Area Forecast Discussion
Issued by NWS Atlanta, GA

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FXUS62 KFFC 181434
AFDFFC

Area Forecast Discussion
National Weather Service Peachtree City GA
934 AM EST Fri Jan 18 2019


.UPDATE...

Updated the Dense Fog Advisory to expire at noon vs 10 AM. Starting
to see some minor improvements in a few places, but overall still
seeing many observations at a 1/4 mile or less. Otherwise, no big
changes to the forecast at this time.

20

&&

.PREV DISCUSSION... /Issued 640 AM EST Fri Jan 18 2019/

..12Z AVIATION UPDATE...

PREV DISCUSSION... /Issued 440 AM EST Fri Jan 18 2019/

SHORT TERM /Today through Saturday/...

A weak upper-level shortwave moved over the SE U.S. yesterday, but
without a surface front, left the area in a warm sector with a drier
surface high pressure in place. With diabatic cooling in the lower
half of the atmosphere, combined with subtle WSWerly flow bringing
even more moisture into the area from the Gulf from weak high
pressure off the Florida coast A thick cloud deck has developed over
the northern half of the forecast area, including the ATL metro. The
cloud deck has lowered in most areas far enough to reduce
visibilities across these areas to 0.5-2.0 miles with several spots
getting below 0.25 mile. This thick cloud layer will also help to
keep us around 10-15 degrees above normal tonight with lows in the
mid-40s overnight. These conditions are expected to persist
throughout most of the morning, with a possible break around or just
after noon before returning tomorrow overnight. Isolated, very light
showers will also be possible throughout this time.

Tomorrow, isolated light showers may sprout up early in the morning,
but the main event will come during the day. Starting around 12Z,
high resolution models bring in a high-shear low-CAPE Quasi-Linear
Convective System into NW GA, moving into the ATL metro by the
afternoon. With very limited but adequate instability around the
region, scattered thunderstorms will be possible, some of which
could produce brief periods of heavy rains and strong wind gusts
from mixing aloft. 0-1km Helicity values up to 300-400 will be
possible head of the line showing a potential for one or two brief,
spin-up tornadoes along the baroclinic front.

Thiem

LONG TERM /Saturday Night through Thursday/...

Fcst period starts out tricky with the influence of the frontal
system mentioned in the short term and some potent upper trough
dynamics. Not much confidence on fine tuned timing of when
heaviest showers/isolated thunderstorms will be pushing across the
area late Saturday due to some discrepancies on the hi-res CAM
solutions. There is the potential for some pre-frontal
propagation, though the main front quickly traverses the area
between 00z and 06z Sunday. As stated in the short term, will
need to watch for the potential of isolated strong to severe
storms given the highly sheared environment if any instability is
attainable, which would have main threats of strong to damaging
winds and a few brief spin-up tornadoes. SPC has updated the Day 2
outlook to have an expanded Marginal Risk that covers the SW
majority of the CWA and includes part of the Atlanta Metro. Little
change at least to the expected QPF as it is still generally
0.5-1.5 inches for storm total with the highest amounts in the
north.

Other pesky part to watch is how much low level moisture lingers
on with the wrap-around energy in north GA as very strong CAA
kicks in. Have trended some light snow shower to flurry mention
with a general blend of the southward extent of the 850mb sub-
freezing zone, which starts a transition after 09z Sunday in the
far north and then farther south possibly including the Atlanta
Metro near 12-15z Sunday. The good news is is that the thermal
columns quickly dry out and majority of sfc temps should be above
freezing so little to no accumulation is expected, though some of
the far northern mountains could get a light dusting.

Perhaps the most sure bet with the system is the enhanced gradient
NW winds that will be on the increase for Sunday with the
abnormally cold arctic airmass quickly setting into the region.
Cannot rule out a possible Wind Advisory for some areas with
future updates or even a Wind Chill Advisory for some of the
mountains Sunday night into Monday as resultant wind chill values
could be in the single digits with teens most elsewhere.

Active wave pattern continues with the upper flow quickly
transitioning to SW for Tuesday and some moisture starts to advect
into the region off the Gulf. Models are not in good consensus
with the evolution of the broader trough axis by Wed/Thu, though
enough agreement at least on where the moisture and possible
elongated front exists to carry some chance to likely pops for
scattered showers in this period.

Baker

AVIATION...
12Z Update...
Low CIGS and VSBYS are in place in northern Georgia with MVFR CIGS
to the south. North GA conditions look to slowly improve to IFR
around 15Z, and marginal MVFR around 18Z. Light SW winds will
yield to light SE winds around 00Z. After 03Z, CIGS and VSBYs are
again projected to fall to IFR conditions with OVC005 forecast at
all sites, with a PROB30 for LIFR CIGs, VSBYs, and possible light
showers. For ATL, wind speeds mostly out of the south will
increase to 17012g20kts and -SHRA can be expected by 18Z.

//ATL Confidence...12Z Update...
Medium to High on all elements.

Thiem

&&

.PRELIMINARY POINT TEMPS/POPS...
Athens          59  47  62  39 /  10  30  80  90
Atlanta         57  50  62  35 /  20  30  90  90
Blairsville     53  42  55  32 /  10  30  90  90
Cartersville    56  49  62  34 /  10  30  90  70
Columbus        63  54  67  39 /  20  20  90  80
Gainesville     56  46  58  36 /  10  30  90  90
Macon           61  52  67  41 /  20  20  70  90
Rome            56  48  62  33 /  10  30  90  60
Peachtree City  59  50  64  36 /  20  30  90  90
Vidalia         64  54  71  47 /  20  20  30  90

&&

.FFC WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until noon EST today for the following zones:
Banks...Barrow...Bartow...Carroll...Catoosa...Chattooga...
Cherokee...Clarke...Clayton...Cobb...Coweta...Dade...Dawson...
DeKalb...Douglas...Fannin...Fayette...Floyd...Forsyth...Gilmer...
Gordon...Gwinnett...Hall...Haralson...Heard...Henry...Jackson...
Lumpkin...Madison...Morgan...Murray...Newton...North Fulton...
Oconee...Oglethorpe...Paulding...Pickens...Polk...Rockdale...
South Fulton...Towns...Union...Walker...Walton...White...
Whitfield.

&&

$$

SHORT TERM...20
LONG TERM....Baker
AVIATION...20



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