Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Peachtree City Georgia
237 PM EST sun Dec 21 2014
Short term /tonight through Monday night/...
drizzly and dreary short-term in store for north and central
Georgia...and will give way to a much more active extended.
Prolonged strong isentropic lift will prevail through the short-term
period under southwest flow aloft to begin the period. Surface high
in southeast Canada has begun to ridge down the eastern Seaboard in
diabatically-enhanced classical cad onset...with damming becoming
firmly entrenched overnight tonight. Widespread low clouds will
continue with drizzle and light rain expanding north with the
intrusion of Gulf moisture over the cold /well...cool/ dome tonight.
Best rain chances will remain on the south and east side of the
forecast area...have gone categorical in these areas for a high-pop
low-quantitative precipitation forecast situation...but drizzle will be possible just about anywhere
with low-level saturation under the strengthening inversion.
Probability of precipitation lower somewhat on Monday as first wave of Gulf moisture exits
but deepening wave over the plains will bring another round of more
significant moisture late into the day into Monday night. Upper
diffluence will add synoptic lift but damming remains firm at least
in the northern half of the area. Should see enough lifting of the
warm/wedge front to allow portions of central Georgia to work into
the warm sector starting late Monday. High temperatures Monday afternoon
will be extremely dependent on how far The Wedge front makes it...
but for now have temperatures below normal for northern 2/3 of the area but
near to perhaps slightly above extreme south. This will obviously
have to be reevaluated tonight to get a better handle on location of
The Wedge front...but obviously some bust potential there.
All guidance keeps SBCAPE values pretty much 0...but MUCAPE values
do increase late Monday night across central Georgia...expanding
north with the warm air advection aloft. MUCAPE values of between
200-300 j/kg spread toward the metropolitan area toward early Tuesday
morning. Have added isolated thunder extreme south toward the end of
the period. Want to reiterate that again this is not surface-based
cape so basically only expect isolated rumbles embedded within the
larger increasing rain shield.
By the end of the period...deep plains trough has cutoff in to a middle-
level low with primary surface low in the upper Midwest but
secondary surface low developing off the Texas Gulf Coast...with
southwest flow aloft over the southeast and continued upper-level
diffluence. Probability of precipitation increase again rapidly late overnight Tuesday night
into early Wednesday morning...with some active weather expected
into the extended. See below.
Long term /Tuesday through Sunday/...
models in more agreement within themselves than previous runs and
have therefore made a few changes to the Tuesday/Wednesday storm
based on the latest data. Have increased probability of precipitation Tuesday into
Wednesday. The focus for severe weather will be in two waves. The
first on Tuesday as instability increases across central Georgia south of
the warm front and feel this will be the area of concern Tuesday.
North Georgia will be in the cold dome on Tuesday and although there is
plenty of isentropic lift and while there could be a few rumbles of
thunder...the cold dome should keep north Georgia out of the severe
weather threat. The second wave will be with the cold front...moving
into the western part of the County Warning Area Wednesday morning. Expect a line of
thunderstorm...mainly a qlcs with isolated tornadoes/damaging winds. This
line should move rapidly across the County Warning Area Wednesday as will the threat
of severe storms. The front/storms will be out of the County Warning Area by 00z
Thursday. With a more prominent wedge Tuesday...have made adjustments to
maximum temperatures Tuesday. Models are now advertising a drier system for
next weekend and have brought down the probability of precipitation just a little. No other
changes are planned to the long term.
widespread IFR to low MVFR at taf time. Expect some brief
improvement this afternoon but ceilings drop again after sunset as -dz
develops. Should see IFR to LIFR before sunrise Monday. Expect
IFR visibility as well. Only minor improvement late Monday morning but
basically IFR everywhere through the end of the period...even atl
through 00z Tuesday. Winds east generally between 5-10kt.
//Atl confidence...18z update...
medium on timing of low cigs/vsby.
High on all else.
Preliminary point temps/pops...
Athens 41 48 43 48 / 70 60 60 100
Atlanta 43 51 45 52 / 40 40 50 100
Blairsville 39 49 42 48 / 60 50 50 100
Cartersville 42 52 45 50 / 30 30 40 100
Columbus 45 58 52 65 / 50 40 50 100
Gainesville 40 48 42 46 / 60 50 60 100
Macon 45 54 49 63 / 70 50 50 100
Rome 41 53 46 54 / 30 30 40 100
Peachtree City 41 52 46 56 / 40 40 50 100
Vidalia 47 57 52 71 / 80 70 50 100