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Area forecast discussion
National Weather Service Birmingham Alabama
652 am CDT Mon may 22 2017

for 12z aviation.


Short term...
today through Tuesday.

The weak wind shift associated with the stalling cold front is
currently located along a Selma to Birmingham to Gadsden line,
while the lower dew points are lagging behind to the northwest.
The front will meander southeastwards some this morning, before
lifting back slightly to to the northwest ahead of the
approaching shortwave, becoming located along the I-59 corridor by
afternoon. At upper-levels, a broad trough will begin to amplify
over the central US today. A strong 100+ kt subtropical jet streak
is located at its base over Texas. This will aid a couple
convectively modulated southern stream waves that will move
eastward through central Alabama today and tomorrow, ahead of the
main trough axis on Wednesday.

Wave #1 looks fairly potent and is responsible for a couple
different areas of convection currently located across Texas and
Louisiana and adjacent coastal waters, and will move into central
Alabama late this afternoon and tonight. Scattered showers and
thunderstorms will develop this morning and this afternoon ahead
of the shortwave, in the warm and moist air mass along and south
of the front. 0-6 km shear values will be generally around 25-30
kts, with a tendency for stronger shear values to be on the cool
side of the front, though some pockets of higher values will be
possible due to convective processes. Profiles will be fairly
saturated and mid-level lapse rates weak, with weak low-level flow
during the daytime hours. Some strong storms with gusty winds
will be possible, and cannot rule out an isolated marginally
severe storm with damaging winds. However, threat looks marginal
and not organized enough to include in the severe weather potential statement/graphics but will
be monitored.

Showers and storms will increase in coverage late this afternoon
and especially tonight from southwest central Alabama to southeast
central Alabama due to the shortwave and an at least loosely
organized mesoscale convective system. The low level jet will increase as well due to a surface wave
moving along the front, which will sag southward into southeast
Alabama behind the wave. If a forward propagating qlcs were to
form, there could be a marginal damaging wind threat in far
southeast central Alabama, but most cam runs do not look that
organized. Shear does not look strong enough for a brief tornado
threat in southeast central Alabama either overnight into tomorrow
morning, unless the surface wave is stronger than expected.

More importantly, the threat to focus on will be heavy rain and
flash flooding, more isolated during the afternoon but ramping up
tonight and tomorrow. Pwats will be near 2 inches with strong
upper-level divergence, lift, and low- level moisture transport.
Wave #2 looks to move through on Tuesday. It may be weaker than
wave #1 and track farther south, as the subtropical jet will be
further south, but guidance is indicating the frontal boundary
will be along our southern border. This may keep convection more
elevated than surface-based (minimizing strong to severe storm
potential), but convergence along the boundary will enhance the
heavy rainfall potential along and north of the front, again
across southern central Alabama. The axis of total quantitative precipitation forecast from Monday
to Tuesday on the high-res guidance is a bit further south than
in the global model guidance. Agree with wpc in using the high-res
guidance which places 2-4 inches across southern central Alabama
with locally higher amounts, over areas that already received 2-8
inches of rain Saturday night. Therefore, the current Flash Flood
Watch looks to be mostly in good shape, but a row of counties
will be added to the northern end based on the latest numbers.


Long term...
Tuesday night through Sunday

Wave #3 will be the main upper-level trough axis, anomalously deep
for late may, and will be accompanied by a seasonably strong cold
front. Shear, forcing, and cooling aloft will all be impressive
with this system. However, moisture quality/instability is very
much in doubt as it will be coming in very quickly behind the
previous system (and trending quicker), with some models
especially the European model (ecmwf) indicating westerly/northwesterly low-level
flow ahead of it and only low 60s dew points. Expect the dynamics
to be enough for a fast- moving line of showers and storms
Tuesday night and Wednesday, some strong with gusty winds and
hail. However, not confident enough that convection will be strong
enough to produce severe winds/hail to mention anything in the
severe weather potential statement. Rainfall amounts should also be lighter with this system as
well. Light rain/drizzle will linger Wednesday afternoon/evening
behind the front due to the cold core aloft, and refreshingly
cooler air will move in. The cooldown won't last very long, with
warmer temperatures and some shower/storm chances returning by the
weekend in west-southwest flow ahead of a trough moving through
the northern tier.



12z taf discussion.

For specifics on the forecast itself please read the short term
discussion for the overall scenario. Specific to flight
information, only restrictions at the moment are at toi and mgm
where they are south of the frontal boundary and low clouds have
developed. IFR conditions will be expected for the next several
hours. The good news is that the front is still drifting south and
wave #1 is beginning to already move into the area and will erode
the low clouds with some higher, but still MVFR clouds by 14z to
to 15z.

Rainfall will be more scattered this morning and then become more
widespread by the afternoon into tonight. Once the rain sets in
the north it will likely stay there with just scattered
thunderstorms. No real good timing here for ts so went with just
thunderstorms in the vicinity until the wave actually works into the area overnight. As
for mgm it appears that the wave will work into this area by 00z
and remain over the area through the rest of the taf cycle. For
toi, this area remains the trickiest as the front looks to
generatea few showers/storms here then lift north. This could
potentially end the rain for toi until the wave starts drifting
southward after 6z. Expect multiple updates and amendments
throughout the day with the ever changing conditions.



Fire weather...

Several rounds of heavy rain and storms are expected beginning
this afternoon and continuing through mid week. Another 3-4
inches of rainfall is possible across southern portions of central
Alabama by Wednesday. Therefore, critical fire weather conditions
will not occur.


Preliminary point temps/pops...
Gadsden 80 60 76 61 72 / 30 60 60 70 60
Anniston 80 62 77 62 74 / 40 80 70 60 60
Birmingham 81 62 77 63 72 / 50 60 60 60 60
Tuscaloosa 81 61 79 62 73 / 70 70 60 60 50
Calera 81 62 78 62 73 / 60 70 70 60 60
Auburn 80 65 77 64 74 / 60 90 80 60 60
Montgomery 83 66 80 66 77 / 80 90 80 60 60
Troy 84 66 78 65 76 / 80 80 80 70 60


Bmx watches/warnings/advisories/...
Flash Flood Watch from 1 PM CDT this afternoon through Tuesday
evening for the following counties: Autauga...Barbour...Bibb...

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