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Area Forecast Discussion (AFD)

Issued by Peachtree City, GA (FFC)

FXUS62 KFFC 191513

Area Forecast Discussion
National Weather Service Peachtree City GA
1113 AM EDT Mon Mar 19 2018

Near term forecast trends continues to look good. Coverage of 
precip confined to mainly across far north GA, and have increased
POPs with chance for thunderstorms there over the next several
hours. We continue to focus on two rounds of storms. First round 
of isolated to scattered cells including supercell storms this 
afternoon after 3pm, which could affect just about any part of 
north and central GA with western areas affected first. Second 
round would be a line of strong/severe storms after 8pm in NW GA 
moving south. This second line would weaken or even dissipate 
after 3am after it moves well into middle GA. Current temperature
and dewpoint trends are on track and have made only minor tweaks 
for this update.


PREV DISCUSSION... /Issued 440 AM EDT Mon Mar 19 2018/ 

SHORT TERM /Today through Tuesday/...
Primary concern is timing and intensity of severe convection. 00Z
and more recent model guidance continue to indicate favorable
parameters for supercells with large hail and a few tornadoes this
afternoon and evening with development of a line of strong/severe
storms after 10pm over NW GA moving SE into ATL metro after
midnight before weakening and perhaps dissipating. Hires models 
still not solid on most likely locations of supercells during the
afternoon. Could be a cluster over northern AL and southern TN 
moving into northern 1/4th of GA and another cluster over 
eastcentral AL and westcentral GA moving into middle GA but our 
confidence remains low. Ingredients for supercells will be in 
place across all of CWA with MLCAPE ranging from 2000 J/kg over 
westcentral GA to 250-500 J/kg over NE GA in area of retreating 
CAD air mass. Vertical wind shear quite strong with 0-6km shear 
ranging from 50kts over middle GA to 75kts in the far north and 
0-1km shear 20kts over middle GA to 30kts in the far north. As 
mentioned night before, 0-1km shear in this range could be keep 
tornado threat confined to weaker, shorter-lived supercell-based 
tornadoes. Significant tornado parameter (STP) values from GFS 
coming up to 4-7 over AL/GA border SW of ATL metro, but keep in 
mind forecast values typically larger than observed (i.e. SPC RAP 
analysis) STP so observed STP values of 2-3 are likely what we 
will end up with.

Supercell threat will diminish greatly after 10p with line of
storms expected afterward. However with diminishing low level
convergence, large scale lift and MLCAPE, even this line should
weaken and eventually dissipate by 1-2a. 

Could see storms refire over far SE counties Tuesday midday and
early afternoon, however parameters for severe storm not as
favorable. One or two storms could be strong to severe with large
hail possible. 


LONG TERM /Tuesday Night through Sunday/...
Much colder temperatures will be advecting into the area on breezy 
northwest winds by Tuesday night behind a cold front. As an upper 
low passes to the north Tuesday night into Wednesday morning, the 
chance for light showers will linger across primarily north Georgia. 
With temperatures dropping near or below freezing by Wednesday 
morning in far north Georgia, the likelihood for a transition to a 
rain/snow mix or light snow showers will increase. Light snow 
accumulations will be possible across the higher elevations by 
Wednesday morning. At this time, generally expecting only up to a 
around a half inch in these higher elevations. Any light snow 
accumulations would melt by midday Wednesday as temperatures warm 
well above freezing. Wednesday will remain a chilly day by late 
March standards with high temperatures largely in the 50s (and 40s 
in far north Georgia). Sustained northwest winds around 20 mph with 
higher gusts will add to the chill.

Thursday and Friday will bring moderating temperatures and will 
remain primarily dry with only a slight chance of a few showers 
sneaking into far north Georgia by late Friday. Rain chances appear 
to begin to increase more appreciably by Saturday as upper pattern 
transitions into a more zonal flow and a low pressure system moves 
into the Ohio Valley, though discrepancies remain with timing and 


12Z Update...
IFR or near IFR cigs a little slower than expected to develop
across ATL metro airports this morning. If and when they do,
should only persist through 15Z. Any chc for TSRA should hold off
until 21Z and may be scattered in nature such that probabilities
remain low enough to keep probs at TEMPO or lower. After 02Z, line
of TSRA will likely affect ATL metro airports but should move out
or dissipate by 06-07Z. KMCN and KCSG should not be affected by
this line. SE sfc winds will become SW as line of storms moves 
thru 03-07Z then become west and increase to 10-15kts with gusts 
to 22-25kts after 15Z Tue.

//ATL Confidence...12Z Update...
Medium on TSRA likelihood and timing this afternoon
Medium confidence in IFR extent and duration this morning 
High confidence on all other elements



Athens          65  56  71  39 /  50  80  40  40 
Atlanta         68  57  68  37 /  50  70  40  30 
Blairsville     59  49  62  33 /  60  80  50  50 
Cartersville    68  53  65  36 /  60  80  50  30 
Columbus        75  61  70  41 /  50  50  30  20 
Gainesville     61  53  67  36 /  60  80  40  50 
Macon           74  62  75  40 /  50  60  40  20 
Rome            68  53  63  36 /  60  80  40  40 
Peachtree City  71  57  69  39 /  50  70  40  30 
Vidalia         75  64  76  46 /  60  50  60  10