Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 271200

Area Forecast Discussion
National Weather Service Birmingham AL
700 AM CDT Sat May 27 2017

Aviation Discussion.


Today and Tonight.

Watching a thunderstorm complex in far northern Arkansas this
morning, moving to the east. There is uncertainty regarding whether
this complex makes more of a turn to the southeast later this
morning or stays to our north, crossing TN.  Although track varies,
models are consistent in showing a weakening trend through the day.
This is partly due to a strong cap over the local area and forcing
associated with the upper level impulse remaining to our north.
However, instability increases significantly across Central Alabama
as temperatures warm into the 80s with dewpoints in the upper 60s to
lower 70s, with bulk shear values around 40-45kts. Stronger storms
could produce damaging wind gusts, and will be adding a low
confidence severe threat to the HWO for the north.

Whether the weakening storm complex makes a turn to the southeast or
convection over north Central Alabama is outflow boundary driven,
either solution will result in isolated to scattered shower/storm
coverage across the north today. Activity will decrease by late
afternoon as the sun begins to set. The upper level pattern remains
active as impulses drive nightly convection, with the next impulse
approaching Central Alabama late tonight.


Sunday through Friday.

The forecast for Sunday will largely hinge on the MCS expected to
approach the area from the northwest late tonight. General
consensus from the latest CAMs is that a chance of isolated to
scattered showers and storms will linger for a couple hours past
12Z as an outflow boundary moves in. These would likely be in a
weakened state due to the time of day and the continued presence
of strong mid-level capping over the area associated with a strong
EML. This outflow boundary would need to be watched for additional
re-development of showers and storms later in the day, but latest
guidance suggests it will retreat north of the forecast area by
afternoon. This would leave the forecast area in a capped air
mass with a lack of forcing to break the cap during the afternoon,
as 700 mb temperatures will remain around +10-11C with dry air
also present at mid-levels and the front well off to the west. For
the late afternoon, will leave just a slight chances of an
isolated shower/storm across the northern half of Central Alabama.
If a storm develops in the afternoon, it would likely be strong
to severe with hail/wind given high CAPE, steep mid-level lapse
rates, and 35-45 kts of 0-6 km bulk shear. But, confidence in
development is too low to include in the HWO at this time.

Greater chances of showers and storms will come after sunset
Sunday night, more likely some time after the 10pm-midnight
timeframe. A decelerating cold front will move in from the
northwest, extending southwestward from low pressure over the
Great Lakes. Weak height falls will occur as the trough over the
north-central US causes the Gulf ridge to be suppressed eastward,
and the forecast area will begin to feel the influence of the
right entrance region of a jet streak over the Ohio Valley.  This
upper-level forcing will result in cooling at 700 mb and allow
the cap to weaken. Will keep an eye on this late Sunday night
period for any strong to severe threat given the instability still
present, but time of day will not be particularly favorable and
bulk shear somewhat marginal.

Numerous showers and thunderstorms are expected for Memorial Day
in the southwest to northeast oriented moisture axis along and
south of the front which will have stalled. These will be aided by
weak shortwaves in the southwest flow aloft to the south of the
strong upper low over the Great Lakes. Strong storms will continue
to be possible, and an isolated severe storm cannot be ruled out.
0-6 km bulk shear values will be marginal at 25-35 kts, mid-
level lapse rates will be weakening, and profiles will be
fairly saturated. Ongoing precipitation may also hamper
destabilization. Therefore, chances of an organized threat are too
low to mention in the HWO at this time.

The forecast area will remain in the moisture axis south of the
old front Tuesday and Wednesday with continued cyclonic upper-
level flow south of the upper low lifting into Ontario. Scattered
showers and storms will continue across the area, with the best
chances around peak heating. For the rest of the week, the upper
low will continue to lift northeastward while ridging develops
over the Plains. A few southern stream waves will try to under-cut
the ridge. A warm and humid air mass will remain in place over
Central Alabama, with continued summertime afternoon scattered
showers and thunderstorms.



12Z TAF Discussion.

A decaying thunderstorm complex and possible outflow boundaries will
push southward into north Central Alabama today. Isolated to
scattered thunderstorms will be possible as these boundaries
encounter an unstable airmass. Given the uncertainty on timing and
coverage, have included VCTS at BHM, TCL, ANB, and ASN. Confidence
is too low on how far south activity spreads, and will not include
VCTS at EET, MGM, and TOI.

Activity will diminish by early evening, with MVFR cigs possible
late in the period. Will also be monitoring the next impulse to
approach the area late tonight, and might have to add TS wording for
early Sunday morning with future forecasts.




A few showers and storms will be possible across the north today and
Sunday, but most areas will remain warm and dry. The best rain
chances hold off until Sunday night into Monday when the next upper
level system and surface front approach. Critical fire weather
conditions are not expected.


Gadsden     85  68  86  67  81 /  10  20  40  60  50
Anniston    87  70  86  69  82 /  10  20  20  50  50
Birmingham  88  73  87  71  82 /  10  20  30  60  60
Tuscaloosa  87  73  87  70  83 /  10  10  20  60  70
Calera      87  71  87  70  82 /  10  10  20  50  70
Auburn      87  70  86  71  83 /   0  10  10  30  50
Montgomery  90  73  91  72  85 /   0  10  10  40  60
Troy        88  70  89  71  85 /   0  10  10  30  50




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