Area Forecast Discussion
Issued by NWS Atlanta, GA
FXUS62 KFFC 261140
Area Forecast Discussion
National Weather Service Peachtree City GA
740 AM EDT Wed Apr 26 2017
.PREV DISCUSSION... /issued 356 AM EDT Wed Apr 26 2017/
SHORT TERM /Today through Thursday/...
The latest regional analysis shows strong upper low that plagued
the local area for the last several days now off the mid atlantic
coast and replaced by some subtle ridging aloft...a welcome
change. Mostly clear skies across the area although there is the
occasional patchy fog to disrupt things this morning across
Central GA. Temperatures falling through the mid 50s and dewpoints
doing the same which is favorable for fog this time of year so
still could see some additional patches develop prior to daybreak.
Will not be much of a respite for the area as next system will
already be entering the western most Mississippi River valley by
this afternoon. We actually realize some weak CAPE over the local
area as well over far North GA in developing south to southwest
flow. Conditions still much to dry however for any pop mention
with precipitable water values remaining well below an inch.
Quite a strong full latitude trough enters the Tennessee Valley by
Wed night with strongly diffluent pattern aloft affecting the
forecast area. Although northern portion of trough becomes closed
off...southern portion should remain fairly progressive and if
anything squall line may be ahead of schedule of what the models
reflect. This will be important as there already seems to be a
disconnect between the entering precipitation and the lagging
instability in the models. Could entirely be the case that alot of
the activity pushes in before we can really
destabilize...especially given the nighttime arrival.
Instability does eventually catch up and may be a case where
Central GA actually sees the greatest threat of SVR storms once
CAPE and lift become more in phase Thu afternoon. Forecast CAPE
values around 2000 J/KG along with modest shear values all point
to isolated strong to severe storms. 0-1km helicity values are
actually higher than any low level jets pointing to the fact we
will need to monitor closely for any spin-ups along the line in
the QLCS environment.
LONG TERM /Thursday Night through Tuesday/...
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.
After a long period of low clouds and periods of SHRA and
TSRA...we finally have some VFR conditions areawide. These VFR
conditions will persist through today with only a wind shift to
worry about at ATL. Winds could actually fluctuate between SSW and
SSE throughout the day but for now will just show one shift at
21Z. Increasing low clouds and threat from TSRA aft 12Z Thu. Have
included Prob30 for now which will eventually need to be
converted to a TEMPO or prevailing.
//ATL CONFIDENCE...12Z UPDATE...
Medium on wind shift.
High on remaining elements.
.PRELIMINARY POINT TEMPS/POPS...
Athens 84 61 80 63 / 0 20 50 30
Atlanta 83 64 79 63 / 0 60 60 20
Blairsville 81 56 75 54 / 0 40 60 10
Cartersville 84 63 79 59 / 0 60 60 10
Columbus 85 66 82 66 / 0 60 60 30
Gainesville 82 61 77 60 / 0 40 60 20
Macon 85 61 82 66 / 0 30 50 30
Rome 85 62 78 57 / 0 70 50 10
Peachtree City 83 61 80 61 / 0 60 60 20
Vidalia 86 63 87 67 / 0 20 30 30