Heavy Rain in Georgia, January 2, 2021

Area Forecast Discussion
National Weather Service Jacksonville FL
936 PM EST Fri Jan 1 2021

...WAVES OF HEAVY DOWNPOURS TO IMPACT PORTIONS OF INLAND SOUTHEAST
GA THROUGH SATURDAY NIGHT...

.UPDATE...

Late evening surface analysis depicts complex low pressure (1007
millibars) over the Ohio and Tennessee Valleys, with a
decelerating cold front stretching southward from this storm
system to the FL panhandle and extending into the central Gulf of
Mexico. Strong high pressure (1031 millibars) over New England was
keeping a wedge of cool air over the Carolinas and the southern
Appalachians, with southerly low level flow to the south of this
wedge over our area. Aloft...deep-layered ridging was centered
over the Bahamas, while a potent shortwave trough was accelerating
east-northeastward across the lower Great Lakes region, with
another potent shortwave trough emerging from the Desert Southwest
into west Texas. A ribbon of deep moisture remains in place ahead
of the upstream cold front over inland southeast GA, with latest
RAP analysis indicating PWAT values of 1.7-1.9 inches for
locations west and northwest of Waycross. Within this corridor of
deep moisture, heavy downpours streamed over the Ocmulgee and
western Altamaha Rivers earlier this evening, and another wave of
downpours currently moving onshore along the FL Big Bend coast was
poised to race northeastward towards these areas after midnight.
Mid and high altitude cloud cover otherwise blankets our region,
with some low stratus hugging the coast from Amelia Island /
Fernandina Beach northward. Temperatures at 02Z were generally in
the 65-70 degree range, with unseasonably high dewpoints in the
mid to upper 60s prevailing area-wide.

Potent shortwave troughing accelerating northeastward through the
lower Great Lakes states will continue to move away from our
region overnight, resulting in the frontal boundary to the west of
our region stalling overnight. Another wave of heavy downpours
will quickly move northeastward across inland southeast GA after
midnight, providing another 0.25 to 0.5 inches of rainfall for
areas west and northwest of Waycross that received up to an inch
of rain this evening. Meanwhile, the large trough aloft entering
west Texas this evening will pivot northeastward across the Ozarks
by Saturday afternoon and will then accelerate across the
Tennessee Valley on Saturday night. This feature will create a
highly diffluent flow pattern aloft over the Gulf of Mexico,
resulting in a wave of low pressure developing along the stalled
frontal boundary towards noon on Saturday. The increasingly
diffluent flow aloft will continue to send waves of convection and
downpours from the Gulf of Mexico northeastward across the FL Big
Bend region and into inland portions of southeast GA through
Saturday night, with only a few showers possible elsewhere as
ridging aloft only slowly slides eastward by late Saturday. Lows
tonight will only fall to the low and mid 60s area-wide, with
highs on Saturday remaining in the low to mid 70s for inland
southeast GA and the upper 70s to lower 80s elsewhere.

&&

.MARINE...

Areas of dense sea fog will remain possible tonight through early
Sunday afternoon over the near shore waters adjacent to southeast
GA and possibly the waters surrounding Amelia Island. Small Craft
should continue to Exercise Caution if venturing to the offshore
waters, where southerly winds will be sustained at 15-20 knots
through at least midnight and 4-6 foot seas will prevail through
Sunday. Otherwise, a cold front stretching from the Tennessee
Valley southward to the central Gulf of Mexico will stall to the
west of the Georgia waters on Saturday as low pressure develops
along the front over the Gulf of Mexico and lifts northward. This
low pressure center will then accelerate northeastward while
slowly strengthening on Saturday night, propelling the frontal
boundary eastward across our local waters during the predawn and
early morning hours on Sunday, accompanied by showers. High
pressure will then build along the northern Gulf coast region
early next week, with prevailing offshore and gradually
diminishing seas expected through Tuesday.

Rip Currents: A moderate risk of rip currents will continue at
area beaches on Saturday, with low risks expected Sunday and early
next week.

&&

.HYDROLOGY...

Some of the 00Z Sat guidance is indicating the potential for 2-4
inches of additional rainfall for locations west and northwest of
Waycross, where a Flood Watch or even a Flash Flood Watch may be
considered for Saturday and Saturday night as downpours continue
to train over increasingly saturated soils, particularly for
locations along the Ocmulgee and Altamaha Rivers. Rises along
these rivers are likely later this weekend and into next week.

&&

.PREV DISCUSSION [704 PM EST]...

.NEAR TERM [Through Tonight]...

