Area Forecast Discussion
Issued by NWS Atlanta, GA

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FXUS62 KFFC 230228
AFDFFC

Area Forecast Discussion
National Weather Service Peachtree City GA
1028 PM EDT Sun Apr 22 2018

.UPDATE...

Current forecast remains on track for the most part as widespread
light to moderate shower activity continues over the forecast
area. The big question thus far has been centered around
previously anticipated QPF values. Up through 00Z, MPE values were
really close to those forecast for this event. The issue is that
between 00Z and 06Z, we should be seeing in excess of an inch to
and inch and a quarter over SW portions. Not sure current radar
however is supporting this as intensity values seem similar to the
last several hours.

To the west over Alabama, there is a line of convection that is
certainly producing heavier rainfall totals but this is a narrow
line with relatively quick movement. After consultation with the
SERFC, have lowered totals anticipated which takes some of the
anticipated flooding sites out and just into action stage for this
event. That said, still an opportunity for some sites to still go
into flood and will maintain watch as is for now.

As far as the winds go, there have been several instances thus far
of gusts to 40 mph to the north and widespread in the 30s this
evening. Wind advisory still looks on target and even without
robust convection, may be able to realize some trees down in this
windy environment.

Biggest change in models is the quicker exit of sustained moderate
rain Monday morning. There will still be activity for the morning
commute but majority will be on the east side of the state by
then. Still think there will be an opportunity for sct storms to
develop mid morning on back side of this system with potential
for isolated strong to severe storms with this activity.

Deese

&&

.PREV DISCUSSION... /issued 758 PM EDT Sun Apr 22 2018/

PREV DISCUSSION... /issued 259 PM EDT Sun Apr 22 2018/

SHORT TERM /Tonight through Monday Night/...
The main change to the forecast was to add a few areas to the flood
watch as the axis of heavier rainfall has shifted a bit.

Mid level and low level low pressure will meander over the Lower
Mississippi River Valley through Monday. Currently, a surface low
pressure system is situated across SW Mississippi. A warm front
extends eastward from the center of the low. A long fetch of
easterly winds off of the Atlantic and isentropic lift over the warm
front will allow for widespread precipitation across the CWFA well
into tomorrow. For much of today, the rainfall has been fairly
light. However, the higher PWATS/deeper moisture and greater ascent
are expected during the evening into the overnight hours. The axis
of heavier rainfall has shifted a bit, but still remains roughly
along and northwest of a line from Macon to Wilkes Counties.

The greatest potential for thunderstorms remains in or nearest to
the warm sector overnight. As the surface low over SW MS lifts a bit
northward, the warm front will lift northward too. A few of the
storms could be strong and an isolated severe thunderstorm is not
out of the question. A little concerned about thunderstorms during
the afternoon tomorrow. Still some lingering moisture on the
backside of the departing widespread precip. Some decent mid level
lapse rates are present, so thunderstorms (a few strong) are
possible during the late morning and early afternoon hours.

Strong east winds are expected overnight. Made no changes to the
wind advisory. The highest impacts/gusts are expected across
elevations aoa 200kft.

NListemaa

LONG TERM /Tuesday through Sunday/...
Extended forecast is on track, overall. Late week dew points were
struggling to match the latest model solutions, so have updated
to reflect current thinking. Following the wet short term,
additional wave of precip is expected late week, and have
adjusted the Thursday evening/Friday morning pop and wx grids,
particularly over the southern CWA, to better reflect secondary
low pressure rotating through the upper level flow.

Otherwise, minimal updates have been made aside from minor min/max
temperature adjustments. Previous forecast discussion follows.

31

/Issued 453 AM EDT Sun Apr 22 2018/

Upper closed low slowly dampens and lifts north into the longwave
energy by late Tuesday. Models indicating a bit more instability
present - parts of east for Monday evening and then again Tuesday
afternoon for the northern majority, so have included slight
chance thunder with shower mention and trend highest pops to the
north. Nam BUFR soundings indicate some decent low level lapse
rates too and given a pretty low freezing level at 10 kft or
lower, wouldn`t be surprised if a few cells resulted in some small
hail or graupel (especially Tuesday afternoon). Will need to
watch for lingering flooding and runoff concerns into the
beginning of the fcst period given the expected 3-4 inches mainly
in the short term.

