Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Peachtree City Georgia
733 PM EST Sat Feb 28 2015
updated grids for tonight and Sunday morning to use latest raw
model guidance. Mav/met MOS guidance too warm given strong cad
event setting up...often see busts of 5 to 10 degrees in wedge
events with precipitation from said guidance based on National Weather Service/Georgia tech study.
Surface winds east-northeast at 10-15kts tonight and plenty of dry low level air
upstream in SC Piedmont. Used nam12/GFS/sref 2m temperatures and
dewpoints and utilized most recent wpc quantitative precipitation forecast guidance which puts up
to 0.10 inch ice in wedge favored areas of NE Georgia. Expanded
Freezing Rain Advisory to include Dawson and Hall counties where
the ice accums expanded...southern half of Hall County should be
fine...but could see light icing in trees even in counties just
outside the advisory.
Will be watching observed temperatures and precipitation closely tonight so
further refinements may be possible.
Previous discussion... /issued 300 PM EST Sat Feb 28 2015/
Short term /tonight through Sunday night/...
southwest flow aloft streaming moisture across the northern
Gulf in the form of middle/high clouds with some light rain
spreading northward into the forecast area this afternoon.
Strong high pressure was in the process of building down the
Appalachians and this wedge will strengthen across north and
central Georgia overnight. Strong flow from the south and southwest will
continue to overrun The Wedge tonight into Sunday. This will produce
light rain and drizzle over much of the forecast area...with
highest probability of precipitation over the eastern zones which should be deeper into The
Wedge. Low clouds and rain combined with easterly winds should keep
diurnal range low for tonight into Sunday. Have lowered maximum
temperatures several degrees area wide for tomorrow. Have kept the
Freezing Rain Advisory in place for the northeast. Temperatures at
or just below freezing still look possible for that area overnight.
Long term /Monday through Saturday/...
active pattern through the extended in terms of precipitation.
Front sinks into the state on Monday but weakens as it moves in.
The models are still showing some differences in terms of timing
and location of precipitation with the front...but better chances
will be across north Georgia.
Main feature of the extended is a cold front that moves into the
state middle week. This will bring back the chance of thunder in the
Tuesday night to Wednesday night time frame with the best chances
on Wednesday with MUCAPE values near 500 j/kg. This will be the
system to watch over the next week for strong to severe weather as
at one point...the better MUCAPE aligns with 40kts of low level
For Wednesday night...the rain continues across County Warning Area...however
temperatures around or just below freezing are forecast to move into
parts of the northwest for the second half of the night. Based on GFS
bufr data have included mention of freezing/rain sleet across the
far northwest across a very small area of the County Warning Area...but with clouds and
precipitation already occurring...forecast and model guidance
temperatures may be a little on the cool side. Precipitation
associated with the front should push out by Friday.
Previous discussion follows.
Previous long term /Sunday night through Friday/...
warmer pattern setting up across the region for the majority of the
extended forecast period with persistent southwesterly upper flow.
This pattern also leans wetter. Minor focus for more organized
precipitation Monday as a weak frontal boundary sags into the state
and washes out. Little upper support and minimal instability so I am
inclined to remove the slight chance for thunder we currently have
for the daytime period Monday. Better chances for thunder Wednesday
into Wednesday night as a much stronger front sweeps into the state
ahead of a fairly strong short wave. Instability is noticeably
better with this system...and with a strong low-level jet prognosticated
across the southeastern U.S. We will need to keep an eye out for
possible severe weather with this system. Cooler weather fills in
behind this system heading into next weekend...but flattening upper
pattern shunts the best cold air east well north of the state.
primary concern is onset of IFR ceilings and precipitation tonight. 4kft
ceilings moving in from the south this evening but should drop to IFR
after 06z at katl when light rain moves in and remain IFR until
after 18z when precipitation moves tapers off. Surface winds will remain east-northeast
10-13kts...decreasing to 5-10kts Sunday.
//Atl confidence...00z update...
medium confidence on IFR ceilings and precipitation timing.
High on all other elements.
Preliminary point temps/pops...
Athens 34 42 39 63 / 70 70 20 40
Atlanta 36 45 43 59 / 50 50 40 40
Blairsville 31 47 42 54 / 50 60 50 50
Cartersville 36 46 44 54 / 40 40 50 50
Columbus 41 52 47 66 / 50 40 40 30
Gainesville 32 43 41 58 / 50 60 30 40
Macon 39 47 45 67 / 70 70 20 30
Rome 36 48 44 52 / 30 30 40 50
Peachtree City 37 48 44 60 / 50 50 40 40
Vidalia 40 50 49 69 / 70 70 10 20
Freezing Rain Advisory from 10 PM this evening to 8 am EST
Sunday for the following zones: Dawson...Fannin...Hall...