Area Forecast Discussion
Issued by NWS Atlanta, GA

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FXUS62 KFFC 211803
AFDFFC

Area Forecast Discussion
National Weather Service Peachtree City GA
203 PM EDT Sun May 21 2017



.PREV DISCUSSION... /Issued 1008 AM EDT Sun May 21 2017/

UPDATE...
Tropical-like airmass continues across the CWFA through today. The
back edge of the heavier rainfall will push eastward through the
remainder of the outlook area into the early afternoon. 24 hour
MPE values have the highest rainfall amounts roughly along and
north of a line from Heard county to Oglethorpe county.

Already seeing some breaks in the cloud cover, especially across
the western and southern CWFA. Surface instability will increase
in those areas throughout the afternoon, and expect sct shra and
tsra to develop. The hi-res models are all over the place with
where/when the convection will develop. So, will keep the chance
pops in the forecast for now.

PREV DISCUSSION... /Issued 731 AM EDT Sun May 21 2017/

PREV DISCUSSION... /issued 437 AM EDT Sun May 21 2017/

SHORT TERM /Today through Monday/...

Flash Flood Watch In Effect For Far NE GA This Afternoon...

A tropical like airmass remains in place across the area with MCV
moving into the Atlanta metro among many foci for abundant
rainfall across the area this morning. We did see a boundary shift
southward across portions of North GA and evidenced on the
terminal doppler velocity. This boundary along with strong
southerly flow off the surface set the stage for the initial
overnight push of heavy showers and storms along the western
portion of I85 and I20. This activity has now progressed east to
affect Athens but has also expanded with time southward all the
way to the CWA border.

For the remainder of today...will see MCV shift east and should
bring additional activity to the eastern portion of the CWA
through midday. With southerly transport...concerned about
rainfall totals along south facing range of the mountains despite
relatively high FFG values. With Lumpkin already receiving 1 to 2
inches and remaining counties of far NE GA quickly catching
up...though it prudent to go ahead and issue a FFA for a handful
of counties.

Remainder of the area could actually see a good respite for the
remainder of today in post MCV environment. Still some scattered
showers and storms around for sure but atmosphere looks to be in
recovery mode for much of today. Another shortwave however pushes
in during the evening hours with precipitable water getting close
to 2 inches across the southern tier once again. Should see
numerous showers and storms redevelop across the area but
primarily along and south of I20. May even need categorical pops
should model trends continue with subsequent forecasts.

Much like today...may actually see less activity on Monday as
shortwave shifts east and we await another perhaps more potent
system Mon evening into Monday night. At some point...another
flood watch will need to be strongly considered for Central and
portions of Northern GA.

Deese

LONG TERM /Monday Night through Saturday/...

Global models in general agreement with a strong jet streak at 250mb
of around 100kts shifting northeast off the upper TX coast toward
the SE states late monday into early Tuesday. In response a surface
wave develops along the stalled/remnant frontal boundary draped
across GA and moves across the region late mon/early tue.  Local hi
res models end at 00z Tuesday but suggest convective cluster will be
approaching GA late Monday associated with the short wave. Model
PWAT values along and south of I-20 running AOA 1.5-1.8 inches or
>90th percentile per sounding climotology Monday into Tuesday.
Given the combination of the stalled front, high PWATs and the
shortwave..would expect very efficient rain producers and a
localized flood risk to increase overnight Monday into early
Tuesday and lasting through the day Tuesday as the main upper trof
digs south into the mid-MS valley. GFS and ECMWF have the same
general idea with the shortwave and pattern evolution but timing
is a little off on the passage of the shortwave through the broad
SW flow. The GFS a bit faster, pre 12z, and the ECMWF a bit slower
but insists on higher QPF values going into the late day Tuesday.
Have trended a bit more toward the ecmwf soln and increased pops
during the day on Tuesday given the overall pattern and energy
moving through ahead of the digging trof.

Timing of subsequent shortwaves and convection beyond Tuesday gets a
bit trickier as models have struggled with day to day convection let
alone convection several days out.   Appears as though a much
stronger lobe rotates around late Tuesday into Wednesday which could
pose both a severe threat and a heavy rain threat.  ECMWF much
higher with shear and instability values going into Wednesday owing
to 50kts of deep layer shear and sufficient CAPE values over
1000j/kg.  Given the overall look to the pattern, cannot rule out a
severe risk going into Wed with the main trof and cold front moving
through...But think Flooding will be at the top of the list given
the several days of rain preceding the main trof.

Big cool down on tap by Thursday as the main trof and upper low
shift into the TN Valley/srn Applchn mountains setting us up for a
good first half of the holiday weekend.

30



&&

.AVIATION...
18Z Update...
No real focus for convective development today or tonight, but the
airmass remains moist and unstable enough for scattered shra/tsra
through the overnight hours. Storms should develop along and south
of a cold front tomorrow. Winds will remain on the west sided
through the period.

//ATL Confidence...18Z Update...
Medium confidence all elements.


&&

.PRELIMINARY POINT TEMPS/POPS...
Athens          80  66  82  64 /  70  50  30  60
Atlanta         79  68  81  64 /  70  50  30  50
Blairsville     75  62  76  57 /  80  50  20  50
Cartersville    79  66  80  61 /  70  40  20  40
Columbus        81  69  84  67 /  40  60  30  60
Gainesville     77  66  79  63 /  80  50  30  60
Macon           82  68  84  67 /  80  60  40  60
Rome            80  65  80  60 /  60  40  20  30
Peachtree City  79  66  81  63 /  70  60  30  50
Vidalia         86  69  85  69 /  60  50  60  60

&&

.FFC WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 8 PM EDT this evening for the following
zones: Lumpkin...Towns...Union...White.

&&

$$



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