Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 221758
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
1258 PM CDT Sat Jul 22 2017

.UPDATE...
Midday Update and 18Z Aviation.

&&

.SHORT TERM...

This afternoon.

Several features to talk about this morning both on the synoptic
scale and the mesoscale. First, synoptically we have the large
upper level ridge of high pressure just off to our northwest with
everything rotating around the periphery of the ridge. One of
those features is a mid to upper level disturbance tracking
westward along the Gulf Coast. That disturbance helped to trigger
storms along the coast yesterday. Afterward, second and third
generation convection was triggered mostly along our southern
counties from outflow boundary interactions. That disturbance is
now just off the Louisiana coastline. Weak surface ridging was
present to our south and east which will continue to produce a
weak southerly flow at the surface. Meanwhile, drier air aloft is
advecting westward from Georgia and the Carolinas, as seen on
water vapor and GOES Total Precipitable Water.

In terms of what was viewed at the mesoscale, several boundaries
were noted across Central Alabama on visible satellite that may
help to trigger afternoon convection. Based on the KBMX 12Z
sounding, we`re more moist across the atmospheric profile than 24
hours ago. The only exception was aloft, where the drier air was
being advected in from the east. A more moist profile means lapse
rates aren`t quite as steep aloft. After modifying the sounding
for forecast high temps today, LFCs are being calculated around
4500 feet which is similar to yesterday. CAPE values of around
4000-4500 j/kg were also being generated this afternoon. In terms
of lift, synoptic lift is expected to be less today with the mid
to upper level disturbance more to our west. Therefore,
opportunity for widespread convection should be more limited today
especially with the drier air advecting in aloft.

So, for the updated forecast, have adjusted PoPs a bit higher
across the far western and southern counties and a bit lower
across the southeast counties. The exception was for the ridgetop
locations where updrafts and cu were already developing as of
11am. As is the case for any hot afternoon in the summertime, if
a strong enough updraft is able to develop we can expect gusty
winds and torrential rainfall. Microburst parameters based on the
modified sounding were coming up with a moderate potential this
afternoon, so something for us to keep an eye on. Highs are
generally on track for today, and we`re still expecting Heat
Advisory criteria (Heat Index 105 or higher) to be met in many
locations.

56/GDG

Tonight and Sunday.

Overall changes to this period are minimal. Much of the afternoon
convection should dissipate within an hour or so of sunset and
held onto some low pops through midnight or so south. A few of the
models try to hold onto or redevelop some convection overnight.
Although mean relative humidity is higher than previous days, do
not see a big lifting mechanism overnight. It should mainly hold
off until the heating of the daytime.

Sunday appears to have a bit more mean moisture as PWATS climb to
near 2 inches area wide. Daytime relative humidities are high
above the surface and expect a decent cumulus field overhead. Low
level thickness and 850 temp are near or just above today and
surface dew points remain 70 to 75 daytime. Think the high
temperatures will be in the 90s most areas but near or just below
todays numbers. This puts most of the heat indices in the 100-104
range. This once again will have to be monitored closely as the
mix of temperature and dew points hour by hour will be key and we
are close to the threshold of 105 degrees.

Sunday will transition to more of a normal summer day as pops will
be in the 40s areawide, which is expect on most summer days. the
mid to upper flow starts to become south to southwest as a trough
digs toward us from the northwest.



.LONG TERM...
Sunday through Friday.

A surface low moves through the Great Lakes region Sunday and
stretches a cold front down through the Ohio River Valley. This
creates a subtle shortwave in the upper levels that moves through
the TN Valley Monday into Tuesday. Therefore, have increased PoPs
both Monday and Tuesday as extra lift could lead to increased
coverage of afternoon storms.

As we go into the second half of the week, scattered thunderstorms
are possible each afternoon with near-normal diurnal temperature
ranges. Troughing over the Eastern US with upper-level ridging over
the Great Plains will set up generally northwesterly flow across
Central AL. Models hint a stronger low pressure system moving into
Quebec that could send a cold front down into the Gulf States by
next weekend, but I`d like to see consistency in timing for a few
model runs before increasing PoPs above 40%.

25/Owen

&&



.AVIATION...
18Z TAF Discussion.

Terminal forecast remain relatively the same and close to a
typical summer day. Cumulus field has started to develop with
convection firing north and east of the area. Rain chances will be
only around 40 percent of so but the highest moisture is north and
left VCTS in 20-24z. The storms will move from east northeast to
the west southwest this afternoon. Models hint at a boundary
moving into the southern areas later this afternoon despite
slightly lower mean humidities. Therefore, have VCTS 22-02z south.
The only other restriction appears to be some MVFR BR around
daybreak. The only change was to make tempo groups due to
confidence in development. Winds remain rather light and keep a
south to west component.

75

&&

.FIRE WEATHER...

Isolated to scattered showers and thunderstorms are expected for
the next several days. There are no fire weather concerns at this
time.

&&

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

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
Heat Advisory until 9 PM CDT this evening for the following
counties: Autauga...Barbour...Bibb...Blount...Bullock...
Chilton...Clay...Coosa...Dallas...Elmore...Fayette...Greene...
Hale...Jefferson...Lamar...Lee...Lowndes...Macon...Marengo...
Marion...Montgomery...Perry...Pickens...Pike...Russell...
Shelby...St. Clair...Sumter...Talladega...Tallapoosa...
Tuscaloosa...Walker...Winston.

&&

$$


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