Area Forecast Discussion
Issued by NWS Atlanta, GA

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FXUS62 KFFC 300128
AFDFFC

Area Forecast Discussion
National Weather Service Peachtree City GA
928 PM EDT Sat Apr 29 2017

.UPDATE...

Current forecast in good shape with majority of the activity now
over North GA. Still some stray SHRA all the way down the South
Metro but overall trend should be reduced activity. Strong
seabreeze/outflow could also allow for brief development for the
next several hours given residual CAPE values across the area.

No change for Sun night into Monday system with SPC still showing
a marginal risk for Day 2 and 3 for the local area. Models still
showing a healthy 50kt 850mb jet but CAPE values will struggle to
get to 200 to 400 J/KG for the initial portion of the line. Will
continue to monitor instability trends as any subtle increase
could result in a more enhanced severe threat.

Deese


&&

.PREV DISCUSSION... /issued 751 PM EDT Sat Apr 29 2017/

PREV DISCUSSION... /issued 345 PM EDT Sat Apr 29 2017/

SHORT TERM /Tonight through Sunday Night/...

High pressure ridge remains anchored off the mid Atlantic coast
through Sunday...and southeasterly flow will continue to bring
moisture into the forecast area. No real triggers for organized
convection except for possible terrain effects and any old outflow
boundaries. A few thunderstorms were forming currently and have
tried to adjust low pops through the evening based on radar. Pretty
much the same situation for Sunday. However...by Sunday evening  the
surface front and associated dynamics have crossed the MS river and
will be approaching GA. So far...the timing for the best chances for
convection look like after midnight Sunday night into Monday
morning. There is at least a marginal risk for severe storms across
a portion of northwest GA late Sunday night with damaging winds and
an isolated tornado possible.

Continued warm temperatures through the period and mav/met looks
okay with a few tweaks.

41

LONG TERM /Monday through Saturday/...

As opposed to yesterday, models are not in as good of agreement for
the long term.  We still have timing uncertainties with the system
on Monday and the overall extent of the system progged to impact the
area the second half of the week.

A storm system and associated cold front will approach the NW tier
of GA prior to sunrise and slowly move east across the area through
Monday afternoon. Model discrepancies with the overall timing are
more apparent today as we are now within the window for high
resolution model output. The high res. solutions illustrate the line
of showers/thunderstorms arriving in NW GA during the early morning
hours Monday, crossing the central GA by mid-afternoon. However,
other models are showing a slower frontal passage. So we will keep
our current thinking, a bit broad for now, of Monday morning through
the afternoon hours. The overall extent of severe weather is still a
bit uncertain as many parameters associated with this system are
still in question. Timing, shear and instability just to name a few.
While the upper level support is most defined across northern GA,
the better instability collocated with some low/mid level shear is
noted further across central GA. The shear is looking a bit more
impressive as compared to yesterdays model runs and the instability
is pushing further north. So, confidence is increasing that we could
see pretty good coverage of thunderstorm activity.  However, given
the timing of the fropa and cloud coverage ahead of this system, not
sure how much instability will be realized until later in the
afternoon across central and easter GA. Also, for locations that are
not observing as much cloud coverage during the morning hours
(perhaps those across central and eastern GA), this is an area where
we may see more development as the front nears in the afternoon. The
shear and helicity are the best ingredients with this system, so
anticipate a HSLC/QLCS scenario for Monday. Given we are within the
spring months and models tend to downplay instability, we will have
to keep our eye on this storm system and see how it evolves through
the weekend.  The main impacts from this line of storms will be
damaging winds and brief tornadoes possible.

Anticipate a reprieve after the aforementioned front exits Monday
evening through Wednesday night; typical diurnal thunderstorms
during Wednesday afternoon is possible. A system will approach
Thursday with models diverging quite a bit by this point.  The GFS
continues to show a closed low and waves of precipitation crossing
the area through the weekend.  Meanwhile, ECMWF is pushing a strong
front through the area Thursday into early Friday morning with
clearing thereafter.  Given this recent change in model
inconsistency, have not adjusted the forecast too much during this
period. Need to see better model continuity. Also, with such
uncertainty, kept thunder in the forecast during this time. 26

26

PREV DISCUSSION... /Issued 336 AM EDT Sat Apr 29 2017/

LONG TERM /Sunday Night through Friday/...

Still looking at two main systems impacting the extended forecast
period, late Sunday night and Monday and then later in the week
generally centered on Thursday.

Initial system Sunday night and Monday continues to look like it
brings at least a marginal risk for severe weather. Strong
southwesterly upper flow ahead of a fairly deep upper low points to
some potential before midnight, especially across the west. Main
threat approaches after midnight entering the far west before
daybreak as the associated cold front enters the state. Cold front is
progged to make steady progress across the forecast area through the
day Monday despite the parent surface low lifting well north into the
western Great Lakes. Medium range models continue to minimize the
instability somewhat as the system approaches Sunday night into early
Monday. MUCAPE values drop below 800 and 850-500mb lapse rates drop
to 6 degrees or less. Some diminished instability is to be expected
in the overnight period, but we will need to watch actual trends
closely. Nevertheless, with moderate low-level shear expected to
accompany the main area of convection, severe threat cannot be
ignored. Deeper layer shear appears to lag a bit behind main system
at this time. Moderate recovery to instability during the day Monday
as the system shifts into the eastern half of the forecast area and
moderate low-level shear maintains the severe threat.

A bit of a break behind this initial system through Wednesday before
the next system approaches by early Thursday. Will likely see a
return of isolated to scattered showers or thunderstorms as early as
Wednesday but main impacts are still centered on Thursday, lingering
into Friday. Still a bit of a spread between the main medium range
models concerning the exact track and timing of the surface and
upper-level features with this system but all show a good shot of
mild to cool air filling in behind the system going into the weekend.

20

CLIMATE...

Records for 04-29

               Max Temperature         Min Temperature
  Station      High        Low         High        Low
  -------  ----------  ----------  ----------  ----------
   KAHN      90 1915     50 1999     66 1975     38 1973
                                        1970        1928
   KATL      88 1894     47 1934     67 2014     39 1992
                                        1989
                                        1970
   KCSG      92 1943     57 1934     71 1970     39 1928
                1927
   KMCN      92 2012     50 1934     68 1970     37 1992
                                        1914

AVIATION...
00Z UPDATE...

Most of the TSRA activity today has been well north of the
terminals and latest radar confirms that trend is continuing. Have
removed the VCSH for the 00Z set and gone FEW for the CU as even
it has diminished greatly. Low level moisture to increase tonight
and MVFR developing around 11Z at ATL looks on target. This will
lift to a VFR cig through mid morning and eventually SCT. No need
for precip mention in this cycle but likely needed with the 06Z
for ATL as next squall line approaches.

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

Medium on MVFR timing.
High on remaining elements.

Deese

&&

.PRELIMINARY POINT TEMPS/POPS...
Athens          67  86  67  76 /  20  10  40  70
Atlanta         69  85  66  74 /  20  10  60  70
Blairsville     62  79  62  69 /  30  30  60  70
Cartersville    67  86  63  74 /  20  10  80  70
Columbus        70  88  68  77 /  20   5  60  70
Gainesville     66  83  65  72 /  20  20  50  70
Macon           67  89  69  79 /  20   5  30  60
Rome            67  86  62  75 /  20  10  80  70
Peachtree City  66  86  65  74 /  20  10  60  70
Vidalia         69  89  70  84 /   0   5  20  60

&&

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

$$

SHORT TERM...Deese
LONG TERM....31
AVIATION...Deese


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