Area Forecast Discussion
Issued by NWS Atlanta, GA

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FXUS62 KFFC 301942
AFDFFC

Area Forecast Discussion
National Weather Service Peachtree City GA
342 PM EDT Sun Apr 30 2017



.SHORT TERM /Tonight through Monday Night/...

A deep closed low lifting out of the Rockies will be tracking to
the Great Lakes region through the remainder of the short term. An
associated cold front moving through the central CONUS will
eventually push through GA Monday. This system will spawn off two
main rounds of convection noted below.

ROUND ONE:

A strong line of thunderstorms moving across Alabama will
continue to track eastward this afternoon/evening.  This line is
pushing out ahead of the main front and is producing strong winds
and heavy rain with isolated tornadoes a possibility.  Given this,
we have issued a Tornado Watch until 7pm this evening across
portions of NW GA. If the line holds together and the threats
continue, we may need to extend the watch in time and further west.
However, at this time, we feel that the line will be moving out
ahead of the main front/dynamics and into a more stable airmass. The
line should diminish as we go through the evening hours.  There will
be a brief lull until the main line starts to redevelop along the
front across Alabama later tonight, eventually moving toward NW GA
by the early morning hours.

ROUND TWO:

Timing of the main line is still a bit uncertain. However, based on
model consensus, the main line of showers/thunderstorms should
approach during the early morning hours across NW GA. The line will
continue to move east through the day Monday, exiting to the SE by
the evening hours. Anticipate showers/thunderstorms to approach and
move through the metro between 5am and 10am.

The overall extent of strong to severe thunderstorms looks marginal
as much of the upper level support/dynamics surge further to the
north. Weak to marginal parameters are expected, with 30 to 40kt
deep layer shear and CAPE values less than 1000J/kg. However, dew
points in the mid/upper 60s would result in higher MLCAPE. Also,
helicity will be fairly elevated right along the front lending to
the possibility of brief embedded tornadoes along the line. Like we
have seen with recent events, would not be surprised if there were
some discrete thunderstorms moving up from the south just ahead of
the line that may show characteristics of brief circulation. But the
overall coverage of severe thunderstorms should be fairly isolated.
The main impact from this system will be damaging winds, isolated
brief tornadoes, frequent lightning and heavy rain (leading to
localized flooding). Large hail will likely become more of an issue
as the line moves into central and eastern GA during the
afternoon hours with better instability and lapse rates.

Given the aforementioned variables (timing of the precipitation,
cloud coverage ahead of the system and marginal convective
parameters), current thinking is the strongest storms may be right
as the line moves into NW GA and across central/eastern GA later in
the afternoon.

Strong gusty winds ahead and just along the front can be expected.
We have placed the far northern counties within a Wind Advisory from
8pm this evening through 4pm Monday. Winds of 15 to 25 MPH with
gusts upwards of 45 MPH at the highest elevations can be expected.

As the front moves out of the area Monday afternoon, clearing will
commence with cooler air moving into the region by the evening.
Winds will also diminish through the overnight hours.





26


.LONG TERM /Tuesday through Sunday/...
No changes have been made to the long term. The location of the
upper low on Friday and Friday night differs a bit between the
ECMWF and the GFS...also the GFS is showing more wrap around
moisture into the state. For now...have left the pops alone for
Friday. Previous discussion follows below.

41


&&


20
.Previous...130 AM EDT Sun Apr 30 2017

LONG TERM /Monday Night through Saturday/...
Cold front will be exiting the CWFA at the very beginning of the
long term period. High pressure will briefly build across the area
on Tuesday, but begin shifting offshore by early Wednesday as the
next low pressure/frontal system moves into the central US. A second
cold front is expected to impact the CWFA Thursday, with high
pressure building in for the weekend.

Any lingering showers and thunderstorms Monday night should move out
of the area and diminish by early Tuesday morning. A dry period with
near to slightly above normal high temps is expected Tuesday through
Wednesday. Good moisture begins returning on Wednesday ahead of the
next system, but most areas should remain dry through the day
Wednesday. This is a slight change from previous forecasts.

Precip chances begin to increase Wednesday night ahead of the front.
Thursday and Thursday night should be pretty wet with on/off shra
and tsra for most of the period. The models have been pretty
consistent with bringing a line of shra/tsra through on Thursday.
There is definitely some potential for strong to severe
thunderstorms as this system moves through the area. It`s too early
to call what the coverage of storms will be, but instability
parameters and forcing along the front/in the mid levels are lining
up. In addition, the potential for locally heavy rainfall exists.

Cooler temperatures are expected behind the front...and for now...a
pleasant weekend is on tap.

NListemaa

AVIATION...
18Z Update...

Most locations have finally lifted to VFR criteria across the TAF
sites with only lingering low level cigs north of the metro.
Anticipate isolated to scattered showers/thunderstorms this
afternoon, but mainly across eastern GA. Models struggling to
illustrate the line of thunderstorms surging east across Alabama
this afternoon. TAF sites across the metro/CSG may see some
residual showers associated with this line. Anticipate the
activity to diminish through this evening ahead of the main line
of showers/thunderstorms associated with the cold front. The main
line of showers/thunderstorms will move through the metro TAF
sites around 09-15Z with lowering MVFR cigs/vsbys. The line will
continue to march eastward through the afternoon hours. Conditions
will improve later in the day as the front exits. Ahead and
behind the front, anticipate gusty winds to continue out of the
south and eventually veer to the west during then morning hours.
Winds will start to diminish during the overnight hours Monday.

//ATL Confidence...18Z Update...
Low Confidence of earlier showers/thunderstorms this evening.
Medium Confidence on MVFR timing and duration.
Medium on timing of the main line of showers/thunderstorms.
High Confidence on remaining elements.

26

&&

.PRELIMINARY POINT TEMPS/POPS...
Athens          68  75  53  78 /  50  70  10   5
Atlanta         66  74  55  78 /  70  70  10   5
Blairsville     63  70  48  72 /  70  70  10   5
Cartersville    63  75  51  77 /  70  70  10   5
Columbus        68  77  55  82 /  70  70  10   5
Gainesville     66  73  53  76 /  70  70  10   5
Macon           69  77  52  82 /  40  60  20   5
Rome            62  75  51  78 /  70  70   5   5
Peachtree City  65  75  51  79 /  70  70  10   5
Vidalia         70  84  57  83 /  30  60  30  10

&&

.FFC WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 8 PM this evening to 4 PM EDT Monday for the
following zones: Catoosa...Dade...Dawson...Fannin...Gilmer...
Lumpkin...Murray...Pickens...Towns...Union...Walker...White...
Whitfield.

Tornado Watch until 8 PM EDT this evening for the following
zones: Bartow...Carroll...Catoosa...Chattooga...Dade...Floyd...
Gordon...Haralson...Heard...Murray...Paulding...Polk...Walker...
Whitfield.

&&

$$

SHORT TERM...26
LONG TERM....41
AVIATION...26



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