Flash Flooding in Georgia, March 12, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
336 PM EDT Thu Mar 12 2015

.Near Term [Through Tonight]...

As of 18Z, regional radars indicated convective initiation near the
Alabama-Florida border, and from TLH southeast to around CTY in the
Florida Big Bend. This area of low-level moisture flux convergence
arcing from southeast Alabama to the Suwannee River basin is
expected to become more convectively active over the next few hours.
Latest objective RAP analysis shows SBCAPE around 1500 j/kg with
effective deep layer shear of 30-35 knots, which is similar to what
was expected based on a modified 12Z Tallahassee sounding. This
combination of moderate shear and instability should be sufficient
to produce some organized thunderstorms this afternoon, and a severe
storm or two can't be ruled out. The main threat would be damaging
wind gusts. With increasing low-level forcing between 21Z and 03Z as
a backdoor cold front arrives from the northeast, thunderstorms
should congeal into clusters with time - and this is when organized
cold pools and damaging wind gusts would become more likely, along
with heavy rainfall. Convection should diminish in coverage and
intensity after 03Z and be focused primarily in the northwestern
parts of the forecast area. The remainder of the overnight period
should see low stratus advancing into the area from the east, a
chance of some patchy fog, and continued above normal lows.

.Short Term [Friday Through Saturday Night]...

Friday, the upper level pattern will consist of a mid/upper ridge
over California, extending into the Great Basin, and another ridge
over much of the Southeast. Much of the country will be under the
influence of a large, broad trough. Embedded within the trough are
three distinct waves: one over the Northern Plains, one over
western Mexico and the Southwest, and a final one over the
Southern Plains and Lower Mississippi Valley. Associated CONUS low
pressure systems at the surface are forecast to be over the Upper
Mississippi Valley and the Lower Mississippi Valley.

Through Saturday night, the Mississippi Valley anomalies will
converge on the Northeast. The wave over western Mexico will
continue to drift south and cutoff a low over Mexico Saturday
night. The northern stream trough will move east over the eastern
third of the CONUS and strengthen as the Mississippi Valley waves
merge with another wave diving out of Manitoba/Ontario into the
Great Lakes region. At the surface, the wedge of high pressure
along the Eastern Seaboard should hold through Thursday as the
Lower Mississippi Valley low moves essentially due north. As the
surface lows begin to merge Friday night and move east, the wedge
should break down. This will clear a path for the cold front
associated with the now merged frontal system to move through the
Tri-State region Saturday through Saturday night.

An area of light to moderate rain should be expected Friday
morning along the Panhandle coast as remnant forcing from a
surface trough moving off the Peninsula overnight, and veering
flow over relatively cooler shelf waters resulting in sloping
isentropic surfaces combine forces. Much of the shower activity on
Friday however, will be focused along the wedge front remaining in
place across the Tri-State region. Though due to an influx of mid-
level dry air, convection should remain rather shallow. Friday
night through Saturday night will feature a gradual west-to-east
increase in PoPs associated with the passing front. There is
consensus that the front will be weakening along its journey, so
the best chance for rain with the front will be west of a line
from Tallahassee to Albany.

.Long Term [Sunday Through Thursday]...

A transition to a drier weather pattern is expected through the
extended range. High temperatures should remain above average, in
the upper 70s to low 80s. Rain chances may increase once again
around mid to late week as a weakening shortwave (remnants of
cutoff over Mexico) approaches from the west.

&&

.Aviation...
[Through 18Z Friday]

VFR conditions are prevailing at our terminals at 18Z, and that
is generally expected through the early evening. Some IFR VIS
restrictions are possible in showers and thunderstorms, with
chances of +SHRA and +TSRA highest between 19Z and 02Z. A cold
front will push into the area from the northeast this evening,
bringing moderate easterly breezes and lowering CIGS. The ceilings
should fall through MVFR range and into LIFR/IFR later tonight.
The low CIGS may persist through much of the morning on Friday.

&&

.Marine...

Moderate south/southeasterly flow will prevail through Saturday.
Behind the front Saturday night/Sunday morning a transition to
light winds and low seas is expected as calm high pressure drops
into the Southeast.

&&

.Fire Weather...

Sufficient low-level moisture is forecast to remain in place over
the next several days, with no fire weather concerns anticipated.

&&

.Hydrology...

Rainfall amounts today through Friday should average around 1".
Localized higher amounts up to 4 inches could be possible across
south Georgia and southeast Alabama. QPF associated with the front
should remain on the order of a half of an inch or less. The
aforementioned isolated higher amounts could have a minor impact
on area rivers should it fall over a basin.

&&

.Preliminary Point Temps/PoPs...

Tallahassee   64  78  62  78  60 /  50  60  40  50  20
Panama City   65  71  64  69  60 /  30  60  60  60  20
Dothan        59  72  60  78  57 /  70  70  50  50  10
Albany        57  70  57  78  57 /  80  60  50  50  30
Valdosta      62  79  61  79  60 /  50  40  30  50  30
Cross City    66  82  63  80  62 /  30  20  30  30  20
Apalachicola  64  70  63  69  63 /  20  60  50  60  20

&&

.TAE Watches/Warnings/Advisories...

FL...High Rip Current Risk until 7 PM CDT this evening for South
     Walton.
GA...None.
AL...None.
GM...None.

&&

NEAR TERM...LAMERS
SHORT TERM...HARRIGAN
LONG TERM...HARRIGAN
AVIATION...LAMERS
MARINE...HARRIGAN
FIRE WEATHER...BARRY
HYDROLOGY...HARRIGAN