Area Forecast Discussion
Issued by NWS Atlanta, GA
FXUS62 KFFC 270042 AAA
Area Forecast Discussion...UPDATED
National Weather Service Peachtree City GA
842 PM EDT Wed Apr 26 2017
Forecast appears to be in good shape this evening. Timing of
convection moving in Thursday morning after 10Z looks OK but may
need some tweaking based on recent hires model runs. Will make
adjustments in next 30 to 45 minutes.
Model CAPE and vertical wind shear a little higher than might be
expected during the morning and may increase further around
18-21Z. Will depend greatly on final storm evolution and how thick
stratus layer will be in the morning ahead of front and how
quickly it erodes.
.PREV DISCUSSION... /Issued 321 PM EDT Wed Apr 26 2017/
SHORT TERM /Tonight through Thursday Night/...
Latest satellite loop shows mainly fair weather CU developing across
the region ahead of the approaching cold front now moving into the
MS river valley. A strong full latitude trough will continue moving
into the Tennessee Valley tonight with a strongly diffluent pattern
aloft affecting the forecast area. Although northern portion of
trough becomes closed off...southern portion should remain fairly
progressive. Current Hi-Res models show a squall line moving into NW
GA between 09z-11z Thu morning. Its not well organized but as it
moves South and East it looks like it will redevelop across Central
GA. The available instability appears to be out ahead of the
approaching system right now. The system does eventually catch up
and it looks like we have a case where Central GA actually sees the
greatest threat of SVR storms as the CAPE and lift become more in
phase. Keeping in mind this also occurs during the prime time
heating of the day Thu. Forecast CAPE values around 2000 J/KG along
with modest shear values all point to isolated strong to severe
storms. We will need to monitor this closely for any spin-ups along
the line in the QLCS environment. This system pushes east of our
area Thu night and not expecting any more precip in the short term.
High temps expected to be in the 70s and 80s Thu and mainly in the
LONG TERM /Friday through Wednesday/...
Only tweaks to the long term based on the latest guidance.
The cold front pushing through the CWA during the short term period
will affect the beginning of the long term period on Thursday night.
The cold front looks to stall across a portion of the area late
Thursday night into Friday...though trying to pinpoint exactly where
it will stall is not very easy. Nonetheless plenty of instability
will be available to sustain lightning activity across what looks to
be mainly central Georgia at this time. Loss of daytime heating in
conjunction with unimpressive shear and absence of upper level
support should gradually allow thunderstorms to diminish during the
overnight hours...with the severe threat very minimal late Thursday
evening and overnight.
Looking upstream from the Southeast on Friday...a closed low will
begin developing across the southern Rockies amidst a broad cyclonic
flow regime. This closed low will not only induce lee cyclogenesis
at the sfc across the TX panhandle...but the digging nature of this
feature will help build a subtropical ridge across the Caribbean
north into the Southeast. These processes will lift the stalled sfc
boundary across our area north as a warm front. Unstable conditions
in the vicinity of this frontal boundary will be the primary
catalyst for isolated thunderstorm development during the day
Friday. Lack of synoptic forcing and increasing influence of
subtropical ridge will keep severe threat pulse-type at best.
Poleward movement of the warm front will usher in a rather moist and
unstable airmass with Gulf influence...with the aid of anticyclonic
flow of Bermuda sfc high by Saturday. Although the subtropical ridge
argues for synoptic scale subsidence...such an unstable lower level
airmass will undoubtedly allow for diurnally driven thunderstorm
development. Given mid level lapse rates forecast to be over
6C/km...some isolated to scattered pulse severe storms with a wind
and hail threat will be possible in this weakly sheared
These same conditions carry over into Sunday with perhaps greater
coverage of thunderstorms ahead of an approaching cold front. Any
pre-frontal area of convergence could enhance thunderstorm activity
with the same threats as Saturday. Right now there is also some
indication of orographically enhanced thunderstorm development as
low level southerly winds kick up and ride upslope the mountains.
Then comes the interesting system Sunday night into Monday.
Aforementioned closed low will have pivoted into the Midwest enroute
to the Upper Great Lakes. Associated sfc cyclone will follow suit
just to its east...which will drag a cold front eastward toward our
local area. Given the timing for entry into Georgia will be just
before sunrise...instability is not overly impressive at this point.
However...it should be enough to support a severe threat as mid
level lapse rates remain over 6C/km. Shear will be stronger with
this system with better upper and lower level jet support...so
storm organization should not be a problem for this system. At
this point front parallel flow points towards a linear MCS
threat..although large displacement of the best forcing to the
north does cast some doubt on the ability of the line to sustain
itself despite an otherwise ripe environment...as is usually the
struggle across this area of the country.
As fropa occurs late Monday...we dry out from northwest to southeast
under the influence of sfc high pressure. EC tries to bring moisture
back in quickly by Wednesday...while GFS holds dry under
continued high pressure. Will monitor trends.
VFR conditions until around 09Z when MVFR stratus will develop.
Stratus could initially come in as high as 2500ft before lowering
to just above IFR as rain moves in around 12-15Z, from west to
east across the state. Most likely timing for SHRA at ATL is
14-18Z with TSRA likely 14-16Z. Will increase TS probs to TEMPO
on next update. Sfc winds will gradually increase tonight from SSE
to 10-12kts with gusts to 22kts just ahead of front. After front
passes around 16-17Z, winds will become SW. Conditions clearing
quickly after 22Z.
//ATL Confidence...00Z Update...
Medium confidence on stratus and SHRA/TSRA timing.
High confidence on all other elements.
.PRELIMINARY POINT TEMPS/POPS...
Athens 60 78 62 85 / 20 40 30 20
Atlanta 64 77 63 84 / 50 60 10 20
Blairsville 57 73 52 80 / 50 60 20 20
Cartersville 63 78 58 84 / 60 60 10 20
Columbus 67 81 66 87 / 50 60 10 20
Gainesville 60 75 60 83 / 40 60 20 20
Macon 61 83 65 88 / 40 40 30 20
Rome 63 78 56 85 / 60 60 10 20
Peachtree City 61 78 61 85 / 50 60 10 20
Vidalia 63 87 67 91 / 20 30 20 10