Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Peachtree City Georgia
229 PM EST Friday Dec 13 2013
Short term /tonight through Saturday night/...
guidance in good agreement with short term so have used general
blend. Dewpoints early this afternoon have been decently lower than
guidance in parts of the north due to a bit better mixing down of
dry air in the boundary layer...resulting in relative humidity values getting below
25 percent for some areas. Fuels are just starting to get spotty 8
percent values near writing of this afd...and considering the
updated forecast trend of dewpoints going back up by middle to late
afternoon...this puts the conditions slightly outside of Fire Danger
Statement criteria...though would not be surprised if a few spotty
areas did end up reaching it if drier air holds on a bit longer.
Otherwise middle to upper level moisture is advecting gradually from
the west ahead of the next disturbance and gradual cloud coverage
should be on the increase and be cloudy area-wide by Saturday
With weakening surface ridge to the east a developing low pressure
system translated from the upper shortwave is set to track east-northeast
across southeast Continental U.S. Saturday. Decent moisture overrunning is on
tap as strong 40-50kt SW 850mb fetch is prognosticated to develop across
the area. Isentropic analysis on NAM 295k Theta surface also supports
some widespread rain with the advance of the low. Have continued to
trend likely to categorical probability of precipitation for rain beginning in the northwest
in the 09-12z period...becoming definite across the majority of the
County Warning Area by 18z...then reducing to chance and slight chance probability of precipitation Saturday
night. The south portion of central Georgia is showing potential for
200-400 MUCAPE Saturday afternoon into evening...and convective
potential is still possible with any weak lingering wedge boundary.
Have therefore continued to include chance for thunderstorms and
rain shower mention in the south...while keeping areas north more in
the stratiform rain coverage. Deep and low level shear values are
still supportive of strong to isolated severe development with
damaging winds and isolated tornado potential as possible
threats...though keeping potential low as available energy looks to
be difficult to attain...especially if mostly cloudy skies fail to
For quantitative precipitation forecast...HPC guidance looks like a reasonable compromise among
guidance solutions...which results in at least 0.50 inches area-wide
and other locations up to 0.75-1.00 inches through Saturday night as
cold front swings through.
Temperatures will be on increase mainly across the south Saturday as
southerly advection component will bring middle 60s south of a Columbus
to Macon line and generally upper 40s to middle 50s for north Georgia. Below
normal temperatures are again in store for the Post-frontal start of the
Long term /Sunday through Friday/...
previous forecast appears to be on track and only a few minor tweaks
to the grids with little in terms of changes to the forecast. See
previous discussion for details.
A cold front will exit the area Saturday night with the main upper
trough over the middle U.S. The bulk of the deep moisture will have
shifted to central Georgia where the highest probability of precipitation will be. The GFS/NAM are
more aggressive moving the deep moisture east than the European.
Only precipitation chances to start Sunday will be over the far southeast. Upper
trough approaches the area with some moisture that could spit some
light precipitation over the far north counties but chances too low to include
in the forecast.
For Monday into Thursday...airmass and upper flow too dry for any
precipitation concerns even though there will be a couple of short waves in
the upper flow during the period.
Next system moves across the middle U.S. On Friday and more significant
moisture increases over the area then.
Temperatures will be transitioning to below normal starting Sunday
then back toward normal late in the week.
initial VFR conditions the afternoon and evening will gradually
lower to MVFR ceilings after near 09z tonight and into IFR/LIFR range
after 12z Saturday with onset of rain. Visibilities look to lower to MVFR
after about 15z Saturday...though may go a bit lower in areas of
heavier rain. Generally southeast winds staying under 10kts then
swinging SW near end of forecast period for katl after 20z Saturday.
//Atl confidence...18z update...
medium confidence on ceilings and visibilities after 12z Saturday.
High confidence all else.
Preliminary point temps/pops...
Athens 41 49 41 52 / 40 100 40 10
Atlanta 44 54 41 48 / 50 100 30 10
Blairsville 36 47 36 44 / 60 100 30 10
Cartersville 41 53 39 47 / 60 100 20 10
Columbus 47 63 47 53 / 40 100 40 10
Gainesville 41 47 40 48 / 50 100 30 10
Macon 45 63 47 56 / 40 90 50 10
Rome 38 53 39 46 / 70 100 20 10
Peachtree City 41 58 41 50 / 50 100 30 10
Vidalia 50 68 55 60 / 20 80 70 20