Flow aloft will continue to veer as shortwave energy shoves the
front and associated moisture into the region late today and
tonight. Current POPs look good with minor adjustments, with
scattered showers moving into southeast GA this afternoon and
better rain chances into the evening and overnight hours. A few
pop-up showers possible this aftn over northeast FL near
convergent lines. Enough instability with MUCAPEs near 300-700
will support a slight chance of thunderstorms late today and
tonight over inland southeast GA. A risk of an isolated strong to
severe storm remains possible this evening and overnight over
inland southeast GA northwest of a line from Baxley to Pearson
where bulk shear values of 40 kt are noted.

Highs today well above normal in the upper 70s to lower 80s, with
near record highs over northeast FL and probably only 2-3 deg shy.
Sfc winds breezy at times from the south near 15 mph and gusty.

.SHORT TERM [Saturday Through Sunday Night]...

The slow moving cold front to our west is expected to become
reinvigorated Saturday afternoon as an upper level +PV anomaly
digs into the Southern Plains and develops a surface low along the
front in the north-central Gulf waters. The upper level low will
then lift into the OH Valley Saturday night, dragging the front
eastward across the southeastern US. Scattered showers and
isolated thunderstorms will continue Saturday with the highest
chances/coverage over inland SE GA, but the majority of the
precipitation is expected to fall Saturday night. Heavy rain is
possible Saturday and Saturday night, mostly across southeast
Georgia. Rainfall amounts of 1-2.5 inches are forecast, and
locally higher amounts of 3-4 inches are possible which could lead
to localized ponding and street flooding. WPC has a slight risk of
excessive rainfall for tomorrow over a portion of Jeff Davis,
Coffee, and Atkinson counties in SE GA. In addition to the heavy
rainfall threat, a portion of the low level jet may still line up
over our inland GA counties and deep layer shear of around 50
knots suggests some of the storms that develop Sat/Sat night could
produce some strong to severe wind gusts (40-60 mph). Even outside
of storms, breezy winds occasionally gusting to around 20-30 mph
will be possible as we've seen today.

The upper low driving this system will continue to lift
northeastward into the Great Lakes Sunday and with the main energy
driving the system lifting so far away, the front will lose much
of its steam as it continues to exit our area of responsibility
Sunday morning. Behind the front, cooler, drier air will begin to
filter into the region. After several days with highs in the upper
70s-low 80s, Sunday will see highs struggling to reach 60 over
inland SE GA and 70 in NE FL. Lows Sunday night will dip into the
mid-upper 30s in the Suwannee Valley and frost is possible early
Monday morning.

.LONG TERM [Monday Through Friday]...

Low level ridging will build in over the southeast early next week
in the wake of the cold front. Around mid-week another upper low
will move into the Plains and develop the next cold front to
approach the region. As this developing system pushes the low
level ridge offshore around Wed/Thu, onshore flow will begin to
return some moisture and low end rain chances ahead of the next
front could return Thursday, with the front crossing through the
area around Friday. Temperatures next week should be near normal
for this time of year- peaking in the 60s in the afternoons and
dipping to the upper 30s/low 40s in the mornings.

.AVIATION...
[Through 00Z Sunday]

VFR conditions will prevail through at least 06Z at the regional
terminals. Brief light showers will be possible at SSI and JAX
through around 04Z, potentially producing ceilings around 4,000
feet. Otherwise, sea fog will possibly set in at SSI after 06Z,
with higher probabilities for IFR conditions occurring after 09Z,
followed by the potential for a brief period of LIFR ceilings
around 300-400 feet towards 12Z. IFR ceilings will likely prevail
at SSI through around 18Z, with ceilings then lifting to MVFR
around 1,500 feet. Elsewhere, MVFR ceilings of 1,200-2,500 feet
will likely develop shortly after 12Z, with brief periods of IFR
to MVFR ceilings at GNV after 09Z. Confidence was too low to
include IFR conditions at GNV at this time. Showers may begin to
impact SSI after 16Z. Southerly low level winds will increase
overnight, with speeds around 30-35 knots expected at the regional
terminals towards 09Z, possibly reaching marginal low level wind
shear criteria at the regional terminals during the predawn and
early morning hours on Saturday as surface winds remain sustained
at or below 5 knots. Sustained southerly surface winds will then
increase to near 10 knots after 15Z.

&&

.PRELIMINARY POINT TEMPS/POPS...

AMG  66  72  54  62  38 /  80  90  80   0   0
SSI  61  71  59  67  42 /  20  30  60  20   0
JAX  64  80  61  70  41 /  20  20  50  20   0
SGJ  65  79  63  71  44 /  10  20  30  30   0
GNV  63  80  62  69  39 /  10  20  60  20   0
OCF  63  81  62  72  41 /  10  10  50  20   0

&&

.JAX WATCHES/WARNINGS/ADVISORIES...
FL...None.
GA...None.
AM...None.
&&