Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 271737

Area Forecast Discussion
National Weather Service Birmingham AL
1137 AM CST Mon Feb 27 2017

For 18Z Aviation.


Today and Tonight.

As of early this morning, southwest upper flow disturbances are
ushering across the Deep South a convective complex.  At the same
time, onshore/low level flow increases across South Alabama but is
slow to make its presence known into Central Alabama with a very dry
airmass in place at present to overcome. Thus, the convective
activity is slow to make progress into our CWA at this time. Even
so, high res models such as the HRRR/ARW are indicating enough of an
increase of lift for an area of precipitation to develop over the
southeast half of Mississippi and feed into the convective complex,
possibly enhanced by our 35-40 KT low level jet moving across the
area. This area will slowly make its way eastward early this AM. So,
isentropic lift precipitation is still expected to begin shortly
before/near sunrise across the far west part of Alabama and spread
across Central Alabama during the morning. Elevated instability is
expected along with some thunder at times as dew points are expected
to slowly increase as onshore flow starts to overcome the dry
airmass. By midday though, the low level jet is gone and will not be
in place once the better dew points arrive across the southern
counties of Central Alabama for this afternoon to aide in any
stronger development. Expecting a large temperature gradient today
with upper 50s far north to lower 70s far south as early
rains/clouds could hamper heating across the north.

As we move into this evening, the upper disturbance and main swath of
convection will be exiting to the east. We should see a lull for
much of the evening and some of the late night between waves. With
our surface ridge by this time completely moved off to the east over
the Atlantic, our warm front will have no problem being ushered to
the north across Central Alabama this evening with low level
southerly flow taking over and dew points continuing to increase.
Will have chance pops in across the north after midnight with a
lingering moisture axis. With much higher dew points tonight
expected than at present, it goes without saying that our overnight
lows won`t be very low.


Tuesday through Sunday.

Forecast area will reside under southwest flow on Tuesday between
the upper-level trough moving from the Rockies to the Plains and
upper-level ridging over the Gulf. A weak wave in the southwest
flow and area of low-level moisture advection will move over the
northwest counties which may serve to enhance rain chances a
little there during the morning. This weak wave and moisture axis
will move north of the area towards afternoon. Some chances of
isolated to scattered showers will remain possible elsewhere with
low- level isentropic lift and sufficient moisture. While shear
and helicity profiles in the far northwest would be conducive to
an isolated strong to severe storm, it looks like the wave and
best chances of precipitation will be just north of the area by
the time the best instability arrives in the afternoon. Morning
low clouds may also put a damper on instability as well. Therefore
strong to severe storms are not expected at this time for
Tuesday. Also trimmed back chances of thunder to just the northern
counties which may need to be trimmed out of all but the far
northwest based on available high-res guidance. With southerly
low-level flow a warm day is expected with highs in the south and
west reaching the upper 70s to low 80s, just under record highs.

Drier air aloft will be moving in from the south by Tuesday
evening with only isolated showers during the evening in the far
north. Shower chances may begin to increase in the northwest
towards sunrise Wednesday morning. Southerly flow will keep low
temperatures very mild Tuesday night, mainly in the low 60s.
Patchy advection fog will be possible in the far southeast but
strong boundary layer winds should prevent anything too dense.

Looking at Wednesday`s severe weather threat, models have trended
faster and further north with the track of the surface low moving
from Michigan quickly northeastward into Canada. Sfc-850 mb flow
is looking more veered especially as the afternoon progresses and
also weakens with time, with hodograph curvature decreasing. Will
continue to mention a threat of an isolated tornado or two across
the north, with helicity around 200 m2/s2 and good low-level and
deep- layer shear, but any tornadoes would likely be weak/short-
lived with the better threat remaining north of the forecast area.
Relatively best tornado chances may be with any storm that gets
going earlier, around mid-day. The main threat with this system
instead will be damaging winds with a large squall line near the
front moving into an air mass with 60-65 dew points, CAPE around
1000 J/kg, steep mid-level lapse rates, and strong wind fields
aloft. Hail will also be possible with WBZ heights around 10k ft
and steep mid-level lapse rates. The GFS remains a bit too fast
with the timing of the front with the ECMWF and NAM looking more
reasonable with the front moving through during the afternoon and
evening. There is no change to the threat areas with the highest
threat being in the northern and western counties.

Thursday through Saturday the forecast area will be under a dry
west to west-northwest flow pattern with more seasonable
temperatures. Moisture may begin to return as early as Sunday with
better rain chances ahead of a cold front just beyond the extended
forecast period.



18Z TAF Discussion.

Morning SHRA with occasional TSRA has produced a cold pool across
our northern TAF sites resulting in a northerly low level flow
while southern sites have experienced southerly winds that are
transitioning to a northerly flow at midday. Expect this wind
shift to be temporary with a southerly low level flow redeveloping
at all sites through the afternoon hours as surface high pressure
is situated off the eastern seaboard. An upper level disturbance
will continue to move east resulting in SHRA with potential for
limited TSRA, particularly near TOI/MGM. Expect decreasing shower
coverage tonight with low CIG`s across most sites. Some reduction
in VIS is expected overnight with greatest potential SE though
confidence currently too low to mention FG at TOI/MGM. More SHRA
expected toward the end of the forecast cycle with improving VIS.




Moisture will increase rapidly today with high rain chances. Rain
chances decrease tonight through Tuesday night but a moist air mass
will remain in place. A cold front will bring a round of showers and
thunderstorms Wednesday afternoon and Wednesday night. No fire
weather concerns.


Gadsden     55  53  72  64  73 /  30  40  50  30  90
Anniston    57  55  75  63  76 /  60  40  40  20  80
Birmingham  56  57  76  65  75 /  40  40  40  30  80
Tuscaloosa  58  57  79  66  78 /  60  30  40  20  80
Calera      57  57  76  65  77 /  60  30  30  20  70
Auburn      62  58  76  63  75 /  80  20  20  10  30
Montgomery  63  60  81  65  81 /  80  20  20  10  40
Troy        66  59  79  65  80 /  80  20  10  10  30




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