Area Forecast Discussion
Issued by NWS Atlanta, GA

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FXUS62 KFFC 210023
AFDFFC

Area Forecast Discussion
National Weather Service Peachtree City GA
823 PM EDT Fri Jul 20 2018


.UPDATE...
Tornado Watch is now in effect for far northwest Georgia. Starting
to see strong development for the storms moving from Kentucky into
Tennessee. Even with the loss of daytime heating, these storms are
moving into a more favorable environment for storm development,
and short term models continue to indicate robust convection
moving into northwest Georgia around/after 10 PM. At this time,
the shear environment over Georgia is not as pronounced, but a
few tornadoes are possible, particularly as these storms will
generally be supercellular in nature. Modeled DCAPE over the
region is high, and expect damaging winds to be the primary
concern, along with potentially large hail. Although storms are
expected to weaken as they move farther into the state, it may be
necessary to expand the watch southward.

31


&&

.PREV DISCUSSION... /Issued 749 PM EDT Fri Jul 20 2018/

PREV DISCUSSION... /Issued 320 PM EDT Fri Jul 20 2018/

SHORT TERM /Tonight through Saturday Night/...

At the surface, there is high pressure building in from the NE with
a departing low riding a residual boundary along the Carolina coast.
This has led to some subsidence across the area limiting
precipitation activity across much of the region today. We are
seeing slightly better coverage of showers and isolated
thunderstorms across our far southern tier with better deep layer
moisture and instability. Expect this activity to wane this
evening.

The main challenge with this forecast will be in regards to a
potential storm system moving into north GA this evening/overnight.
A somewhat rare pattern is setting up as an upper level low drops
south toward the mid-atlantic over the next couple of days. This
will result in mid/upper level NW flow with shortwave activity
rounding the trough. Given this, there may be a series of
complexes coming out of the north through the weekend. The first
potential complex may quickly move through this evening. Given
that the atmosphere is not exceptionally unstable, the potency of
this system looks fairly weak by the time it arrives toward the
metro around 02-04Z. I think the far NW tier of the CWA may see a
bit more strength with this system as it moves south at or around
sunset. Elevated instability and shear might be enough to overcome
some stable air at the surface. It should be noted that some
guidance is hardly showing any activity at all, while others show
showers/thunderstorms through late tonight. So the model
solutions are all over the place. Either way though, current
feeling is that this complex should diminish as it moves south and
the overall impacts should be fairly limited in coverage.
Whatever impacts there are will be in the form of damaging wind
gusts, small hail, heavy rain and frequent lighting.

Once this complex moves through, at least whatever is left of it as
it crosses the region, we should have a brief reprieve in
precipitation activity...likely through tomorrow morning.
Thunderstorm development tomorrow afternoon does look a bit more
widespread with better instability, deep layer moisture and upper
level support. It should be noted however that the overall
activity may be tempered a bit if we have better cloud
coverage/precipitation activity lingering into the morning hours.
So we will will have to keep an eye on the trends. There is a
slight risk for severe thunderstorms with the main impacts being
damaging wind gusts, hail, heavy rain and frequent lightning. The
aforementioned shower/thunderstorm activity may even linger into
the evening/overnight with shortwave activity crossing the region.

A lot of questions remain with this forecast package, but the
them is the same...it will be an unsettled pattern through the
short term.

26

LONG TERM /Sunday through Friday/...

For much of the extended period, the forecast area will be under the
influence of upper level troughing associated with an upper level
cutoff low currently located over the Great Lakes region. As the
period begins on Sunday the cutoff low will be moving southeastward
into the Ohio Valley and will then drop southward towards the
southeastern CONUS, with the center of the low positioning over
Georgia by Monday evening. Model guidance is beginning to come into
agreement that a dry slot could limit precipitation chances
somewhat in central Georgia on Sunday. After that, deep moist
convergence will take hold over the area on Monday and Tuesday.

Early next week, the low will slowly deteriorate as it meanders
around the area before opening up and getting absorbed into the
northern westerlies. There is still disagreement on the positioning
of the low. However, a belt of warm and moist southwesterly flow and
isentropic lift on the eastern side of the low is nonetheless
expected to set up over the area during the early parts of the week.
Numerous showers and thunderstorms are expected during the afternoon
and evening each day from Monday until there is a change in the
overall pattern. As such, maintained likely PoPs for each afternoon
and evening through Thursday. Model guidance becomes more divergent
by the end of the period, with the GFS solution indicating dry air
building into the area Friday and the ECMWF solution maintaining
moist conditions and widespread precipitation. Due to these
inconsistencies, I have ended the likely PoPs after Thursday.

High temperatures are forecast to be a few degrees below normal
throughout the extended period, with afternoon highs ranging from
the mid 80s in north Georgia to the low 90s further south. Low
temperatures are forecast to be near normal with morning lows in
the upper 60s to low 70s.

King

AVIATION...
00Z Update...
Mostly VFR ceilings expected through this TAF period with some
brief periods MVFR ceilings in and around SHRA/TSRA development.
Expecting a complex of storms to move south out of KY/TN tonight
but it should diminish as it begins to move close to the TAF sites
by 06z-08z tonight. Thinking we may see some showers across the
area but not expecting much more than that. Will see another round
of SHRA/TSRA Sat afternoon as well. Winds are starting out of the
SE but will turn to the SW and then NW through Sat afternoon.
Winds speeds will stay 10kt or less except in and around
convective activity where we could see some gust to 20kt. VSBYs
will also stay VFR except in and around thunderstorms.

//ATL Confidence...00Z Update...
Confidence medium to high on all elements

01

&&

.PRELIMINARY POINT TEMPS/POPS...
Athens          70  88  70  88 /  20  50  50  40
Atlanta         73  90  72  88 /  40  50  50  40
Blairsville     64  81  63  80 /  40  50  50  60
Cartersville    72  89  69  87 /  40  50  50  40
Columbus        75  93  74  92 /  40  50  50  30
Gainesville     70  88  70  86 /  40  50  50  40
Macon           72  90  73  91 /  30  50  50  50
Rome            72  90  70  88 /  40  50  50  50
Peachtree City  71  90  70  89 /  40  50  50  40
Vidalia         73  91  75  91 /  40  40  50  50

&&

.FFC WATCHES/WARNINGS/ADVISORIES...
Tornado Watch until 4 AM EDT Saturday for the following zones:
Catoosa...Dade...Murray...Walker...Whitfield.

&&

$$

SHORT TERM...01
LONG TERM....King
AVIATION...01


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