Area Forecast Discussion
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112
FXUS62 KTAE 220955
AFDTAE

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
455 AM EST Sat Nov 22 2014

...Severe Thunderstorms Expected on Sunday...

.Near Term [Through Today]...

The water vapor imagery early this morning depicts a potent
shortwave digging through the southwest states. Quite a few CG
lightning strikes have been detected with this feature in northern
Mexico, which is another sign that it`s a potent disturbance.
Closer to home, a warm front will start to develop over the Gulf
this afternoon with gradually increasing moisture and lift across
the area. We expect light rain to gradually develop from south to
north through the afternoon hours, primarily affecting the Florida
big bend, but most of it is not expected until after 4 pm EST.
Farther north and west into Alabama and Georgia, PoPs are lower
with most of the rain expected to hold off until tonight. Amounts
today will generally be on the order of a tenth of an inch or
less. Highs are expected to range from the mid 60s across the far
north to the lower 70s across the southeast big bend.


.Short Term [Tonight Through Sunday]...

Confidence has increased in a severe weather event for our
forecast area on Sunday - particularly over the western half of
our forecast area (southeast Alabama and the western Florida
Panhandle). The Storm Prediction Center has highlighted those
areas in an "Enhanced Risk" of severe weather on their latest
Day 2 Convective Outlook.

Prior to the severe weather risk, we should have a round of rain
(and some embedded, elevated storms). This will occur primarily
from late this afternoon, as discussed above, into tonight and
will be forced by low-level WAA and isentropic ascent to the north
of the surface warm front over the Gulf. There is not much
instability, even for elevated parcels, so the sensible weather
was worded more for a steady rain with the possibility of isolated
storms. We are forecasting an areal average of around 1" of QPF
overnight, although some locally higher totals are possible.

There is fairly good model consensus on clearing much of this rain
north of our area quickly between 12Z and 15Z Sunday as the
surface warm front surges inland in response to rapid deepening of
the surface low in the Mid Mississippi River Valley. Some of the
global models generate some convection near the coast in the
morning, but we are leaning towards a brief clearing at this point
given that the strongest low-level forcing will be displaced
north, with mid-upper level forcing yet to arrive from the west.

The potent shortwave / PV anomaly referenced in the near term
discussion will eject quickly east-northeast from coastal Texas
tonight, to central and northern Georgia by Sunday evening. Both
global models and convection-allowing models indicate vigorous
convection along this lobe of strong forcing, and the timing is
relatively similar across the board. Therefore, we expect that
there will be some sort of QLCS in progress near coastal Louisiana
and into the NW Gulf of Mexico by daybreak Sunday, and that should
quickly translate east into our area by early afternoon. While
some CAMs show a continuous line of storms, others indicate mixed
modes of line segments and supercells. Given the strong forcing
associated with the ejecting wave, we anticipate primarily linear
modes, but some fore-running supercells, line mesovortices, or
broken line segments will also be possible. Given the model
consensus timing, the primary severe weather risk should be
between about 18Z Sunday and 03Z Monday.

Probabilistic guidance, as well as the SPC forecast, indicate the
greatest threat of severe weather will be over the western half of
our forecast area. The environment will be favorable for both
damaging winds and tornadoes. Strong low-mid level wind fields are
expected with 850mb winds around 50 knots and 700mb winds around
60 knots. These should help facilitate fast storm motions, and
downward momentum transfer of some of the stronger winds supports
the damaging wind threat. Meanwhile, by 18Z Sunday an average of
the NAM, GFS, and ECMWF show MLCAPE of 600 j/kg, 0-1km SRH of 190,
and 0-6km shear of 55 knots - certainly a favorable environment
for supercell thunderstorms and a tornado risk. The threat of
tornadoes would be higher if discrete cells can form. However,
there will be a significant component of the 0-3km (low-mid level)
shear vector normal to the expected line of storms - likely around
40 knots or so. Such values tend to correspond to an increased
risk of tornadoes and enhanced wind damage from QLCS mesovortices.
To summarize: the primary threats will be damaging winds and
tornadoes, and we think the severe weather could be a little more
widespread and/or significant than a typical wintertime event.


