292
FXUS62 KFFC 201135
AFDFFC

Area Forecast Discussion
National Weather Service Peachtree City GA
635 AM EST Mon Jan 20 2025

...New 12Z Aviation Discussion...

.SHORT TERM...
(Today through Tuesday)
Issued at 414 AM EST Mon Jan 20 2025

At a glance:

    - Extreme Cold Warning in effect for the Atlanta Metro and
      points north through noon, Cold Weather Advisory in effect
      for the remainder of north and central Georgia through 11AM

    - Chances for next round of wintry precipitation increase
      beginning mid-morning Tuesday, uncertainty remains regarding
      accumulation and northward extent

    - Winter Storm Watch in effect beginning 10AM Tuesday

The combined effects of a reinforcing shot of dry air at the mid-
levels, the southern and eastern fringes of a strong Arctic surface
high situated across the Southeast, clearing skies allowing for
additional radiational cooling, and lingering gusty winds post-front
has set the stage for the coldest night of the season thus far. Lows
this morning will be in the single digits to 20s areawide.
Windchills as low as zero are expected as far south as metro
Atlanta, with single digits to teens for all other locales within
our forecast area. An Extreme Cold Warning is in effect until noon
today for most areas along and north of the I-20/I-85 interchange,
and a Cold Weather Advisory is in effect until 11AM for areas
further south. If planning on spending any time outdoors this
morning, layer up, wear a hat and face covering, and endeavor to
minimize exposed skin. Highs today will be far from warm in just the
mid-20s to lower 40s, and areas north of I-85 are unlikely to breach
the freezing mark. An additional forecast concern this afternoon is
the potential necessity for a Fire Danger Statement for at least the
southern portion of the forecast area. Critical thresholds for
relative humidity will be met or exceeded, but fine fuels have held
on to moisture in the wake of the rainfall we received on Saturday.
Expect a short-fused decision to be made on the day shift today.

Our unseasonably cold airmass will play a large role in the onset
and evolution of our next potential winter weather event. By early
Tuesday, flow at the mid-levels is solidly from the southwest --
owing to the base of a broad trough being situated across southern
CONUS -- while surface flow remains dry and out of the north. A
swath of moisture (accompanied by a Gulf low at the surface) will be
nudged eastward by a lifting mid-level shortwave, forcing it to
glide atop and overrun our very cold, very dry surface airmass. The
aforementioned dry lower levels (dewpoints in the single digits to
teens, at/below the 10th percentile per SPC Sounding Climatology)
will preclude precipitation accumulating as soon as moisture
arrives, and it will likely take a few hours of evaporating frozen
precipitation to nudge our temperature profiles toward sufficient
saturation.

For now, it looks like we could begin to see light snow showers move
in from west to east as early as mid-morning Tuesday and potentially
as late as early afternoon. Questions remain regarding how far north
our snowfall footprint will be, and how much actually falls -- much
of which hinges upon on how much wetbulbing occurs. At present,
we`re forecasting highs areawide to be in the upper 20s to lower
40s. However, with such a wide dewpoint depression, wetbulb
temperatures will be in the 20s throughout the day all the way to
our southern CWA border. Resolution of the wetbulbing process can be
tricky, and it can lead to "surprise" higher accumulations if
handled poorly. With HiRes guidance coming into the purview of this
forecast, this is a huge caveat to remember. A colder and more moist
environment will support higher snow totals.

NAM-3km and HRRR model soundings resolve a stout dry layer between
875-850 mb that lingers through the entire duration of our
precipitation event, and this appears to be a true floor for the
event. If falling frozen precipitation encounters an extremely dry
layer aloft, evaporation and removal of sensible heat will work
towards saturating that layer, and the dry air at the surface is
left untouched. This would result in effectively no totals -- snow
or otherwise. NBM guidance has trended notably lower today, and this
is likely due to the drier HiRes solutions being incorporated. All
this to say, despite being within 36 hours of onset, there are still
many variables to consider that could have major impacts on the
forecast. The important thing to keep in mind: even if snow totals
trend lower, our presiding cold airmass and cold ground/roadway
temperatures will support travel impacts ramping up as soon as
precipitation begins making it to the ground. For details regarding
our accumulation forecast, see the long term portion of the AFD.

96

&&

.LONG TERM...
(Tuesday night through Sunday)
Issued at 414 AM EST Mon Jan 20 2025

As the extended period begins on Tuesday night, a strong (1040+ mb)
Arctic high will be positioned to the northwest of Georgia near the
Tennessee Valley region. Meanwhile, longwave upper troughing will
extend through the Lower Mississippi River Valley and along the
northwestern Gulf Coast. A shortwave perturbation and associated
surface low will be over the central Gulf of Mexico, and traversing
east-northeast along the flow ahead of the trough. Mid-level
moisture will continue to overrun this Gulf low to the north,
spreading northward into the Gulf Coast states. As the low continues
its trek through the Gulf, the shield of precipitation will shift
eastward through the overnight hours. Temperatures will be below
freezing across the area shortly after sunset, continuing to
decrease throughout the overnight hours. This combination will lead
to more prolonged light snow in central and portions of east Georgia
overnight before coming to an end by daybreak on Wednesday morning.
There will also be some potential for sleet and freezing rain in the
far southeastern portion of the forecast area during the early
morning hours. Forecast ice accumulations remain light at less than
0.05 inch in locations where freezing rain occurs.

