Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 271308
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
808 AM CDT Thu Jul 27 2017

.UPDATE...
MESOSCALE UPDATE.

&&

.SHORT TERM...

12Z BMX sounding indicates a fairly moist profile from the surface
aloft to around 660 mb where a subsidence inversion exists.
A more defined subsidence inversion exists further aloft around
460 mb. A radiation inversion was present from the surface to
around 950 mb. The wind profile is characterized as generally
light and variable with speeds generally less than 10 knots from
the surface to around 450 mb. Winds were generally out of the
north and northeast from the surface to around 830 mb then
transitions from the west and northwest from 830 mb to around 600
mb. An easterly flow develops from above 600 mb aloft with speeds
increasing from 20 to 40 knots.

The vertical profile contains a precipitable water (PWAT) amount
of 1.71 inches with a DCAPE value of 1326 J/kg. Convective
temperature is 87 degrees with surface-based Convective Available
Potential Energy (CAPE) of 1355 J/kg with forecast values around
2833 J/kg. Lifted index values are -5 with readings around -8
expected this afternoon.

Much of our forecast area was experiencing mostly sunny skies
early this morning outside of some patchy fog south and east that
was largely confined to near bodies of water and areas lower in
elevation. Expect fair skies to persist through much of the
morning, permitting maximum solar insolation with temperatures
expected to rapidly approach the 90 degree mark toward noontime.
Best convective development time frame looks to range from 10 am
to 1 pm dependent upon local temperature trends.

Surface, radar and satellite analyses indicate lower surface
pressures across the northwest corner of the state while a
localized extent of slightly higher surface pressures exist across
our southern and central counties. A low level boundary was
identified in regional NEXRAD mosaic imagery from southwest Macon
county south through western Bullock and into central Pike County
around 06Z and was drifting west. This boundary was identified at
sunrise from satellite imagery and extended from southeast
Montgomery County through eastern Butler County and into central
Covington County and continued to drift westward. Additional low
level boundaries were analyzed across the eastern counties from
southern Cherokee County through eastern Calhoun County and
extended southwest across Talladega County and into northeast
Shelby County and was drifting to the southeast.

The larger scale picture shows a surface low across southern
Kansas along a cold front that extends from western Oklahoma
northeast into northern Missouri and central Illinois while a warm
front extended from northern Indiana southeast across southwest
Ohio and into eastern Kentucky further south into east Tennessee,
Western North Carolina and into northeast Georgia. An outflow
boundary was analyzed to the southeast of the cold front from
southern Missouri through southern Illinois and into southern
Indiana.

DCAPE values over 1000 J/kg along with the notable volume of drier
air aloft will encourage the development of wet microbursts with
locally a high risk for wet microbursts today. Instability is more
than sufficient for initial robust updrafts that will encourage
formation of strong pulse convection. A reminder that ground
conditions remain moist in some spots and some trees are weaker
than normal due to drought conditions last year. Winds much lower
than severe criteria will be capable of causing tree root systems
to lose grip and allow them to fall.

The combination of relatively high PWAT values of 1.71 inches
coupled with model-derived storm motion vectors indicating
expected storm motions from 5 to 20 knots generally from the north
and northeast. Relatively slow storm motions will allow convection
to efficiently produce heavy rainfall over localized areas. Areas
that have recently experienced heavy rains and flooding will be
highly prone to additional flooding as ground conditions in these
areas remain saturated.

05

Previous short-term discussion:Today and Tonight.

