Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Peachtree City Georgia
250 PM EST Thursday Dec 18 2014
Short term /tonight through Saturday/...
Weak upper disturbance and lingering moisture will continue to allow
just some slight probability of precipitation for showers in the northwest into early this
evening then should see some scattering out of midlevel moisture
though cirrus likely hanging around. Models in good consensus with
zonal flow aloft into Friday then good moisture advection off the
northern Gulf ahead of the next shortwave for Friday night into
Saturday. NAM 295 Theta surface indicating strong isentropic lift from
the southwest by late afternoon/early evening Friday...overspreading
majority of area after midnight. Have trended probability of precipitation accordingly and
start a bit earlier than models suggest as they typically are too
slow with upglide onset. Bring definite probability of precipitation in after midnight
through much of Saturday morning then begin diminishing from the
west into the afternoon as prognosticated trough axis and enhanced moisture
field push east. Should be a nice soaking and nearly steady rain.
Quantitative precipitation forecast values do not look that impressive with storm totals generally
near a half inch. Trended low temperatures and dewpoints a bit higher than
guidance for some locations given the likely cloudy and moist
Long term /Saturday night through Thursday/...
still looks to be an active long term period with generally
unsettled weather Sunday into Monday and a stronger storm system
pushing into the region for Tuesday night and Wednesday. Did not
make any huge changes from previous long term issuance. Probability of precipitation were
increased a bit Sunday night into Monday as models agree on a wave
pushing across the area from the southwest. Otherwise...still
uncertainty regarding impacts from the Tuesday/Wednesday system.
GFS indicates higher chances for some snow across far north
Georgia for Wednesday morning...but still too far out to put much
confidence in that solution. Will continue to evaluate both
thunderstorm and winter precipitation potential with this system
over the coming days. See the previous discussion below for
Previous long term discussion... /issued 336 am EST Thursday Dec 18 2014/
spell of dry weather to end beginning this weekend and next week. Flow
over most of Continental U.S. To become more zonal Friday. Model forecasts in
good agreement on shortwave over Southern Plains to move east to MS
River Valley Friday night. Solid warm/moist advection pattern kicks
in well in advance of wave. Based on track of wave...rain chances
appear categorical Friday night and Sat morning. Have bumped probability of precipitation up
from model consensus blend. Wave fairly open so zonal flow and no
strong subsidence or surface ridging behind it. Could be good pattern
for stratus...fog and drizzle if we get overrunning. Kept slt chance
probability of precipitation going for measurable drizzle or light rain Sat night through Sun
Pattern becomes more amplified over Continental U.S. As strong flow digs into
large trough over the plains. First in a series of short waves to
lift east-northeast along Gulf Coast out of south Texas Sun night. Models have a
little higher rain chances Monday middle Georgia and points south and
east as wave quickly lifts and dampens. Again...southwest flow in
wake of this wave so even better opportunity for fog/drizzle Monday
Tuesday...large upper trough with low centered over Iowa. Lobe of
vorticity over western Tennessee to lift NE then north over Midwest states.
Timing and location of upstream wave will be key to precipitation...both
intensity and type on Tuesday night through Wednesday night /xmas eve/. 00z
European model (ecmwf) coming more in line with previous GFS runs and other medium range
models with main upper low remaining over Midwest states and
somewhat less sharp trough axis than previous ec runs...moving into
middle-south Tuesday night. Main threat if any Tuesday night could be high-
shear Low-Cape storms but likelihood of that diminished from previous
ec runs. Dry slot should end precipitation quick Wednesday but some very light
rain showers or even snow showers poss in far north Georgia as very
cold air aloft rotates through Wednesday and Wednesday night. Again...00z ec
backing off on amount of cold air and upper low formation further
south. Did keep small chances of light rain/snow showers in forecast
mainly in higher elevations. No real chance for any precipitation in atl
metropolitan or middle Georgia after 12z Wednesday. Will continue to monitor.
VFR conditions expected for this taf period with initial ceilings near
6 kft building in from west then some scattering out to mainly
cirrus after about 06-09z tonight. Ceilings should gradually lower
again to near 10 kft by 21z Friday. Winds initially northwest near 5 kts
becoming light to calm overnight...then becoming NE around 5 kts after
about 16z Friday though katl and nearby sites could see some
wavering to northwest at times. Precipitation chances increasing from SW just
beyond the end of the period.
//Atl confidence...18z update...
medium on timing of overnight ceiling scattering and timing of NE
wind shift Friday.
High all else.
Preliminary point temps/pops...
Athens 35 56 41 47 / 0 10 100 100
Atlanta 37 55 44 50 / 0 20 100 100
Blairsville 31 52 38 46 / 0 5 90 90
Cartersville 30 53 40 49 / 0 20 100 100
Columbus 40 60 47 56 / 5 50 100 100
Gainesville 36 53 41 47 / 0 10 100 100
Macon 36 61 44 55 / 0 40 100 100
Rome 30 52 39 50 / 0 10 90 100
Peachtree City 32 56 42 51 / 0 30 100 100
Vidalia 41 63 48 57 / 0 20 100 100