Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Peachtree City Georgia
222 PM EST Sat Dec 27 2014
Short term /tonight through Monday/...
Short term begins with strong southwest flow aloft...with a front
stretching from the middle-level trough and surface low in the Great
Lakes down to the surface low in the lower Mississippi Valley. Deep
Gulf moisture being advected into the southeast this afternoon. We
had a slug of moisture move across Alabama this morning into north
Georgia early this afternoon with some isentropic lift at the 300-
305k surface...but main precipitation shield remains down near the
Gulf and will approach the area this evening. Continued trend of
categorical probability of precipitation especially up north as the heaviest precipitation is
shunted northeast with the flow aloft. New 18z wpc quantitative precipitation forecast has over 2.5
inches in some areas especially west of the Atlanta metropolitan but
generally around 2 inches for most of north Georgia. Considered
issuing a Flood Watch but it seems for now that the quantitative precipitation forecast will be
spread out enough through the short term to avoid too many rapid
rises. Will have the midnight shift reevaluate if trends are
As far as thunder potential GOES...MUCAPE values tomorrow and Monday
are really on the low side...like 50 j/kg or less. Could possibly
argue for more thunder chances southeast Monday afternoon but only
one model is giving US much MUCAPE to work with. That said...it
could very well end up not being upright instability but more
slantwise instability that results in isolated claps of thunder...
but not confident enough to really be concerned...so kept wording at
just showers for now.
First wave of deep moisture pushes through tonight into Sunday
afternoon...then a bit /not much/ of a lull before a second round
works its way up the southwest flow aloft...driven by the trough
itself. This should affect the area Sunday night into Monday
morning. Shortwave really damps by this time as the surface front
elongates...but it finally pushes through most of the area on Monday
ushering a drier and cooler period behind it. Overall for the short
term...expect temperatures to be above-normal for this time of year
due to the southwest flow aloft and plentiful moisture.
Long term /Monday night through Saturday/...
Not much change to the previous long term portion with the
loading in of 12z grid data. Still looks like for the next storm
system Thursday night into Friday that there will be some cold air
trapped in the valleys to start Friday morning. Chances are this
cold air will retreat prior to onset of precipitation but enough
of a close call to warrant keeping slight chance freezing rain in
there for now. Otherwise...did increase probability of precipitation for the event with
likely now for most areas.
With upper level low forecast to lift across the upper Mississippi
Valley...there will be an opportunity for a surface southerly flow
to develop locally from the Gulf of Mexico Friday night into
Saturday. Dewpoints from the GFS increase to middle to upper 60s as
a result and associated convective available potential energy go to almost 1000 j/kg over the
extreme southeast corner of the area. Best shear remains north of
the greatest instability but there is a section of overlap over
central Georgia where strong storms would be possible should this
forecast trend continue. No additional changes were made to the
Previous discussion... /issued 303 am EST Sat Dec 27 2014/
Long term /Sunday night through Friday/... GFS/European model (ecmwf) in pretty
good agreement with the exiting system early in the long term. As
the next low digs over the southwest U.S. A nearly zonal flow
develops with a low level north to northeast flow. This should
keep a steady stream of upper level clouds moving across the
southern U.S. This will also keep temperatures at or just slightly below
normal through much of the week. Differences develop in the models
with the timing of the next system. The European model (ecmwf) is faster with
bringing moisture into the County Warning Area Thursday night but it also has a
deeper low over the southern rockies. This brings in ample
moisture Thursday night with a warmer southerly flow while the GFS
is slower and a little cooler as it holds on to the surface high
over the County Warning Area. Will go with a compromise on timing of probability of precipitation but at
least both indicate moisture moving into the County Warning Area so will go with
low chance Thursday night...and high chance to likely beginning
Friday. There still remains the question of winter precipitation Thursday
night. Both models push the 0c 850 temperature north of the County Warning Area with the
European model (ecmwf) pushing the 0c line the furthest north...into Kentucky. Low level
thicknesses indicate all liquid precipitation. At this time...will go
with a chance of r-/zr- in the mountains late Thursday night.
Confidence in this happening is low.
continued VFR this afternoon with scattered showers across north
Georgia...with include vcsh for a couple of hours to start. Expect
increasing middle-level cloudiness this evening...dropping to MVFR
between 06-09z depending on location...with IFR expected just
about everywhere between 10-12z. Do not see much to indicate
improvement on Sunday so kept IFR ceilings in 22z /for atl/. Rain showers
should move in shortly after 00z for northern tafs but may not
make it to southern tafs...at least not enough to include at this
time. Front pushes through just before noon at metropolitan tafs bringing
southeast winds around to SW.
//Atl confidence...18z update...
medium-high on timing of elements.
High on occurrence of elements.
Preliminary point temps/pops...
Athens 49 63 53 60 / 90 90 80 40
Atlanta 51 62 52 61 / 100 90 90 30
Blairsville 46 57 47 57 / 100 100 100 30
Cartersville 50 61 48 59 / 100 100 100 20
Columbus 53 70 57 66 / 70 60 70 30
Gainesville 48 58 50 59 / 100 100 90 30
Macon 51 70 58 66 / 30 50 70 50
Rome 49 60 47 58 / 100 100 100 20
Peachtree City 51 64 53 63 / 100 80 80 30
Vidalia 53 73 60 69 / 10 20 40 40