Wednesday should be a brief reprieve ahead of a continued active
pattern with only some slight to low end shower chances in the
north ahead of another shortwave/frontal system for Wednesday
night into Thursday (Euro a bit less progressive than the GFS). A
second disturbance could impact the CWA as a quicker cold front
with less enhanced moisture present for Friday (GFS) or slower
system that cuts off farther south and taps into more Gulf
moisture to make for a rainy Saturday (Euro). Have not bought into
the less consistent Euro solution so for now have kept chance pops
of showers for Thursday, slight pops Friday, and dry Saturday.
Will fine tune given hopefully better consensus in future updates.

Temps should stay generally a bit cooler than climo norms through
period though expect some later period temps to be adjusted given
aforementioned model inconsistency with active jetstream pattern.

Baker

HYDROLOGY...
The Flood Watch has been expanded to include a larger portion of
north and central Georgia based on the latest QPF guidance from
WPC. This new guidance has shifted the highest rainfall amounts
farther northeast than the previous forecast. Within the watch
area, rainfall amounts of 2 to 4 inches are possible, with the
highest amounts over portions of northeast Georgia, generally
along the I-85 corridor and south of the Appalachian ridgeline,
where 3 to 4 inches are likely. Although some of the watch area
now have lower rainfall amounts than previously expected, will
hold off on canceling the watch for any counties until later in
the event.

AVIATION...
00Z UPDATE...
Widespread SHRA continues to move across the terminals this
evening but no TS as of yet to accompany it. May be a break later
tonight as there is a back edge to the precip over Alabama but
this should be short lived as additional SHRA will develop late
tonight. What has changed is it looks like there will be a quicker
end to the sustained SHRA activity by 17Z and possible sooner.
Will still need to carry a Prob30 for TSRA for the afternoon hours
but otherwise just VCSH. Winds will continue to be an issued
increasing to 20 and gusting to over 30 knots tonight. Cigs will
largely remain IFR and then MVFR by tomorrow afternoon.

//ATL CONFIDENCE...00Z UPDATE...
Medium on TS potential.
High on remaining elements.

Deese

&&

.PRELIMINARY POINT TEMPS/POPS...
Athens          55  68  56  72 / 100 100  50  50
Atlanta         56  71  56  71 / 100  50  30  40
Blairsville     50  60  51  66 / 100  70  70  80
Cartersville    55  69  54  71 / 100  50  40  50
Columbus        62  77  57  75 / 100  30  20  20
Gainesville     54  64  55  69 / 100  90  50  60
Macon           60  78  56  75 / 100  80  30  20
Rome            55  70  54  71 / 100  40  40  50
Peachtree City  57  73  54  73 / 100  50  30  30
Vidalia         62  77  59  77 / 100 100  50  20

&&

.FFC WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Monday afternoon for the following zones:
Baldwin...Banks...Barrow...Bartow...Bibb...Butts...Carroll...
Catoosa...Chattooga...Cherokee...Clarke...Clayton...Cobb...
Coweta...Crawford...Dade...Dawson...DeKalb...Douglas...Fannin...
Fayette...Floyd...Forsyth...Gilmer...Gordon...Greene...
Gwinnett...Hall...Hancock...Haralson...Harris...Heard...Henry...
Jackson...Jasper...Jones...Lamar...Lumpkin...Macon...Madison...
Meriwether...Monroe...Morgan...Murray...Newton...North Fulton...
Oconee...Oglethorpe...Paulding...Peach...Pickens...Pike...Polk...
Putnam...Rockdale...South Fulton...Spalding...Talbot...
Taliaferro...Taylor...Towns...Troup...Union...Upson...Walker...
Walton...White...Whitfield...Wilkes.

Wind Advisory until 7 AM EDT Monday for the following zones:
Banks...Barrow...Bartow...Bibb...Bleckley...Butts...Carroll...
Catoosa...Chattahoochee...Chattooga...Cherokee...Clarke...
Clayton...Cobb...Coweta...Crawford...Crisp...Dade...Dawson...
DeKalb...Dooly...Douglas...Fannin...Fayette...Floyd...Forsyth...
Gilmer...Gordon...Gwinnett...Hall...Haralson...Harris...Heard...
Henry...Houston...Jackson...Jasper...Jones...Lamar...Lumpkin...
Macon...Madison...Marion...Meriwether...Monroe...Morgan...
Murray...Muscogee...Newton...North Fulton...Oconee...Paulding...
Peach...Pickens...Pike...Polk...Pulaski...Rockdale...Schley...
South Fulton...Spalding...Stewart...Sumter...Talbot...Taylor...
Towns...Troup...Twiggs...Union...Upson...Walker...Walton...
Webster...White...Whitfield...Wilcox.

&&

$$

SHORT TERM...Deese
LONG TERM....Merritt
AVIATION...Deese


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