Scientific Forecaster Discussion
fxus62 kffc 180535
Area forecast discussion
National Weather Service Peachtree City Georgia
1235 am EST Mon Dec 18 2017
Previous discussion... /issued 1032 PM EST sun Dec 17 2017/
updated forecast to add dense fog to forecast grids mainly over
areas north of the fall line (columbus to athens). Vsbys may go
below 1 mile but widespread vsbys below 1/4 mile not expected at
this time. Will issue Special Weather Statement to address this
Previous discussion... /issued 711 PM EST sun Dec 17 2017/
updated forecast to include more mention of fog and drizzle
tonight and Monday. Low clouds, fog and drizzle likely to persist
much of the day now and may affect temps as well.
Previous discussion... /issued 221 PM EST sun Dec 17 2017/
Short term /tonight through Monday night/...
broad cyclonic flow over the western 2/3rds of the country has
allowed for the transport of deep Pacific moisture via the
subtropical jet over the southeast US amidst a southwest mid level
flow regime. A weakening mid level shortwave trough currently over
the Midwest is helping provide additional lift in the mid
levels...while at the sfc strong warm air advection ahead of a subtle wave is
also contributing the deteriorating conditions this afternoon.
Rest of today and tonight: rather widespread rain is ongoing across
much of north Georgia. This is expected to slowly move into portions
of central Georgia through the evening before dissipating to into
isolated light showers. Instability is minimal with this activity so
no thunder is expected through tonight. Given the rapid transport of
lower level moisture...fog is expected overnight. In
addition...hrrr/rap/nam12 sounding data also suggests the potential
for drizzle through much of the night into Monday morning.
Overall...expecting damp conditions over the next 12 hours. Temps
have been struggling across the north to even make 50 today...so
have adjusted high temps down across this area of the County Warning Area and
adjusted future trends /mainly met guidance/ to represent this.
Given ample cloud cover and fog potential...do not think temps will
fall that much overnight.
Monday is tricky. A warm front associated with the aforementioned
sfc wave will stall across the local area and become a stationary
boundary early Monday. The exact location of this boundary through
the day tomorrow will determine where the most moisture resides and
hence where highest rain chances will be confined. Right now think
it will be in the Columbus to Macon vicinity. Multiple weak
perturbations traversing through the mid level southwest flow will
provide additional ascent atop the stalled boundary to allow for the
development of rain through the day. Some MUCAPE is noted across
central Georgia so isolated thunderstorms could be embedded within
the broader rain shield. The stalled boundary will continue into the
long term...with a wet pattern to continue.
Long term /Tuesday through Sunday/...
unsettled weather still expected for much of the long term period of
the forecast. In addition, the potential for locally heavy rainfall
is increasing across portions of northern Georgia.
Stationary frontal boundary will set up across portions of north Georgia
by Tuesday night. This old front will serve to focus precip for
Tuesday night into early Thursday with the models coming into
agreement that this period will be wet. Several perturbations in the
mid level flow will interact with the stalled boundary, potentially
causing several rounds of precipitation. A stronger wave aloft along
with a surface low should move along the stalled front during the
day on Wednesday, with periods of locally heavier rainfall. For now,
amounts between one and three inches are possible, with some locally
higher amounts, especially across areas north of the Atlanta Metro.
Since the axis of heaviest rain will depend upon where the old
boundary will stall, am not confident on pinpointing on exactly
where it will set up. Currently, models are progging an area just
south of the Tennessee/NC/GA line.
Another frontal boundary/low pressure system should impact the County warning forecast area
for the late part of the week into the weekend.
Above normal temps expected through the period.
Low clouds are on the rapid increase across the terminals with IFR
at most all of the atl sites. Visibility has been a different
story and slow to lower at the terminals but more so over northwest Georgia.
Do expect both cig and vsby to continue to trend downward through
sunrise with conditions bottoming out in the life range. Certainly
possible for some vlifr as well but thinking this will be largely
outside of the atl terminal. Winds still expected to shift to the
west side by 16z with no changes to timing for this cycle.
//Atl confidence...06z update...
medium on LIFR timing and vlifr potential.
High on remaining elements.
Preliminary point temps/pops...
Athens 61 48 68 56 / 30 30 30 60
Atlanta 61 51 66 58 / 30 30 40 60
Blairsville 59 40 62 50 / 10 10 60 80
Cartersville 58 46 64 56 / 20 20 50 70
Columbus 66 56 71 60 / 60 40 20 30
Gainesville 60 48 64 56 / 20 20 50 70
Macon 64 53 71 58 / 50 40 10 30
Rome 57 45 63 54 / 10 10 60 80
Peachtree City 62 50 68 57 / 50 40 30 50
Vidalia 68 59 74 60 / 50 40 5 20