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Area Forecast Discussion (AFD)

Issued by Peachtree City, GA (FFC)

FXUS62 KFFC 190028

Area Forecast Discussion
National Weather Service Peachtree City GA
828 PM EDT Sun Mar 18 2018


00Z Update...
The first of 3 precipitation waves is now impacting several TAF
sites in association with a warm front moving over the region. 
Fortunately much of it is non-convective and cigs are not lowering
as much as forecast earlier. Winds have been tricky as they relax
and shift with warm front...but most model guidance suggest some 
veering through the evening and by 06z to a easterly or 
southeasterly direction...but differ on when that may occur. Cigs 
should continue to lower through the night into MVFR by 08z and 
possible IFR by 12z at most sites and hold for a couple hours 
before getting back up into MVFR levels. Some forecast models 
suggest a thunderstorm complex or scattered storms moving through 
central GA early AM into the early afternoon. Not entirely sold on
it...however...As a result, have added some prob30 TSRA into the 
Central GA sites beginning 14z to 16z. Will have to monitor metro 
sites during this period as it could be a lull period but may also
have to include TS if storms develop further north. Round 3 will 
have the greatest chance of severe storms including large hail. 
Have delayed the onset of round 3 for metro sites until after 20z 
with a 6 hour window of 20z to 02z...but could be more like 21z to
03z depending on which hi-res model has the best handle. Appears 
it will begin as a line that starts developing in Northern AL 
during the mid to late afternoon hours and works south and east 
into GA.

//ATL Confidence...00Z Update...
medium on TSRA and IFR duration
high on all others.



.PREV DISCUSSION... /Issued 320 PM EDT Sun Mar 18 2018/ 

SHORT TERM /Tonight through Monday Night/...

Regional radar view shows considerable activity presently over SW
Alabama riding the energy provided by embedded shortwave in
largely westerly flow aloft. Expect the most robust of this
activity to ride along the best instability axis this evening
which is roughly from Columbus to Vidalia and points southward
currently. Some CAPE values just south of the area running close 
to 1500 J/KG so some strong to severe activity not out of the 
question and SPC slight risk for just a few counties looks on 

For the remainder of the overnight period, rain chances will
increase rapidly as isentropic processes combine with shortwaves 
to produce widespread rain with embedded thunderstorms for 
southern sections. Previous model runs had indicated the potential
for some isolated flooding across the southern tier. Latest runs 
though indicate around an inch to an inch and a quarter which 
Central GA should be able to handle through the overnight period. 
As far as severe potential into the overnight, developing easterly
flow should inhibit expansion of instability axis and even 
elevated CAPE looks manageable through 12Z Monday so prospects 
look low at this time for any robust convection.

Very difficult call for Monday afternoon as some models indicate
low clouds and stable conditions will stay with the entire area
for the balance of the day. But there is a very sharp instability
gradient with in excess of 2000 J?KG just along and south of our
southern CWA border. Shear not the best but sufficient for some
rotating discreet cells should activity be able to develop across
the southern tier. 

Final wave which looks to be of the most concern will be Monday
evening into the first part of the overnight period. Even
instability associated with this feature is less in the models
than this time a few days ago. But with positive CAPE values, feel
activity that develops over Alabama will be able to maintain
itself over the local area and take advantage of very strong shear
values to produce strong to severe storms. Pretty much everything
on the table for this one including damaging winds, isolated
tornadoes and large hail. Some hi res guidance wants to weaken
this feature as it gets south of the metro but thinking this may
be premature given similar events in the past and will advertise
severe risk well into Central GA for this one.


LONG TERM /Tuesday through Sunday/...
Mainly only tweaks to the long term based on the latest guidance.
Thunderstorm chances will be over portions of central GA mainly
E and S of MCN on Tuesday and then just chances for showers
into Wednesday favoring N and E GA.



Athens          54  65  54  70 /  80  60 100  40 
Atlanta         55  68  54  67 /  90  50 100  40 
Blairsville     49  61  47  62 /  70  60 100  50 
Cartersville    53  69  51  66 /  80  80 100  50 
Columbus        58  74  58  73 / 100  40  80  30 
Gainesville     52  62  52  67 /  70  60 100  50 
Macon           57  72  59  75 / 100  60  60  50 
Rome            53  69  51  64 /  80  80 100  50 
Peachtree City  55  71  54  69 /  90  40 100  40 
Vidalia         59  73  63  77 /  90  60  40  60