Special Weather Statement, NWS Atlanta, GA

Area Forecast Discussion
National Weather Service Peachtree City GA
737 PM EDT Wed Aug 19 2020

.PREV DISCUSSION... /issued 348 PM EDT Wed Aug 19 2020/

SHORT TERM /Tonight through Thursday Night/...
Primary concern in the short term period is heavy rainfall/
isolated flash flooding and diurnal convection. PW on KFFC morning
RAOB was only 1.3in and 19Z RAP analysis 1.4in over all but far SE
counties, yet deep convection in north GA so far today have been
very efficient and moving just fast enough for some training/
redevelopment with favorable orientation and boundary interaction.
PW progged to increase significantly to 1.75-1.85in this time
tomorrow. Only hope for not seeing enhanced heavy rainfall and
flooding threat is if clouds will diminish sfc heating and
instability, which is a strong possibility. Vertical wind shear
will be limited as well, so storms should be less intense
Thursday. Guidance also indicating that convection Thurs night
will persist past midnight, which is also very possible if deep
moisture materializes to the extent forecast. Tried to modify
max/min temps to decrease diurnal range but NBM max temps already
quite cool tomorrow.

SNELSON

LONG TERM /Friday through Wednesday/...
Wet weekend ahead as the long term picks up on Friday with stalled,
sheared PV anomaly and associated 500 mb trough axis still to the
west of the CWA. Broad ascent is forecast to continue to
overspread the region focused in part by any vorticity maxima
rotating around the trough as well as general WAA the the low
levels which will also be bringing in plenty of gulf moisture.
Models are supportive of likely or higher PoPs across the region
both Friday and Saturday, though final coverage details may be
influenced by mesoscale details, such as remnant outflow
boundaries and leftover cloudiness from previous day's/overnight
convection, that courser global models won't properly resolve.
700-500 mb lapse rates will remain rather lack luster on Friday,
though ample day time heating and marginal shear will support some
strong thunderstorm potential. Lapse rates improve a bit on
Saturday as colder core of trough is expected to rotate through
the CWA on its way to being absorbed within polar jet, making for
somewhat better CAPE profiles and decent shear profiles, so some
potential will once again exist for stronger thunderstorms across
the area, though plenty of uncertainty remains with regards to
timing as well as placement of aforementioned mesoscale features
that could disrupt the larger scale set up.

Trough begins to dissipate on Sunday as PV anomaly gets sheared
out between large, persistent western ridge and another ridge
sliding in from the Atlantic. Still, plenty of moisture will be
wrapping into the CWA thanks to being on the western flank of that
Atlantic ridge, so while PoPs may not be as prolific, still
expect typical summer like coverage across the north, and possibly
even better coverage to the south thanks to outflows and
seabreeze type fronts. Similar situation on Monday, though more
uncertainty begins to creep into the forecast. 12Z GFS shows
tropical wave moving into the gulf during this time period.
Location of this tropical wave will be highly dependent on how
organized current system over the MDR of the tropical Atlantic
becomes as well as details of the ridge sliding to the west over
the Atlantic. Will carry this uncertainty in the PoPs, though
still expect some coverage given tropical airmass will still be in
place across the CWA.

Lusk

&&

.AVIATION...
00Z UPDATE...

Line of TSRA sliding past ATL are terminals at this hour but still
affecting AHN which will require a continued TEMPO group. Feel
activity will remain north of CSG and MCN through this evening and
will not include any mention of TS there. A brief period of IFR
possible toward daybreak Thu and will continue the mention in the
TAF. Have converted the predominant TSRA to SHRA for Thu afternoon
and then a Prob30 for TSRA. Winds look to remain on the east side
and generally light.

//ATL CONFIDENCE...00Z UPDATE...

Medium on IFR potential.
High on remaining elements.

Deese

&&

.PRELIMINARY POINT TEMPS/POPS...
Athens          68  84  68  81 /  50  80  80  70
Atlanta         69  83  70  80 /  60  80  80  70
Blairsville     63  78  64  74 /  50  80  80  80
Cartersville    68  85  69  80 /  60  80  80  70
Columbus        70  88  71  83 /  50  80  80  70
Gainesville     67  82  68  79 /  50  80  80  70
Macon           70  87  71  82 /  50  80  80  60
Rome            67  85  69  81 /  60  80  70  70
Peachtree City  67  84  69  79 /  60  80  80  70
Vidalia         71  89  72  85 /  40  70  70  60

&&

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

SHORT TERM...SNELSON
LONG TERM....Lusk
AVIATION...Deese