Area Forecast Discussion
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869
FXUS62 KTAE 211423
AFDTAE

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
923 AM EST Sun Dec 21 2014

.Near Term [Through Today]...
No significant changes are currently planned to today`s forecast.
May tweak PoPs a little based on current radar trends, but
otherwise the forecast reasoning below remains unchanged.

A stalled frontal zone across the Northern Gulf of Mexico will
serve as the focus for the development of additional areas of
rain across the area. Coverage and intensity of the rain will
increase as a mid level impulse moves toward the Central Gulf
States later this afternoon. While rainfall amounts will not be
particularly heavy, coverage should be fairly high, especially
across the eastern portion of the forecast area. With cloudy skies
and cool northeasterly flow in place across much of the area on
the north side of this boundary, have undercut guidance for max
temperatures in Alabama and Georgia, where highs may not rise out
of the mid 50s.

&&

.Prev Discussion [458 AM EST]...

.Short Term [Tonight Through Wednesday]...

The +PV anomaly currently positioned along the Texas/Mexico border
will elongate in the fast upper-level southwesterly flow and
spread through the Southeast tonight through tomorrow. This will
strengthen the high pressure wedge sprawling down the Appalachian
mountain chain, and subsequently reinforce the cool airmass in
place across the northern half of the Tri-State region. At the
same time, low to mid level southwesterly flow will surge an
unseasonably warm and moist airmass out of the southern Gulf into
the northeast Gulf. Although the two features will not directly
link up, a broad area of low pressure is expected to develop in
the northeast Gulf overnight. What will start as light to moderate
rain over the Big Bend and south-central Georgia (generated along
sloping low-level isentropic surfaces in the 295-305K layer) will
eventually become heavy rain as the low level convergence
strengthens a warm front along the northeast Gulf coast between
the aforementioned cool land based airmass and the warming Gulf
airmass. The surface and mid-level features will likely link up
best across the southeast Big Bend of Florida. In the unseasonably
tropical airmass (1.5"-2"PWAT), 1 to 2 inches of rain may fall in
this area tonight through Monday. Although there will likely be a
brief respite late Monday afternoon through Monday evening, light
to moderate rain will likely increase in coverage overnight as
low-level isentropic ascent continues over the strengthening and
nearing warm front. On Tuesday, a low-level jet will gradually
strengthen under the stratiform rain shield. At the same time, a
subtle southern stream anomaly will slide through the central Gulf
and approach the northeast Gulf coast. In addition a very strong
northern stream low pressure system will be digging into the
Southern Plains. Diverging upper flow and the approaching southern
stream wave will increase deep synoptic lift, while the
subtropical wave will strengthening deep layer shear. These features
will all combine some-time on Tuesday afternoon to support
discrete thunderstorm development along the aforementioned warm
front. Model consensus would suggest somewhere on the order of
50-60kts of deep layer shear, coupling with near 30kts of low-
level shear to warrant a threat for supercell thunderstorms. The
primary threat with these storms will be tornadoes and possibly
damaging wind gusts. Tuesday night into early Wednesday morning a
potent squall line associated with the northern stream trough will
pass through the same favorable environment for severe
thunderstorms. The primary threats along the squall will be QLCS
tornado development as well as damaging straight line winds. The
severe threat will probably continue into Wednesday morning/early
afternoon across the southeast Big Bend as the cold front makes
its way through the region. Between the storms on Tuesday and
Wednesday, another 2-4 inches of rain will be possible.


.Long Term [Wednesday Night Through Sunday]...

In the wake of the aforementioned front, temperatures will fall to
slightly below seasonal averages Wednesday night and Thursday,
before warming to near normal to end the week. Next weekend will
bring another frontal system, though this one appears much weaker
with much less rain expected.


.Aviation...
[Through 06Z Monday]

With a frontal zone in place just south of the Florida, an
extensive period of IFR ceilings are expected across the area
through at least 18z. These ceilings may lift to low MVFR in the
afternoon in between periods of moderate rain. After 02z, ceilings
should return to IFR regionwide through the end of the TAF cycle.


.Marine...

Cautionary level northeasterly winds will weaken through the day
today and remain rather light through Monday. Then winds will turn
southerly as a warm front moves into the northern Gulf. Expect
advisory levels winds by Tuesday ahead of a cold front, persisting
through Wednesday night in the wake of the front. Gust to gale
force may be possible Tuesday night through Wednesday. Winds and
seas will then subside once again to finish the week.


.Fire Weather...

Low level moisture will be abundant over the next few days with
drier air moving in on Wednesday. Red flag conditions are not
expected over the next week.


.Hydrology...

While there is still some uncertainty with respect to the exact
placement of heaviest rainfall with the storm system on Tuesday and
Wednesday, confidence is increasing in a period of significant
rainfall occurring across the region. Storm total accumulations of 3
to 5 inches with isolated heavier amounts are possible with this
system.

Rainfall totals of this magnitude will increase the potential for
localized flooding across the region, particularly during times of
heaviest rainfall on Tuesday Night into Wednesday Morning.

With area rivers and associated tributaries at or below normal flows
for December, the currently predicted rainfall amounts are generally
expected to produce only rises to action stage on most of the river
forecast points. The notable exceptions to this may be within the
Ochlockonee and Withlacoochee River Basins where there`s increased
potential to reach minor flood stage later this week.

&&

.Preliminary Point Temps/PoPs...

Tallahassee   60  56  67  62  67 /  70  80  50  70 100
Panama City   63  57  66  64  68 /  60  70  40  70 100
Dothan        56  51  61  57  66 /  50  50  40  60 100
Albany        55  49  59  56  66 /  60  70  50  60 100
Valdosta      57  53  65  61  69 /  70 100  70  60  90
Cross City    69  62  69  64  71 / 100 100  70  70  90
Apalachicola  63  61  68  65  69 /  70  80  50  70 100

&&

.TAE Watches/Warnings/Advisories...

FL...None.
GA...None.
AL...None.
GM...None.

&&

$$

NEAR TERM...DVD/GODSEY
SHORT TERM...HARRIGAN
LONG TERM...HARRIGAN
AVIATION...GODSEY
MARINE...HARRIGAN
FIRE WEATHER...GODSEY
HYDROLOGY...GODSEY






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