AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
617 AM EST Fri Dec 28 2012

…PATCHY FROST THIS EARLY MORNING…

…RAIN AND ISOLATED THUNDERSTORMS TONIGHT INTO SATURDAY…

.NEAR TERM [Through Today]…
The large scale longwave pattern remains both progressive and
amplified. This is highlighted by ridging across extreme Ern Pacific
and West Coast, a broad positively tilted trough pivoting Ewd into
the Srn Plains and ridging over Ern states weakening from N-S.
At surface, main features are inverted trough over Ern TX starting
to develop a low just offshore with cold front SWWD and warm front
extending SEWD, and a high over Cntrl Appalachians with axis into SE
region.

Under calm winds, sunrise will see inland temps bottoming out in the
low to mid 30s with coldest values across the eastern most counties
where a few sites will drop into the upper 20s. higher dew points
will favor areas of frost until around 9 am est. During rest of
today, upper trough will move ENE. Inverted trough will continue to
develop into weak 1008mb surface low (cyclogenesis) that will
lift NEWD into LA and S/Cntrl MS by sundown. All this will shunt Ern
ridge/high further EWD towards Atlc. The combination of approaching
trough and lifting low will push cold front SEWD to LA/MS by
sundown. Locally, expect increasing mainly higher level clouds, WAA
and a more mild airmass. Dew points in the 30s this afternoon will
rise to the mid 40s inland and mid 50s at the coast by sundown. Will
go with 30-0% SW-NE pop gradient (rain mainly over Wrn waters).
Under veering winds, highs will warm to around 60 north to mid 60s
elsewhere.

&&

.SHORT TERM [Tonight Through Sunday]…
For Tonight and Saturday, The progressive flow pattern will bring
the next rainmaker across the region. The surface low continues to
race NEWD to across our area tonight reaching the GA Atlc coast
before sunrise and Carolina coast in the morning. Deep and
increasing isentropic lift is maximized as warm front lifted NWD
over our area. Dew points will increase to around 60 at the coast
and these values will lift NWD thru the night. This will be followed
by the cold front early on Sat. Guidance shows upper trough lifting
newd across OH/TN Valleys thru Sat then lift across Great Lakes and
NE states with low opening up taking much of energy with it. Still,
some increase in upper level support/divergence and ML cape ahead of
trough. Trajectory of low focuses on SE third of local area and
especially the waters. Conversely, limited amount of land to be in
warm sector so pops/instability and chances of storms mainly over
water and Big Bend counties. Thus mainly a cool rain event north of
FL border and little overall concern for severe weather. QPF
low…generally about 1/2 inch for the event. Highest rainfall
amounts will be closest to low where locally heavy rain possible.
Will go with 70-80% N-S POP gradient tonight and 0-60% W-E POP
gradient on Saturday. Moderate west winds become breezy and gusty
northwest winds following the frontal passage. Highs top out at
upper 50s SE AL to mid-upper 60S SE Big Bend.

By Sat night, low well NE and cold front well SE of our region with
a strong and cold ridge of high pressure building SEWD allowing
strong CAA and clear skies to overspread area from NW-SE. A freeze
is possible, particularly in the northwest half of our forecast area
with lows from upper 20s north to lower to mid 30s south. This Ridge
is expected to strengthen and be situated right over the CWA on
Sunday. Expect highs only in the low 50s SE AL to upper 50s SE Big
Bend.

&&

Long Term [Sunday night through next Friday]…
The period begins in a rather benign zonal flow regime aloft, with a
trough departing the east coast, and another digging through the
Southwest. At the surface, high pressure is firmly in control. The
western trough will approach the local area with its attendant
surface low forecast to drag a cold front through the Tri-State area
Tuesday through Wednesday. Although there is some disagreement as to
how dynamic this system will be (locally), there is 100% agreement
at this time that instability will be too weak to even support
thunderstorm development. Wednesday night, another shortwave will
exit the Southern Plains and spark Gulf cyclogenesis along the
eventual stalled front discussed above. As the low rides along the
front, it will bring the local area another round of showers to end
the week.

Temperatures will remain near average to begin the period, then fall
to below average under the influence of the weather systems
mentioned above.

&&

.Aviation…

Updated 617 AM:

MVFR visibilities will continue at KVLD for another hour or so.
Otherwise, VFR conditions will persist through the day as mid-level
clouds move in ahead of the next weather system. Restrictions are
expected tomorrow night as clouds and rain pass over the Tri-State
region.

&&

.MARINE…
With the coming passage of the next cold front today into early
Sat, winds are expected to reach advisory levels over the
western waters early Saturday morning. Elevated wind speeds will
build eastward and remain around 20 to 25 knots until Sunday
afternoon. Waves will be correspondingly high during the weekend,
peaking around 8 feet for the offshore waters and decreasing Sunday
afternoon.

&&

.Fire Weather…
Red Flag conditions are not expected for the next several days.

&&

.HYDROLOGY…
No Flood Concerns expected from the recent rainfall. Widespread 2
to 3 inches of rain occurred over Southeast Alabama and portions
of Southwest Georgia on Tuesday and Wednesday of this week. The
rain primarily occurred in the upper headwaters of the major
basins affecting the Pea and Choctawhatchee basins in Alabama,
Flint River basins in Georgia and the Chattahoochee River basin
along the Alabama-Georgia Border. 1 to 2 inches fell in the upper
Ochlockonee River basin in South Georgia.

Soils continue to moisten up with each passing system, and the
smaller creeks and streams are becoming more responsive with each
event. With this most recent event, there were minor rises on
smaller creeks and streams to near or slightly above action stage
in SE Alabama & SW Georgia. There is enough runoff to begin
generating minor rises downstream on the lower Choctawhatchee and
Ochlockonee Rivers. Flows on the Flint and Chattahoochee Rivers
are so low, that the runoff has had little impact on downstream
flows affecting the lower Flint River and discharges from
reservoirs on the Chattahoochee into the Apalachicola River.