Area Forecast Discussion
Issued by NWS
FXUS62 KTAE 010114
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
914 PM EDT FRI JUL 31 2015
...Heavy rain expected across portions of the forecast area this
.Near Term [Through Tonight]...
The 8 pm EDT regional surface analysis showed weak frontal waves
just off the FL Panhandle coast and around Columbus, GA. The
boundary layer along and on the warm side of this boundary was extremely
moist, with dewpoints in the mid to upper 70s. (There were even
some low 80s dewpoints in central GA this afternoon). Numerous
thunderstorms developed across central GA this afternoon, and
formed a large surface cold pool which was translating south
across south GA and AL this evening. These storms will likely
continue to re-develop despite the setting sun, given such a
moist boundary layer. Rainfall rates have been heavy, in the 4-6
inch per hour range. The fact that this was occurring this evening
boosts our confidence in our Flash Flood Watch, though that threat
will shift to the southeast portions of our forecast area
overnight and Saturday. We expect most of the rain to diminish
after midnight, except over Apalachee Bay and the adjacent coastal
waters, where convection is likely to linger.
[Through 00Z Sunday] TSRA will diminish later this evening,
followed by generally VFR conditions. A period of MVFR vis/cigs is
possible at KVLD and KABY around dawn. Numerous TSRA are expected
Saturday, especially during the afternoon. The tropical nature of
the rain could result in IFR Vis at times.
.Prev Discussion [411 PM EDT]...
.Short Term [Saturday Through Sunday Night]...
An area of low pressure over the northern Gulf waters will translate
northeastward across our forecast area through the weekend. This
system will bring with it plenty of Gulf moisture to our already hot
and humid local environment, bumping up precipitable water values to
2-2.5" in a swath centered along a line from Apalachicola, FL to
Savannah, GA. In addition to the forcing from the low pressure system
(and our local sea breeze front in the afternoons) and plentiful
Gulf moisture, we will also have some instability on tap with CAPE
around 1500-2000 J/kg and mid-level lapse rates around 6 C/km. With
all of these ingredients coming together, we`re expecting a high
coverage of thunderstorms Saturday and Sunday, especially in north
Florida and southern Georgia.
As is typical for this time of year, some of these storms will
become strong enough to produce downbursts of damaging winds. The
main concern with these storms, however, will be heavy rainfall that
could potential cause flash flooding. Local hi-res ensemble guidance
shows a 20-40% chance of the Florida Big Bend and south-central
Georgia receiving 9"+, and even a 20% chance of exceeding 12" in the
eastern Big Bend. While 20% may not seem like a lot, it is actually
very unusual to see chances that high for rainfall amounts that high-
in other words, we have high confidence that there will be some
locally heavy rainfall totals in the Big Bend and south-central
Georgia. Therefore, we have issued a flash flood watch for tomorrow
and tomorrow night that extends from Bay county, FL to Berrien
county, GA and southeastward to allow for some buffer space for
where the heaviest rainfall might occur. North and west of the
watch, widespread rainfall totals through the weekend are expected
to be around 1-3". Where the watch is in effect, rainfall totals
will be 3-6", with isolated areas in the watch likely to receive
even higher totals.
.Long Term [Monday Through Friday]...
Above normal rain chances will continue Monday across north Florida
and south Georgia as the low pressure system slides out of our area.
By mid-week we will return to a pattern where our area sits on the
eastern edge of deep layer ridging with upper level +PV anomalies
swinging across the eastern CONUS just to our north. This pattern
along with our daily afternoon sea-breeze front rolling through will
keep rain chances around 20-40% across the area through the period.
Highs will be in the low 90s with seasonably warm lows for this time
of the year, staying in the mid 70s.
As a low pressure system in the Gulf lifts northeastward across
north Florida and southern Georgia this weekend, the gradient will
tighten and elevate winds and seas to advisory levels. After the
system exits early next week, winds and seas will quickly return to
Humid conditions with above-average rain chances are expected
through this weekend.
It will be a rainy weekend, with widespread areas of 1 to 3 inches
expected in southeastern Alabama and southwest Georgia. Most of the
heavy rainfall will be in the eastern Big Bend where 4 to 6 inches
rain are possible. With slow storm motion, these amounts could be
higher depending on where the thunderstorms set up. All rivers are
currently below flood stage.
.Preliminary Point Temps/PoPs...
Tallahassee 76 92 74 91 75 / 50 80 60 60 50
Panama City 78 89 76 88 77 / 40 70 50 50 50
Dothan 74 93 73 93 74 / 70 60 30 40 30
Albany 75 93 73 93 74 / 80 70 30 40 30
Valdosta 74 91 74 91 74 / 50 80 50 60 40
Cross City 75 87 75 88 75 / 60 80 60 70 50
Apalachicola 78 87 77 87 78 / 40 80 60 60 50
FL...FLASH FLOOD WATCH from 8 AM EDT /7 AM CDT/ Saturday through
Sunday morning FOR Calhoun-Coastal Bay-Coastal Dixie-
Coastal Franklin-Coastal Gulf-Coastal Jefferson-Coastal
Taylor-Coastal Wakulla-Gadsden-Inland Bay-Inland Dixie-
Inland Franklin-Inland Gulf-Inland Jefferson-Inland Taylor-
GA...FLASH FLOOD WATCH from 8 AM EDT Saturday through Sunday morning