A similar set-up today as yesterday with central Alabama on the
eastern periphery of a large upper ridge over the central United
States. Drier and more stable air aloft will advect eastward into
west Alabama this morning with a narrow axis of mid level moisture
across east Alabama.  Any convective activity today should be east
of I-65. The NAM/RUC models indicate high microburst potential
across east Alabama this afternoon, and will mention isolated severe
storms possible east of I-65. Warmer low level air will continue to
feed into Alabama today in westerly low level flow. 1000-850mb
thickness values support mid 90s across west Alabama with lower 90s
east of I-65.  With surface dewpoints in the lower 70s this
afternoon across west Alabama, heat indices will approach 105
degrees.  Based on values from yesterday, will go ahead and issue a
heat advisory for the afternoon hours for areas generally west of a
line from Bear Creek, to Brookwood, to Six Mile, to Selma. Isolated
storms will linger into the evening hours across east Alabama. A
surface low moving across the mid Mississippi Valley region late
tonight will trigger a band of convection which could enter
northwest Alabama between 3 am and 6 am Friday, and small rain
chances continued across the far northwest counties.

58/rose

.LONG TERM...
Friday through Thursday.

We begin Friday with a cold front stretched through the Ohio River
Valley pushing southward into Northern AL. This cold front is
associated with a broad surface low near the base of a developing
longwave trough. The upper level low pressure system is offset well
to the north, over the Great Lakes. We could see some early
shower/thunderstorm activity across the far northern counties Friday
morning. The cold front slowly moves southward during the early part
of the day Friday. By Friday evening, the surface low strengthens
and troughing pattern amplifies over the Eastern US. This
strengthens the northerly flow and pushes the cold front through the
area quickly Friday evening into early Saturday morning. There are
still timing differences in the models, so specific arrival times
will need to be re-evaluated with future model runs. Expect some
decrease in storm coverage after sunset Friday night, increasing
again Saturday morning across the southern most counties as the cold
front finally pushes south of the area. Ahead of the cold front,
PWATs are still showing up in the 2.3-2.4" range, so expect very
heavy rainfall with any storms that do develop. Due to the increased
forcing from the cold front and increased precip loading, I can`t
rule out the potential for some storms to produce strong to severe
downburst winds. 850mb temps are around 20C, making lapse rates very
low; therefore, not expected a large hail threat. We could see some
small hail in the strongest storms, but otherwise the main threat
will be isolated damaging winds and localized flooding. Have
continued mention for low confidence severe for these threats.

The low pressure system slides southeastward Saturday and
essentially stalls out over the Tidewater Region of Coastal Virginia
through early next week. This keeps us under northerly flow on the
back side of the deep trough, leading to drier and cooler weather
through at least mid-week. Models hint at another impulse digging
through the main trough towards the end of next week that could
bring another round or showers/storms. Otherwise, expect below
average temps and precip coverage for most of the upcoming week.

25/Owen

&&

.AVIATION...
12Z TAF Discussion.

VFR conds will dominate this period with exception of lower vsbys
associated with afternoon tstms. High pressure aloft will result
in minimal cloud cover and only small chances for thunderstorms.
Isolated to sct tstms will develop by early afternoon across east
Alabama, and possibly impact KMGM and KTOI. Surface winds will be
generally from the west and southwest due to a surface high
centered over south Louisiana.

58/rose


&&

.FIRE WEATHER...

Isolated to scattered rain chances today, with best chances in the
east. Hot and humid weather expected today. Cold front approaches
on Friday, increasing rain chances. Cooler and drier weather
behind the front through the weekend. No fire weather concerns at
this time.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     93  73  88  71  87 /  20  10  60  50  10
Anniston    92  73  89  72  87 /  30  10  50  50  20
Birmingham  94  76  90  73  87 /  20  10  50  50  20
Tuscaloosa  96  75  92  74  89 /  10  10  50  50  20
Calera      93  75  90  74  88 /  20  10  40  50  30
Auburn      91  74  91  73  86 /  30  10  30  40  40
Montgomery  94  76  94  75  89 /  30  10  30  40  50
Troy        93  73  94  74  87 /  30  10  20  40  60

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
Heat Advisory from noon today to 7 PM CDT this evening for the
following counties: Bibb...Dallas...Fayette...Greene...Hale...
Lamar...Marengo...Marion...Perry...Pickens...Sumter...Tuscaloosa.

&&

$$


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