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

Although the storms are likely to exit quickly Sunday evening, the
synoptic cold front (and upper level longwave trough axis) will
still be positioned well west of the area. This should allow for
continued showers in the east/southeast parts of our forecast area
through Tuesday. As the trough begins to push east on Tuesday
Night and Wednesday, a shortwave rounding the base of the trough
could lead to coastal low development along the cold front off the
southeast Atlantic coast. This may lead to one final round of rain
over the area on Tuesday Night and Wednesday before we finally dry
out on Wednesday Night. Cooler temperatures will arrive on Tuesday
and should linger through the end of the work week.

&&

.Aviation...

[Through 12z Sunday] VFR conditions will prevail through the early
afternoon hours, but as low level moisture continues to increase
and a warm front begins to lift northward from the Gulf, ceilings
are expected to lower to MVFR levels this afternoon from south to
north with some showers beginning to stream northward from the
Gulf, possibly affecting ECP, TLH, and VLD before sunset. After
sunset, rain is expected to overspread the area with ceilings
continuing to lower to IFR.

&&

.Marine...

Buoy and offshore tower observations support the current Small
Craft Advisory, and easterly winds of 20-25 knots are expected to
persist through this evening over most of the coastal waters. The
winds will begin to veer to the south late tonight and into
Sunday, with 20-25 knots spreading all the way to the Apalachee
Bay and Big Bend coastline. The majority of model guidance is
indicating gale force gusts later tonight and on Sunday, with some
models as high as 40 knots or so. Therefore, we have issued a Gale
Warning for all but the far eastern legs beginning 06Z tonight.

The latest extratropical surge guidance, based on the 00Z GFS
winds (which seem reasonable), has reduced surge forecasts on
Sunday morning in Apalachee Bay. Flooding is not expected, but
water levels may be higher than usual following the early morning
high tide. High surf is expected on Sunday, though, with our
current forecast calling for 7 foot surf. A High Surf Advisory will
likely be issued later today for our beaches.

&&

.Fire Weather...

Rain will overspread the area from south to north starting late in
the day today through Sunday with wet flags likely on Sunday.

&&

.Hydrology...

Storm total rainfall amounts Saturday through Tuesday will be on
the order of 2 to 3 inches, with most of the rainfall coming over
a 24 hour period from Saturday Night into Sunday. Low river
levels will likely prevent any river flooding even with these high
totals. Some localized flooding will be possible in the more
urbanized locations that receive higher rain totals over a short
period of time, but the threat of flash flooding does not appear
to be sufficiently high to warrant a Flash Flood Watch.

&&

.Preliminary Point Temps/PoPs...

Tallahassee   70  61  74  63  77 /  60  90  70  40  40
Panama City   67  64  74  66  73 /  40  90  70  30  20
Dothan        67  58  73  61  74 /  30  90  90  20  20
Albany        67  58  72  62  75 /  30  90  90  40  30
Valdosta      69  60  75  63  76 /  50  90  90  40  50
Cross City    72  64  77  64  76 /  70  90  80  40  50
Apalachicola  70  65  74  67  75 /  60  90  60  30  30

&&

.TAE Watches/Warnings/Advisories...

FL...High Risk of Rip Currents for coastal parts of Bay, Franklin,
     Gulf, and Walton Counties from 1 PM EST this afternoon through
     late Sunday Night.

GA...None.
AL...None.

GM...Small Craft Advisory from 1 AM to 7 PM EST Sunday for Apalachee
     Bay-Coastal waters from Suwannee River to Keaton Beach FL
     out 20 NM.

     Small Craft Advisory until 1 AM EST Sunday for Coastal waters
     From Ochlockonee River to Apalachicola FL out to 20 NM-
     Coastal waters from Apalachicola to Destin FL out 20 NM-
     Waters from Suwannee River to Apalachicola FL from 20 to 60
     NM-Waters from Apalachicola to Destin FL from 20 to 60 NM.

     Gale Warning from 1 AM to 7 PM EST Sunday for Coastal waters
     From Ochlockonee River to Apalachicola FL out to 20 NM-
     Coastal waters from Apalachicola to Destin FL out 20 NM-
     Waters from Suwannee River to Apalachicola FL from 20 to 60
     NM-Waters from Apalachicola to Destin FL from 20 to 60 NM.


&&

$$

NEAR TERM...DVD
SHORT TERM...LAMERS
LONG TERM...LAMERS
AVIATION...DVD
MARINE...LAMERS
FIRE WEATHER...DVD
HYDROLOGY...LAMERS






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