Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Peachtree City Georgia
934 PM EDT Thursday Aug 28 2014
no significant changes to the forecast with clear or nearly clear
skies through the overnight. Forecast low temperatures running 2-5
degrees below normal.
Previous discussion... /issued 710 PM EDT Thursday Aug 28 2014/
Short term /tonight through Friday night/...
mainly dry short term forecast trend continues with one more cool
night before moisture return begins in earnest. Afternoon highs
Friday will be quite warm again as the upper ridge remains
relatively strong and skies remain predominately cloud-free. Upper
ridge shifting east late in the period allows rain chances to creep
back into the western edges of the forecast area late Friday night/
early Saturday morning.
Long term /Saturday through Thursday/...
some uncertainty continues for the first part of the extended but
confidence is increasing in the overall pattern especially for
Saturday night. Both the 12z GFS and 12z European model (ecmwf) have continued
trend of widespread moisture across the area especially 18z
Saturday through 00z Sunday...lingering towards 06z Sunday. /For
the record the NAM remains a bit of an outlier./ Precipitable water values prognosticated
to be near to just slightly above 2 inches...definitely above
average and even above the 75th percentile but still below the
99th percentile. Overall though it's not like we're talking
significant quantitative precipitation forecast amounts. It's one of those high pop-low quantitative precipitation forecast type
events /at least basin-average quantitative precipitation forecast/ so certainly not expecting an
all day /or all-evening/ washout or anything like that...but
potential for scattered to numerous showers and thunderstorms
remains very high.
All guidance is showing north and central Georgia in a cape
minimum through early Saturday but with cape values increasing to
between 2000-2500 j/kg Saturday afternoon. We are still removed
enough from the upper support that there still isn't much in the
way of organized potential but of course cannot rule out an
isolated strong to severe pulse thunderstorm. The problem is that
it might be enough to ruin outdoor plans. Likely probability of precipitation still look
good since we are two days out but probability of precipitation will probably be increased
as we get closer just because potential remains high and
confidence is increasing. But of course as with summertime
thunderstorms...pinpointing timing or location will be difficult
at best even on Saturday. Anyway...this will be monitored closely
in subsequent forecasts.
Sunday looking a bit better but that is partially due to continued
model disagreement...with the European model (ecmwf) keeping the bigger quantitative precipitation forecast impacts
in northwest zones but the GFS transitioning to south and east
zones. Have kept trend of likely probability of precipitation north and but increased to
slightly higher chance south than previous forecast. Overall very
few changes to the extended other than to mainly refresh some of
the periods 4 and 5 data.
Long term /Friday night through Wednesday/...
/issued 341 am EDT Thursday Aug 28 2014/
extended period still contains quite a bit of uncertainty but at
least some marginal improvements to model consistency noted this
run. We begin the extended in a transition period as surface high
over the Ohio Valley shifts east off the Atlantic coast in
response to upper level trough kicking out of the Midwest states
and into the Ohio Valley. This will be a key component in our
local weather as low level winds shift to a southerly direction and
pumps moisture northward out of the Gulf of Mexico. This will
bring precipitable water values back above 2 inches for most of
the area on Saturday and this combined with modest shortwave
energy over the northwest zones should lead to likely coverage of
storms along and west of the Interstate 75 corridor. Westerly
steering flow may bring these storms further east than what is
currently depicted in grids and will need to monitor with
subsequent model runs for a possible adjustment upward.
After a few days of suggesting Sunday as the wettest day of the
weekend...models now agree on slightly less coverage. First wave
of shortwave energy lifts north of the area leaving local area
without large scale lift for Sunday. In addition...in the wake of
the shortwave...some drier middle level air works in which the European model (ecmwf)
really picks up on. Still feel far northwest zones will see better than
average rain chances given moisture in place and will go with
likely but transition with a sharp gradient to just low to Middle
Range chance for the remainder of the forecast area.
Middle and upper level ridge build back over the area through next
week but features are not particularly strong. Likewise...deep
layer moisture not anything to write home about and all this
points to climatology probability of precipitation for the remainder of the extended.
VFR conditions are expected to continue overnight and Friday.
Patchy MVFR visibilities are possible 06-12z...but no impacts are
expected at any of the taf sites. Winds will be light and variable
or calm overnight.
//Atl confidence...18z update...
high confidence all elements into Friday.
Low confidence for wind direction during day Friday.
Preliminary point temps/pops...
Athens 63 93 67 91 / 0 10 10 50
Atlanta 68 91 71 88 / 0 10 10 60
Blairsville 59 88 62 84 / 5 10 10 50
Cartersville 62 92 67 90 / 0 10 10 60
Columbus 67 94 72 92 / 0 10 10 60
Gainesville 66 91 69 88 / 0 10 10 60
Macon 64 94 72 92 / 0 10 10 60
Rome 63 93 67 90 / 0 10 20 70
Peachtree City 61 93 67 89 / 0 10 10 60
Vidalia 67 96 72 92 / 0 10 10 50