surface and gusty S/SE winds are expected for
much of the day with gusts upwards of 20-25kts. By 18Z cigs lift back
to VFR but a BKN deck remains around 5,000ft.

&&

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

$$

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


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