Area Forecast Discussion
Issued by NWS -- Remove Highlighting --
-- Discussion containing changed information from previous version are highlighted. --
FXUS62 KTAE 211546
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
1046 AM EST Fri Nov 21 2014
.Near Term [Through Today]...
-- Changed Discussion --Current fcst over land looks well on track with high temps
expected to recover into the middle to upper 60s. Over the coastal
waters, even though a Small Craft Advisory is already in effect as
of 1 PM EST this afternoon, winds and seas are increasing out of
the E-NE a bit ahead of schedule, so did tweak winds up 2-4 kts,
and seas up 1-2 feet over the bulk of the marine area through
-- End Changed Discussion --
.Prev Discussion [416 AM EST]...
-- Changed Discussion --.Short Term [Tonight Through Sunday]...
The catalyst for our changes in the weather along the Gulf coast
in the short term period is a potent mid-upper level shortwave
trough evident in early morning water vapor loops over southern
California. Model analyses indicate the dynamic tropopause (PV1.5)
is down around 500mb associated with the PV anomaly, and there was
even a few CG lightning strikes noted near the San Francisco Bay
area. All of these are indicators of a strong wave, and models are
in good agreement with digging it ESE to central Texas by Saturday
afternoon. The consensus then has the wave ejecting ENE with a
negative tilt into the Deep South on Sunday.
The strength of the ejecting shortwave (500mb height anomalies as
low as 2 to 2.5 std deviations below normal LA-AL) will lead to a
strengthening low-mid level wind field on Sunday, with 850-700mb
winds above the 95th percentile for this time of year. Those
increasing wind fields will lead to strong northward moisture
flux. When combined with overall strong QG forcing and above
normal precipitable water values (+2 std deviations), the rain
chances are quite high from Saturday Night into Sunday.
It appears as though an area of rain, and perhaps embedded
elevated thunderstorms, will take shape over the northern Gulf on
Saturday Night in the low-level WAA / isentropic ascent regime
north of a surface warm front. Model forecast soundings indicate
very little instability with a moist adiabatic profile through
much of the troposphere and a shallow inversion near the surface.
This should limit coverage and intensity of thunderstorms
initially, but moderate to occasionally heavy rain appears likely.
As the surface warm front surges north on Sunday morning in
response to strong surface pressure falls from the Ozarks into the
Deep South, our forecast area should enter the warm sector. Most
model guidance builds MLCAPE over 500 j/kg over at least part of
the area (some close to 1000 j/kg) coincident with about 50-60
knots of deep layer shear and 30-35 knots of 0-1km shear. This is
a favorable environment for severe thunderstorms, and a second
round of convection is expected to develop. The latest SPC Day 3
Convective Outlook places a Slight Risk across our entire area.
.Long Term [Sunday Night Through Friday]...
Most models agree that a lagging mid-upper level trough axis over
the Plains will keep at least moderate QG forcing in place over
our forecast area through Tuesday. Therefore, rain chances were
maintained in the forecast through that time, although we removed
the mention of thunder as instability should be lower by that
time. Cooler temperatures are expected on Tuesday and Wednesday
before slow moderation toward the end of the week.
[Through 12z Saturday] VFR conditions are expected to prevail
through the period with ENE winds just under 10 knots developing
during the mid-morning hours.
We have issued a Small Craft Advisory, except for the far eastern
nearshore legs, valid through the weekend. Winds should gradually
increase today and could hit 20 knots offshore by this afternoon.
For that reason the advisory was started at 18Z. Easterly winds of
20-25 knots will persist through Saturday Night before veering to
the south on Sunday. It will be around that time of the wind shift
that winds should finally begin to increase in the far eastern
legs. So SCEC headlines and then an advisory are likely at some
point for those areas. Model wind gust guidance indicates some
gusts to near gale force will be possible over the coastal waters
from late Saturday Night into Sunday, but as of right now it seems
too limited in scope to warrant any gale headlines.
Related to the increasingly strong onshore flow on Sunday, the
current forecast also calls for surf heights at area beaches to
peak in the 6-7 foot range which would warrant a High Surf
Advisory. The extratropical surge guidance, based on the GFS
model, does indicate an increase in water levels in Apalachee Bay
on Sunday morning to action stage (just shy of flood levels).
However, (1) the GFS model may be overestimating surface winds at
that time -AND- (2) any surge appears likely to happen after high
tide which would reduce the possible water levels. This will be
monitored as the event approaches as specific timing and wind
speeds will make a big difference.
Low level moisture will gradually increase starting today with
widespread rain overspreading the region Saturday night through
Moderate to heavy rainfall looks likely from Saturday Night into
Sunday, with some rain possibly persisting into Monday and
Tuesday. Rainfall totals from Saturday into early next week look
to be mainly between 2.5 and 3.5 inches, on average. This should
not be sufficient to create widespread flooding given current low
flows on area rivers. Still, some minor flooding on a few of the
rivers cannot be ruled out if a more concentrated area of heavier
rainfall were to fall in a large part of a drainage basin. In
urbanized areas, minor flooding problems will also be possible.
-- End Changed Discussion --
.Preliminary Point Temps/PoPs...
Tallahassee 69 44 70 63 73 / 0 0 20 90 90
Panama City 66 50 69 65 72 / 0 0 30 90 80
Dothan 67 43 67 60 73 / 0 0 20 90 80
Albany 66 41 67 60 72 / 0 0 10 90 90
Valdosta 66 45 70 62 73 / 0 10 10 90 90
Cross City 70 46 75 65 74 / 0 10 20 90 80
Apalachicola 68 51 69 66 73 / 0 0 30 90 80
-- Changed Discussion --FL...None.
GM...Small Craft Advisory until 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.
-- End Changed Discussion --