Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 281311

Area Forecast Discussion
National Weather Service Birmingham AL
811 AM CDT Fri Jul 28 2017




12Z BMX Sounding indicates a slightly drier profile overall
compared to yesterday and this is resulting in a lower
precipitable water (PWAT) value of 1.59 inches. The sounding is
characterized by a series of subsidence inversions off the surface
around 900 mb, around 770 mb, around 660 mb, around 455 mb and
around 335 mb. Cloud cover was increasing during the launch time
this morning but the sounding does contain a radiation inversion
around 985 mb. The wind profile is largely unidirectional from the
west from off the surface to around 500 mb with speeds between 15
and 25 knots followed by a northwest flow from around 500 mb aloft
to around 370 mb with speeds between 10 and 15 knots. Above 370 mb
a north wind was measured at speeds ranging from 20 to 45 knots,
generally increasing with height.

Convective Available Potential Energy (CAPE) values were slightly
over 1100 J/kg with a forecast surface based lifted parcel
yielding around 1800 J/kg. With increasing cloud cover already
across the north-central counties, believe this value will be
tough to achieve except across our far southern counties.
Downdraft CAPE was found to be near 1250 J/kg with a convective
temperature of 89 degrees. There is more in the way of wind shear
present with bulk shear values between 15 and 20 knots generally
and storm relative helicities between 33 and 81 m2/s2 in the 0-1
and 0-3 km layers off of the surface respectively.

Model-derived storm motion vectors indicate generally west to
northwest flow expected with speeds between 10 and 30 knots. Wet
microburst analysis shows overall a moderate risk of wet
microbursts today with the primary concern further south where
drier air is expected to be more prevalent through the vertical

Morning surface, radar and satellite analyses indicate highest
surface pressures reside across the southwest portion of the state
with lower surface pressure readings across the Tennessee Valley
region. An outflow boundary from shower activity dropping
southward was analyzed from northern Tuscaloosa County east across
central Jefferson County and through central St. Clair and into
southern Cleburne County.

The synoptic scale analysis indicates a surface low pressure area
located just west of Fort Smith, Arkansas that is along a surface cold
front extending from southern Oklahoma into northern Arkansas that
extends northeast along the Ohio River up into southern Ohio.
Another low pressure center was analyzed near Cairo, Illinois and
near Wilmington, Ohio.

The analysis supports the ongoing forecast with the surface cold
front continuing to advance southeast toward the state today and
tonight. Across our southern counties, mostly clear skies will
permit strong surface insolation through late morning before cloud
cover increases from the north. These areas are expected to have
the higher potential for enough instability to support a few
strong thunderstorms with a lower end moderate wet microburst
risk. The presence of wind shear looks to remain too low to
support organized strong to severe convection.

Further north, ample cloud cover from ongoing convection across
the Tennessee Valley is helping reduce atmospheric
destabilization. This will hamper instability values and reduce
the potential for organized strong to severe storms. High
precipitable water values observed to the north at Nashville,
Tennessee is supporting efficient heavy rain producing convection
across portions of the Tennessee Valley at this time and the
potential will be monitored for heavy rainfall that may produce
flash flooding, especially in urbanized and poor drainage areas.
Thankfully, precipitable water values observed are a bit lower
than yesterday and storm motions are faster than yesterday and
these factors will help to reduce the risk of flash flooding to an
extent. The local atmosphere will modify through the day with
higher PWAT values expected and surface convergence and mechanical
lifting mechanisms aiding in convective development despite lower
surface-based instability values.


Previous short-term discussion:Today and Tonight.

The first wave of showers have entered northwest Alabama early
this  morning. This activity will likely weaken as it encounters
more stable air downstream. This will be a very progressive upper
flow pattern, with most of the rainfall associated with lift ahead
of the upper trof axis. The cool front will lag the upper trof
axis with the actual surface front not forecast to move into the
northern counties of central Alabama until the 9 pm to midnight
time frame. Widespread rainfall expected across areas along and
north of I-20 today due to PWAT`s over 2 inches and forcing ahead
of short wave trof, but the models are showing a wide range of QPF
amounts. While there will likely be some pockets of heavier
rainfall, difficult to say with any confidence where the heavier
rainfall will occur. It could be argued that a Flash Flood Watch
over all of north Alabama may be overdone given that any flash
flooding will be isolated, but uncertainty in model guidance
dictates a broader threat area. SPC has removed any threat of
severe storms across the area for today, but still believe there
could be an isolated severe storm for areas along the I-20
corridor this afternoon as CAPE values are forecast to climb to
2000-2500 J/KG. The threat of heavy rainfall will diminish this
evening as the air mass stabilizes and upper forcing weakens.
Despite less forcing, there will still be enough forcing along the
surface front to warrant likely pops along and south of the I-20


Saturday through Thursday.

A rare August cold front will continue moving southward on
Saturday with showers and storms likely across the southern third
of the forecast area during the latter half of the morning and
into the early afternoon. Farther north, deep-layer northerly flow
will usher in significantly drier air marked by PWAT values below
1 inch, effectively shutting off the faucet for several days and
providing huge relief from the recent excessive humidity.

The upper-level trough will remain in place across the eastern
CONUS through the weekend with residual dry air lingering into
Monday and Tuesday. Rain chances should return for Wednesday and
Thursday in response to yet another unusually strong mid-summer
trough taking shape over the ArkLaMiss region.



12Z TAF Discussion.

Increasing chances of showers and thunderstorms today, mainly
affecting the northern TAF sites, as an upper level trof
approaches the area from the north. Cigs are forecast to stay
above 3000 feet agl thru 06z, with lcl cigs blo 3000 feet agl in
thunderstorms. Low vsbys in heavier showers and tstms will be the
main aviation hazard, with IFR vsbys becoming more likely after
18z. Southwest winds will increase to 8-12 knots by 17z as a cool
front approaches from the north. IFR cigs will become more likely
after 09z, but confidence too low to include in this TAF cycle.




High rain chances are expected along and ahead of a cold front
which will move through the area over the next 24-36 hours.
Cooler and drier weather air will move in behind the front
Saturday afternoon with no chance of rain until the middle of the
upcoming week. There are no fire weather concerns at this time.


Gadsden     86  70  86  63  88 /  70  60   0   0   0
Anniston    87  72  86  65  88 /  60  60  10   0   0
Birmingham  88  73  87  67  88 /  60  60  10   0   0
Tuscaloosa  91  74  89  67  90 /  60  60  20   0   0
Calera      89  73  87  67  88 /  50  60  20   0   0
Auburn      90  73  87  67  87 /  30  60  50   0  10
Montgomery  94  75  90  68  90 /  30  60  60   0   0
Troy        94  73  87  67  88 /  20  50  80  10  10


Flash Flood Watch through this evening for the following
counties: Bibb...Blount...Calhoun...Cherokee...Clay...Cleburne...
Randolph...Shelby...St. Clair...Talladega...Tuscaloosa...


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