Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Peachtree City Georgia
1232 PM EST sun Dec 8 2013
Previous discussion... /issued 953 am EST sun Dec 8 2013/
strong high pressure wedge continues down along The Spine of the
Appalachians this morning. Warm front is situated near the Georgia/Florida
border this morning. The difficult forecast issue for today will
be to determine how much northward progress the warm front will
make. The NAM keeps bl winds below 15kt for the afternoon/evening
which would be an indicator that there will be very little
northward progression of the warm front.
May have to make some adjustments to the high temperatures in the far
southern County warning forecast area early this afternoon...but hourly temperatures are
currently on track.
Previous discussion... /issued 641 am EST sun Dec 8 2013/
Previous discussion... /issued 333 am EST sun Dec 8 2013/
Rain to increase in coverage and intensity across north
Short term /today through Monday night/...
latest surface analysis shows high pressure anchored from the
Great Lakes into the northeast states with ridging through the middle
Atlantic and edging through the Carolinas and East Georgia. Main
influence of The Wedge thus far has been increased low level
moisture and in a change from the norm...those locations outside
the influence of The Wedge are colder. Regardless of the location
relative to The Wedge...the one constant has been the widespread
low clouds and at times reduced visibility which should persist
for the balance of today.
As wedge continues to build in today...south and southwest flow
will increase just off the surface and through 10k feet allowing
for a favorable isentropic upglide profile by late morning. Result
will be an increase in both precipitation coverage and intensity
with north central Georgia targeted initially but quickly
transitioning to just north Georgia this afternoon and evening.
Difficult to tell how much will be moderate rain...light rain...or
just drizzle but could be a bit of all three. Due to the
uncertainty with intensity...stuck close to HPC quantitative precipitation forecast projections
for this cycle. Temperatures will be very challenging today as
locations within The Wedge should see temperatures actually go down today
while other areas will see a modest increase...especially along
the extreme south.
Adiabatic Omega processes become maximized by 00z through 09z
tonight and in fact this is where sref targets isolated thunder.
Feel this fits well as 30 to 40 kts of upglide should produce at
least a few ticks of lightning and will include in latest grid set.
Main front looks to move through late Monday although nam12 is
much slower with its progression than the GFS. If this is the
case...areas to the south will be able to warm further with
possible thunder not yet accounted for in grids and will need to
be monitored closely. Will not resemble anything close to a clean
frontal passage with this system as secondary vorticity maximum begins to
exert its influence as surface low develops along the South
Carolina coast. Will therefore keep probability of precipitation high for the northern
tier for the remainder of the short term period.
Long term /Tuesday through Saturday/...
upper trough axis is pushing through the eastern third of the
country as we begin the extended...with most precipitation moving out of
the southeast by late Tuesday. GFS and European model (ecmwf) are in fairly good
agreement through the first part of the period. Elongated front
should stall across south Georgia or northern Florida which will
keep slight chance probability of precipitation across southern zones but for the most
part the middle part of the week should be dry...albeit cool with
the passage of the front /although even that is relative...
expecting average to just slightly below average temperatures so not
particularly cool for this time of year...just cooler than Monday
temps/. A reinforcing wave and mainly dry cold front pushes
through Wednesday night with a colder surface high sliding
southeast from the northern plains...so Thursday temperatures should be
just a tad cooler than Wednesday.
Weak ridging builds in aloft Thursday night into Friday with an
incremental increase in temperatures in advance of a strong shortwave
over the Desert Southwest. As this moves into the Southern
Plains...it should induce surface low development in the northwest
Gulf. This is where the GFS and European model (ecmwf) begin to really diverge. The
GFS has one main system and begins overspreading precipitation across the
southeast Friday night. The European model (ecmwf) has a weak shortwave lifting
into the plains Friday night...bringing light precipitation to the
southeast over the weekend...but holds off on the main system
until after the end of the period. So...Saturday could be really
wet /as per the GFS/...or it could just be a little wet /as per
the ECMWF/. As is typical for that far out and played the middle
of the Road by painting chance probability of precipitation everywhere. Stay tuned for
the latest storm total quantitative precipitation forecast indicates less than 2.5 inches over
the far north through Tuesday. Confidence in these totals is low
however based on variance we could see with rainfall intensity
rates as wedge builds in. Will continue with esf for now and let
day shift reevaluate based on how coverage and intensity evolve
high pressure ridge remains situated across much of Georgia this
afternoon. Light rain and patchy drizzle will continue into the
overnight hours...along with the low ceilings and 1-2sm visibilities. There
is also the potential for some patchy fog less than 1sm between
03z and 09z...and may have to add a tempo group later. Ceilings will
come up slightly during the afternoon tomorrow as the widespread
moderate rain/front move in.
//Atl confidence...18z update...
high confidence all elements.
Preliminary point temps/pops...
Athens 47 41 63 55 / 100 70 80 50
Atlanta 48 44 62 50 / 100 70 80 60
Blairsville 42 40 55 46 / 100 100 90 70
Cartersville 48 45 56 46 / 100 80 90 80
Columbus 56 55 72 60 / 60 30 80 50
Gainesville 45 39 56 49 / 100 70 80 70
Macon 51 48 72 61 / 50 30 50 40
Rome 48 47 54 45 / 100 90 90 80
Peachtree City 49 46 65 52 / 100 50 80 50
Vidalia 55 55 78 65 / 30 20 30 30