Area Forecast Discussion
Issued by NWS Atlanta, GA

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FXUS62 KFFC 280215
AFDFFC

Area Forecast Discussion
National Weather Service Peachtree City GA
1015 PM EDT Sat May 27 2017

.UPDATE...

...Severe Thunderstorm Watch In Effect For Portions of North GA...

Always a tough call with these NW flow events and in this
case...WNW flow event. First wave moved through this morning as
just clouds mainly which really stabilized things longer than the
models had projected. Making up for lost time now though with CAPE
values pushing 2500 J/KG. There has also been an extensive cap to
overcome as evidenced by impressive warming on 00Z FFC sounding.
Hard to discern how far this extends into North GA and honestly
not likely to matter for the first tier of counties given momentum
of initial MCS. But certainly could see some weakening prior to
entering the southern half of the watch but enough uncertainty
there that thought it best in coordination with SPC to go another
tier of counties.

After that...depends more on further upstream convection and if
it makes a late night run at us which would have a better chance
of impacting the metro. For now have kept Atlanta area in the
chance pops except northern burbs where likely will be in place.
Will be a fluid situation with updates to pop grids likely to
continue into the overnight period.

Deese

&&

.PREV DISCUSSION... /issued 757 PM EDT Sat May 27 2017/

PREV DISCUSSION... /issued 247 PM EDT Sat May 27 2017/

SHORT TERM /Today through Monday/...

A rather complicated forecast as a series of disturbances moves
across the CWA beginning tonight and through Sunday night. The MCS
that was over TN early this morning has dissipated and has not put
out an outflow. The hi-res models eventually caught on to this, but
they still struggle with developing convection and moving it into
GA. The HRRR has flip-flop several times this morning and the
WRF/ARW continue to develop convection over TN and move it
southeast. The problem with this is that showers that have developed
over TN are moving northeast. The other issue is the strong cap over
the CWA between 800 and 700 mb. Unless there is forcing, areas in
north Georgia would need to rise to near 100 degrees before breaking
the cap. Latest models suggest the cap doesn`t break down until
Sunday morning.

Have opted to go with GFS/NAM deep moisture and associated short
waves to forecast the best areas for thunderstorms, which at this
time both the GFS/NAM agree on extreme north Georgia tonight and
Sunday with the better chances in north Georgia Sunday night. As for
severe potential, if any storms develop, there is still the risk of
isolated severe storms as CAPES continue to run high as well as 0 -
6km bulk shear.

17

LONG TERM /Monday Night through Friday/...

A largely unsettled period can be expected through the long-term.
Monday continues to exhibit the best chance for strong to severe
thunderstorm activity as a front enters the area with abundant
instability in place by Monday afternoon. The remainder of the
extended will continue to see chances for convection each day with
plentiful moisture and southwest flow aloft. Refer to the previous
long-term discussion below for additional details.

RW

PREVIOUS LONG TERM /Issued 356 AM Sat May 27 2017/

Main forecast concern in long term period is chance for
strong/severe convection on Monday. 00Z model guidance in pretty
good agreement with Monday forecast. Front progged to continue to
sag into the state Monday with above normal sfc temps and
dewpoints. SBCAPE rises to fairly high values even for late May,
GFS 2500-3500 J/kg. Deep layer shear should be sufficient for
severe convection, 0-6km bulk shear mags > 30kts over much of the
CWA, however low level shear quite weak with 0-1km bulk shear
around 5-10kts. In such environments, we typically see severe
storms with large hail and wind threats but very low risk of
tornadoes. PW will be fairly high but not too abnormally high for
this time of year (GFS values around 1.6 in). With large scale
lift not too high, QPF also less than 1 inch in most locations
Mon, so will likely see periods of heavy rain but flash flooding
risk somewhat low.

After front sags into middle or south GA, essentially washes out
Tuesday. Moisture remains plentiful but without any forcing, any
storms would be fairly isolated thru Thursday. Have kept PoPs
fairly low but unlikely we will see a dry period with weak SWly
flow aloft. By Thurs Night or Friday, warm advection should kick
in with increase in moisture as well. Have bumped up PoPs then.

SNELSON

AVIATION...
00Z UPDATE...

VFR conditions to start out in the short term although do expect
to continue with periods of mid level cigs for the next several
hours. Biggest issue will be trying to time potential TSRA
episodes with guidance all over the place with possibilities. For
now...into the overnight looks like the best chances but
confidence so low that will only include VCSH for now. Intrusion
of lower clouds from the SW overnight will require MVFR cigs aft
10Z for all sites.

//ATL CONFIDENCE...00Z UPDATE...
Medium on TSRA chances.
High on remaining elements.

Deese

&&

.PRELIMINARY POINT TEMPS/POPS...
Athens          70  89  70  87 /  30  40  30  50
Atlanta         71  87  69  84 /  30  20  20  60
Blairsville     62  84  63  79 /  70  50  60  60
Cartersville    70  86  69  83 /  60  40  50  60
Columbus        71  90  73  87 /  10  10  10  60
Gainesville     69  86  68  83 /  60  50  50  50
Macon           69  93  72  89 /  10  10  10  40
Rome            69  86  69  83 /  60  50  70  60
Peachtree City  69  88  71  85 /  20  20  20  60
Vidalia         70  93  72  92 /   5  10  10  20

&&

.FFC WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Deese
LONG TERM....20
AVIATION...Deese


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