Area Forecast Discussion
Issued by NWS Atlanta, GA

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FXUS62 KFFC 221131
AFDFFC

Area Forecast Discussion
National Weather Service Peachtree City GA
731 AM EDT Mon May 22 2017

.PREV DISCUSSION... /issued 419 AM EDT Mon May 22 2017/

SHORT TERM /Today through Tuesday/...

Latest surface analysis shows a very weak pressure pattern across
the area as frontal boundary tries to ease its way into the NW
sections of the forecast area. Over Central GA
meanwhile...continue to see weak waves of low pressure form along
old boundary there. Soupy conditions in place south of the front
with upper 60s for most areas while temps struggle to get into the
60s. Given this moist environment along with the fact that we
continue to see subtle shortwave activity...isolated showers have
developed but are very shallow to this point and short lived.

Transition day today as we await the main shortwave for the next
system coming out of Louisiana. Even prior to this though...some
weaker impulses will make their way into SW potions this
afternoon. With modest cape values approaching 1500 J/KG and PWATS
of 1.75 inches...should see at least high end chance pops along
and south of the I20 corridor. Could easily see this as a repeat
of Friday though with more activity than models were indicating so
day shift will need to monitor for possible upgrade to likely.

Thunderstorms this evening will transition to more of a
widespread moderate rain with embedded storms event as low
pressure develops across Alabama and tracks across North and
Central GA. The one thing we know is there will be an axis of
heavy rainfall that will line up roughly where 850mb jet
establishes itself. The problem is the models differ in their
positions on this feature and this will have large implications on
how much flood potential is realized.

GFS is the further north solution while the NAM12 and to
lesser extent the ECMWF keep activity further south along and
south of I85. Went mainly with a blend but favored northern
solution slightly as such dynamic systems typically spread highest
QPF further north provided limited Gulf cutoff. Regardless of
exact position...should see PWATS approach 2 inches for a large
portion of the area requiring categorical pops.

Although this will be an extended rainfall event...gotta believe
there will be a break at some point between systems. All models at
least hinting at this with the GFS indicating the most drier air
filtering in behind the departing low. Will not reduce pops as
much as GFS does but will show a period of just chance pops across
North GA before starting with likely pops again in extended
period.

Hydrology...

As mentioned above...all indications are there will be an axis of
substantial rainfall that develops but it is just a question of
exactly where. Overall favored the WPC solution with 3 to 4 inch
totals from the SW corner of the forecast area up to the I85
corridor. This axis could easily be further north requiring
amendment to flood watch but for now went with FFA from Polk to
Banks counties and points south to Crisp. Left SE CWA for now
given relatively little rain they have received up to this point.

Deese

LONG TERM /Tuesday Night through Sunday/...

Interesting see-saw kind of a week setting up across the area
that will make you scratch your head and look at the calendar when
all is said and done. The extended period starts off with the
remnants of early Tuesday convection and rain likely shifting
south of the area while the northern part of the state sees a
brief respite from the heavier rainfall. The ECMWF suggests that
there will still be scattered storms around late Tuesday and given
the increasing mid- level instability forecast by all the
model...cannot rule out additional development during the
afternoon Tuesday as the main trof axis approaches. Interestingly
as the main trof approaches late Tue...mid-level lapse rates
increase to near 8 deg c/km across the atl metro area and gt 7 deg
c/km area wide. Combined with 30- 40kt of deep layer shear and
modest CAPE...cannot rule out some severe chances, including hail
and wind, late Tuesday through the overnight into early Wed. The
GFS and NAM both increase deep layer shear going into the morning
hours Wed which is juxtaposed with the left exit region of the
250mb jet. Models are more sporadic and broad with QPF in this
period so confidence is medium on how things will
evolve...especially given the unknown impact of the first wave
moving through Tuesday AM. For now have kept pops in the likely
category and included thunder as well. May see a fairly stout
line of convection develop in the eastern half of the state late
Wed AM into the afternoon as models forecast additional
destabilization during the day.

Of greater note is the 00z ecmwf soln with an upper low cutting off
much further south than previous solns over Central AL and shifting
eastward across GA on Thu.  GFS is much further north with the cut
off portion...but does extend the base of the trof into the same
general area.  GFS and ECMWF 500 mb temps with the low/trof drop to
-21 to -22 across the region on thu. Sounding climatology
 suggests that those temps would be one of and very close to the
 coldest on record for May at ATL should it occur...and most
 certainly a record for the date. In response..afternoon highs
 both Wed and Thu are forecast to remain in the low 70s which is
 right around record low max temps. Overnight lows drop into the
 low 50s Thu AM and Fri AM as well and while much below normal and
 cool...records are generally in the 40s.

Nice weather on tap through Sunday before another trof digs into the
central US and rain chances again increase in response.  Models vary
on the evolution of the trof...but agree that one is forecast to
return.

30

.AVIATION... 12Z UPDATE...
Have seen some isolated activity pop up but so far remaining
outside the terminals. VFR had been the the rule thus far but
IFR and LIFR to the north and east coming precariously close to
ATL terminals and already affecting AHN. All this should lift
into MVFR and then VFR deck this afternoon. Difficult to pin down
best chances for SHRA/TSRA as there will be several opportunities
the next 24 hours. For now have gone tempo initially and then
transitioned to just predominant SHRA aft 00Z. Could certainly be
some embedded thunder in this activity as well and will cover with
VCTS. Winds will be tough as well as low pressure looks to move
right across the terminals resulting in highly varied flow
although favored the NAM this cycle.

//ATL CONFIDENCE...06Z UPDATE...
Medium on winds and TSRA timing.
High on remaining elements.

Deese

&&

.PRELIMINARY POINT TEMPS/POPS...
Athens          83  65  75  62 /  60  80  60  60
Atlanta         81  65  75  64 /  60  90  50  60
Blairsville     76  59  71  57 /  40  70  50  60
Cartersville    80  63  75  61 /  50  80  50  60
Columbus        84  69  78  66 /  70  90  60  60
Gainesville     80  63  73  62 /  50  80  50  60
Macon           85  69  78  65 /  70  90  60  60
Rome            80  62  76  61 /  40  70  40  60
Peachtree City  82  64  75  63 /  60  90  50  60
Vidalia         86  71  80  68 /  60  80  80  70

&&

.FFC WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from this evening through Wednesday afternoon
for the following zones: Baldwin...Banks...Barrow...Bibb...
Bleckley...Butts...Carroll...Chattahoochee...Cherokee...Clarke...
Clayton...Cobb...Coweta...Crawford...Crisp...DeKalb...Dooly...
Douglas...Fayette...Forsyth...Glascock...Greene...Gwinnett...
Hall...Hancock...Haralson...Harris...Heard...Henry...Houston...
Jackson...Jasper...Jones...Lamar...Macon...Madison...Marion...
Meriwether...Monroe...Morgan...Muscogee...Newton...North
Fulton...Oconee...Oglethorpe...Paulding...Peach...Pike...Polk...
Pulaski...Putnam...Rockdale...Schley...South Fulton...Spalding...
Stewart...Sumter...Talbot...Taliaferro...Taylor...Troup...
Twiggs...Upson...Walton...Warren...Washington...Webster...
Wilkes...Wilkinson.

&&

$$

SHORT TERM...Deese
LONG TERM....30
AVIATION...Deese



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