Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Peachtree City Georgia
951 PM EST Wednesday Dec 17 2014
Afternoon forecast grids remain on track so I have made minimal
changes...mainly a few minor tweaks to hourly temperatures/dew points
and skies. Ample cirrus spilling across the region should keep
overnight temperatures right around seasonal normals.
Previous discussion... /issued 625 PM EST Wednesday Dec 17 2014/
Short term /tonight through Thursday night/...
abnormally dense and persistent low cloud deck finally starting to
diminish in the northwest County Warning Area from strong shallow moisture with cold air
advection below an impressive 850mb inversion. Overnight should be
only dealing with some scattered cirrus in the mostly zonal flow
aloft. Went a bit warmer than guidance for lows tonight thinking the
models may be overdoing the radiational cooling a bit.
Prognosticated thermal profiles and time height relative humidity field indicating gradual
moistening down into middle levels for Thursday with advance of next
enhanced moisture wave from the west. Relatively weak impulse along
the upper flow looks to traverse near far north Georgia...providing
enough forcing to warrant a slight chance of showers centered around
18z. This is supported by the consensus of some short term hi-res
simulated reflectivity. Otherwise keeping majority of area under
mostly to partly cloudy skies and dry. Highs should have a decent
gradient from only reaching the upper 40s in the far northwest to
low 60s in much of central Georgia. Thursday night looking to have
overnight lows slightly above normal with partly cloudy skies.
Long term /Friday through Wednesday/...
long term still looks to be quite active. First things first...the
system moving across the area Friday night into Saturday will
bring widespread rainfall. Instability still looks lacking so
thunderstorms are not expected in our area. Still a slight chance
of a brief very light rain snow mix on Sunday morning in the
extreme northeast. Temperatures will remain warm enough that
accumulations are not expected.
The next big story remains the system that will approach the area
during the Tuesday time frame. This system will be interesting to
watch in the coming days... as it has the potential to bring both
thunderstorms and perhaps even frozen precipitation to parts of
the area Tuesday into Christmas evening morning. Have introduced a
rain snow mix across some areas of the north on Christmas evening to
account for this possibility. Stay tuned. See the previous long
term discussion below for further details.
Previous long term discussion... /issued 333 am EST Wednesday Dec 17 2014/
still good agreement amongst the long term models with the
weekend system. Models are somewhat divergent on the Christmas
week system...and there is potential for wet travel conditions
Monday through Wednesday.
Isentropically induced precipitation should begin to overspread the County warning forecast area
late Friday afternoon or early evening. Models typically too slow
to produce isentropic precipitation...so have bumped up probability of precipitation late Friday
afternoon in response. The best chances for precipitation remain
overnight Friday into Saturday...with the rain tapering off
Saturday night. There is still some potential...albeit very
small...of some rasn in the far northeast Georgia mountains before the
precipitation cuts off.
Sunday and Sunday night should be dry as high pressure builds in
from the west.
Models begin to diverge on Monday into the middle part of the
week. The weekend cold front is expected to stall across northern
Florida and begin pushing north as a warm front by late Monday
into Tuesday. Several waves of low pressure are expected to
develop along the old boundary and move east...with the strongest
later Tuesday ahead of an approaching cold front.
Sharp cold front is expected to move through early Wednesday. Both
the GFS and the European model (ecmwf) are bringing a slightly negatively tilted
500mb trough through the County warning forecast area overnight Tuesday/early Wednesday.
Will have to continue to monitor...thunderstorms may have to be
added to the forecast...especially down south. Model trends lately
have the systems starting out strong...then dampening out the
features by 3-4 day timeframe...so a conservative forecast seems
prudent at this time. The European model (ecmwf) and GFS disagree on quantitative precipitation forecast...with
the GFS being drier. Have leaned more towards the European model (ecmwf) with
regards to probability of precipitation. Last nights runs had cooler temperatures for overnight
Wednesday into Thursday morning...temperatures have warmed a few degrees
from the previous runs.
VFR conditions are expected to predominate across the forecast area
through this forecast period. Local MVFR or lower visibilities are
possible 06-14z...in traditionally fog-prone areas...however no
restrictions are expected at any of the taf sites. VFR ceilings
lower into the 5-7kft range by 15-18z. Northwest winds 6kts or less
through the period.
//Atl confidence...00z update...
Preliminary point temps/pops...
Athens 35 57 38 58 / 0 0 0 10
Atlanta 38 56 38 58 / 0 0 5 30
Blairsville 32 52 34 53 / 5 20 5 10
Cartersville 33 52 34 55 / 5 10 5 40
Columbus 39 61 43 61 / 0 0 10 40
Gainesville 37 55 40 55 / 0 5 5 10
Macon 35 61 39 61 / 0 0 0 30
Rome 33 51 33 53 / 5 10 5 40
Peachtree City 33 57 37 59 / 0 0 5 40
Vidalia 40 63 43 64 / 0 0 0 20