Now it`s time to talk about our biggest mode of uncertainty: Snow
accumulations!
Overall, guidance continues to trend towards drier solutions, and
thus lower accumulations overall. The Latest ECMWF and CMC
ensembles have trended towards lower snowfall amounts, bringing
them closer in line with drier GFS ensemble solutions. Hi- res
solutions are also favoring drier solutions as they come into
focus, with some models like the HRRR keeping precipitation
confined to the southern portions of our forecast area and further
south. It is worth noting that many of these hi-res solutions are
blended into the NBM, further contributing to lowering
deterministic outputs. For awareness, below are some probabilities
at a few locations for >1 and >2 inches of snow from the 01Z NBM
run:

Probabilities of 1+ inch of snow:
Columbus: 55%  Macon: 50%  Atlanta: 35%  Athens: 30%

Probabilities of 2+ inches of snow:
Columbus: 40%  Macon: 40%  Atlanta: 25%  Athens: 20%

The spread within the ensemble members has also decreased from
yesterday with respect to amounts. While the "reasonable lowest
amount" (90% or 9/10 exceedence) probabilities still indicate no
measurable snowfall, the "reasonable highest amount" (10% or 1/10
exceedence) probabilities indicate 3-4 inches of snow in a swath
from Columbus to just south of Atlanta.

With all that in mind, it is fair to wonder exactly how well hi-res
guidance is handling the low-level dry air. As alluded to in the
short term discussion above, it is possible that this guidance could
underestimate the wet-bulb effect as precipitation falls through the
layer of dry air near the surface. The evaporative cooling effect
(with 2 phase changes) will still cause temperatures through the
column to cool and dewpoints to rise. Dewpoint depressions at colder
temperatures also tend to be nullified more quickly than we are
often acclimated to here in the Southeast, as air at these lower
temperatures already have a lower capacity of moisture carrying
capacity per degree. This would make the wet-bulb effect more
efficient and serve to saturate the dry layer more quickly. This is
the means by which similar winter weather events, including the Snow
Jam of January 2014, have overperformed the guidance in terms of
forecast snowfall amounts.

Forecast snowfall amounts have been lowered slightly from the
previous forecast to align with the overall trend in the guidance,
with 1-2 inches forecast in central Georgia and progressively lower
amounts to the north. However, we kept snow in the forecast within
the Winter Storm Watch area and avoided major reductions in
accumulations to account for the possibility of overachieving wet-
bulbing, mentioned above. Thus, the Winter Storm Watch has been
maintained for portions of central and north Georgia. This watch
will be in effect from 10 AM Tuesday until 7 AM Wednesday and covers
an area roughly along and south of a line from Carroll county to
Hall county, and also includes the Atlanta metro area, Columbus, and
Macon. Considering the multiple hours near or below freezing in
advance of this system, it is likely that any wintry precipitation
that occurs will stick to roads instantly. Thus, it will not take
much accumulation to create hazardous travel conditions.

Low temperatures on Wednesday morning will be the coldest of the
long term period, in the teens across the majority of the forecast
area, with the exception of single digits in the higher elevations
of far north Georgia and low 20s in east-central Georgia. These lows
will range from 12 to as much as 20 degrees below climatological
normals. High temperatures on Wednesday are forecast to be in the
mid to upper 30s across the majority of the area. Many locations in
north and central Georgia, particularly to the north of I-85, could
have experienced well over 48 consecutive hours of below freezing
temperatures starting within the short term period early Monday. 48
consecutive hours is important because this is typically when
problems begin with exposed pipes freezing and bursting.

Late Wednesday into Thursday, the high pressure system will begin to
weaken and move off towards the central Atlantic coast. The airmass
will begin to gradually moderate over the Southeast through the end
of the week into the weekend, with highs warming a few degrees each
day. Temperatures on Wednesday and Thursday are nonetheless forecast
to rise above freezing for only a few hours each afternoon, only to
drop into the teens and low 20s Wednesday night into Thursday
morning and low to mid 20s on Thursday night into Friday. Snow and
ice that do occur will likely hang around given the limited time for
melting and a certainty of refreezing during the overnight and
morning hours. This is especially true in sheltered or shady areas
that do not see a lot of sunlight.

King

&&

.AVIATION...
(12Z TAFS)
Issued at 631 AM EST Mon Jan 20 2025

VFR and SKC to continue thru the TAF pd. Expect SCT to BKN cigs at
10-20kft to filter in early Tuesday morning. Winds remain out of
the NW/NNW at 8-15kts with isolated gusts to 20-25kts, gradually
slackening to 6-8kts by 08-09Z.

//ATL Confidence...12Z Update...
High confidence all elements.

96

&&

.PRELIMINARY POINT TEMPS/POPS...
Athens          37  21  37  17 /   0   0  50  30
Atlanta         31  21  34  16 /   0   0  40  30
Blairsville     26  11  30   5 /   0   0  30  10
Cartersville    30  16  34  10 /   0   0  30  10
Columbus        38  23  39  17 /   0   0  60  50
Gainesville     33  20  36  15 /   0   0  40  20
Macon           39  23  39  19 /   0   0  70  60
Rome            30  16  34  11 /   0   0  20   0
Peachtree City  33  19  36  14 /   0   0  50  30
Vidalia         43  29  44  25 /   0  10  60  80

&&

.FFC WATCHES/WARNINGS/ADVISORIES...
Extreme Cold Warning until noon EST today for GAZ001>009-011>016-
019>023-030>034-041>045.

Winter Storm Watch from Tuesday morning through Wednesday
morning for GAZ022>025-027-031>039-042>062-066>076-078>086-
089>098-102>113.

Cold Weather Advisory until 11 AM EST this morning for GAZ024-
025-027-035>039-046>062-066>076-078>086-089>098-102>113.

&&

$$

SHORT TERM...96
LONG TERM....King
AVIATION...96