Area Forecast Discussion
Issued by NWS Atlanta, GA

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FXUS62 KFFC 181132 AAA

Area Forecast Discussion...UPDATED
National Weather Service Peachtree City GA
732 AM EDT Sun Mar 18 2018

Near term forecast in pretty good shape. Low clouds and some
patchy fog affecting areas south of a Newnan to Louisville line
with mainly high clouds elsewhere. These low clouds should burn
off by 11a. Still on track for light precip to move in from sw
after 5p tho recent HRRR runs and 06Z 3km NAM showing a little
more coverage than earlier runs. Current PoPs fairly high tonight
so in good shape.



.PREV DISCUSSION... /Issued 435 AM EDT Sun Mar 18 2018/

SHORT TERM /Today through Monday/...
Primary concerns in the short term period are onset of warm
advection showers tonight and timing, identification of threats
and intensity of thunderstorms monday afternoon and evening.

Warm/moist advection to kick in this evening with scattered to
numerous showers likely spreading across the forecast area from
southwest to northeast after 6pm. Instability should be
insufficient at this time for much deep convection but could see
elevated thunderstorms in the evening. This batch of warm
advection convection should push out of the area or at least
diminish greatly in coverage after 3am give or take. Stratus and
some fog will be pretty widespread after that time.

As vigorous shortwave moves east over TN valley, broad and
unusually strong westerly flow to the south remains in place. In
my experience with severe weather forecasts both here and in the
southern plains, this pattern has a great deal of potential to
produce atypically high instability and very strong deep vertical
wind shear with supercells the dominant mode. It has been over a
year since this area has seen supercell storms of this magnitude
(Jan 22 2017). Large hail and a few tornadoes will be likely.
Given MLCAPE values 2000-2500 J/kg and 0-6km bulk shear of
60-70kts, hail will likely be much larger than the Jan 2017 event
with sizes of 2 to 3 inches likely. If there is a bright side to
this story, it could be the low level shear, 0-1km bulk shear is
only progged of 30-35kts. This is still quite strong and should be
sufficient for a few tornadoes, but unlikely to see the larger
/long-tracked tornadoes that we saw with the Jan 2017 event or
other notable supercell-based tornado events and outbreaks.
Interestingly, CIPS top 15 analogs based on 00Z NAM valid 00Z Tue
do not contain any events in the last 10 years, so could be some
analogs with lower storm report density and thus lower analog
probabilities. Top analogs is Apr 3 1998 which produced widespread
hail over north GA but other dates have hit or miss coverage of
reports and not too many tornado events though there area some.

Other bright side of supercell events is that coverage/probability
of any point seeing severe storms will be lower than with linear
events. Not something you want to take a chance on however, plan
accordingly. Have kept PoPs closer to guidance and perhaps lower
with mainly 40-60pct Monday afternoon and evening.

Will certainly keep a close watch on this and will continue with
steady pace of impact briefings.


LONG TERM /Monday Night through Saturday/...
As a surface low moves across Tennessee Monday evening, widespread
thunderstorms will be ongoing and rapidly progressing eastward
Monday night ahead of the attendant cold front. The severe
thunderstorm threat will have increased by this point with
substantial surface instability and favorable shear in place.
Supercellular convection with the threat for large hail, damaging
winds, and tornadoes will be a concern during the evening hours. The
highest severe threat will exist across portions of north and
central Georgia outside of the influence of the wedge which, though
retreating, will help limit instability across northeast Georgia.
The overall severe threat will then quickly diminish from the
northwest as the cold front sweeps southeastward through early
Monday morning.

Additional showers will return on Tuesday as a followup upper wave
and cold front push into the region. Northwest winds will pickup
late Tuesday behind this secondary cold front, and significantly
cooler temperatures will advect into the state. Before the light
rain ends, some brief rain/snow mix may be observed across the high
elevations of far north Georgia during the early morning hours
Wednesday; however, accumulations or impacts would not be a concern.

Wednesday will bring clearing skies and below average temperatures
with a sunny and cool Thursday also on tap. Temperatures will
rebound by Friday with the next chance of rain likely holding off
until Saturday morning at the earliest.



12Z Update...
IFR cigs south of ATL metro airports should dissipate by 16Z,
otherwise VFR conditions thru 00Z Mon. Rain/clouds will spread
from sw to ne after 22Z tonight bringing ATL metro airports down
to MVFR by 01Z and 1500-2500ft cigs by 08Z and IFR by 11-12Z. Sfc
winds will remain NW 6-8kts today but switch back to SE 3-5kts
after 03Z in most areas.

//ATL Confidence...12Z Update...
Medium confidence on cigs/vsby likelihood and timing.
High confidence on all other elements.



Athens          74  53  65  53 /   5  70  70  80
Atlanta         73  55  68  54 /  10  80  70  70
Blairsville     67  48  61  47 /   5  80  70  80
Cartersville    71  54  69  51 /  10  70  70  70
Columbus        78  58  74  58 /  30 100  50  50
Gainesville     70  52  62  51 /   5  70  70  80
Macon           78  57  73  59 /  10  90  60  60
Rome            71  53  70  51 /  10  70  70  70
Peachtree City  74  55  71  54 /  10  80  60  60
Vidalia         80  58  73  63 /   5  90  70  70




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