Scientific Forecaster Discussion
Area forecast discussion...updated aviation
National Weather Service Peachtree City Georgia
1238 am EST Monday Dec 29 2014
have continued to update the forecast...generally the hourly
pop and weather grids...as necessary. Mist or drizzle continues
across portions of the area...and the next wave of light to
moderate rain is expected to push through overnight...beginning in
northwest Georgia in the next hour or two.
Although the flooding threat has diminished somewhat...will
continue with the Flood Watch through Monday morning. The flooding
threat remains non-zero and due to some variability in modeled quantitative precipitation forecast
overnight...and the potential for a few flood prone creeks to rise
to above bankfull levels...feel the watch is still warranted.
Overall...widespread quantitative precipitation forecast amounts should be below 0.5 inches.
Previous discussion... /issued 228 PM EST sun Dec 28 2014/
Short term /tonight through Tuesday/...
short term still dominated by southwest flow aloft as shortwave
over the Southern Plains and lower Mississippi Valley moves east
and continues to damp. Gulf moisture still being pumped into the
southeast with anomalously high precipitable water values...12z kffc sounding was
1.6 inches which is above the 99th percentile and not far from the
record values. Second wave of precipitation should push into north
Georgia tonight as the shortwave continues to damp and push east
into the smvfr and possible IFR expected Monday night.
Southeast. Attendant surface front will push most of the
rain out of the area by Monday afternoon...but the front will
elongate and stall allowing some lingering precipitation across extreme
southeast zones Monday night into Tuesday. Additionally...wrap
around moisture on the back edge will allow for some lingering
light rain showers across extreme northeast Georgia.
Still pretty much zero MUCAPE to deal with and while we cannot
completely rule out an isolated rumble of thunder here and there
especially in the heavier rain...not enough of a concern to add
thunder at this time. Forecast quantitative precipitation forecast through the short term is
pretty small...with generally less than half an inch additional
rain expected. This is despite the high precipitable water values and very
efficient rainfall processes. Might be hard-pressed to get any
flooding as this system wraps up but will leave the Flood Watch in
Temperatures will continue above-average especially for overnight
lows and for highs in southern zones ahead of the front. Went with
a general blend through the period. We will start noticing the
effects of the cold front more drastically in the extended.
Long term /Tuesday night through Sunday/...
next system for late Thursday night into Friday morning is
shaping up to be an interesting one from the outset. We had been
advertising temperatures right around freezing for some of the valleys as
the precipitation moves in and now the new guidance comes in even lower
with some temperatures around freezing even along our western tier of
counties. Taking a look at the model Cobb output however for
Brasstown Bald...it is evident that the low temperatures will be around
03z...just ahead of the precipitation shield and rise through the night
into the middle 30s by 12z. There still could be some cold air locked
into the valleys so would like middle shift to get a look at another
run before pulling frozen precipitation on this cycle.
Severe potential has not really changed from this time yesterday
as cape builds to a high of around 1000 j/kg over the southern
tier Sunday afternoon with best shear remaining to the north. Do
have 30 to 35 kts at 850mb across the southern tier and there is
actually an earlier lower jet around 925mb that pushes a 40 knots maximum
across the Atlanta metropolitan so will continue slightly enhanced
wording in the severe weather potential statement.
Timing issues remain with the 12z GFS maintaining a slower
solution in clearing things out. Have yet to see the European model (ecmwf) for the
12z run but will likely just keep the low end chance probability of precipitation in there
as a hedge for the extended until more agreement is reached. Based
on the last week...am leaning toward a less progressive front and
a wetter pattern to continue.
expect prevailing MVFR ceilings to lower to IFR and LIFR by 09z this
morning and persist through 17-18z today... when ceilings should improve
to MVFR. May see brief improvements at times with the onset of
occasional -ra... but overall ceilings and visibilities should generally remain
MVFR or lower through the morning. Expect improvement by middle to late
morning as the areas of -ra diminish. Could see low VFR ceilings during
the afternoon. Guidance shows ceilings settling back down to IFR later
tonight. Expect northwest winds at 5 kts or less this morning... then northwest at
5-7kt by 18z today.
//Atl confidence...06z update...
medium on timing and duration of LIFR and IFR ceilings this morning.
High on all other elements.
Preliminary point temps/pops...
Athens 60 51 59 45 / 100 90 60 20
Atlanta 61 51 60 46 / 100 100 40 10
Blairsville 58 46 56 42 / 100 100 50 30
Cartersville 59 46 56 42 / 100 100 40 10
Columbus 67 55 63 48 / 100 80 60 20
Gainesville 56 49 57 45 / 100 100 40 20
Macon 68 56 64 47 / 80 80 80 30
Rome 61 46 55 41 / 100 100 30 10
Peachtree City 61 51 61 43 / 100 90 40 10
Vidalia 78 60 67 53 / 10 40 70 60
Flood Watch until 7 am EST Monday for the following zones: