Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Peachtree City Georgia
633 PM EST Tuesday Dec 1 2015
rain shield is roughly along and north of Interstate 85. Hi-res
models keep the rain shield nearly stationary overnight. It really
doesn't begin to kick out until shortwave trough moves through
late in the afternoon tomorrow. Have tweaked probability of precipitation a
bit...according to current radar trends and hi-res model trends.
Wedge is building southwestward...bringing low ceilings and fog. Have
held onto the southeast winds a little longer than the models have
prognosticated. They just don't have a great handle on The Wedge and when
it will erode.
Also...have tweaked hourly temperature/dew grids.
Previous discussion... /issued 236 PM EST Tuesday Dec 1 2015/
Short term /tonight through Wednesday night/...
Flood Watch remains in effect for most of north Georgia through 7pm
Wednesday. See hydrology section for additional details.
Active/soggy short term in store. Strong...nearly stacked...closed
low over the Midwest will be responsible for pushing a cold front
across the County Warning Area during the short term. Radar imagery reveals a broad
swath of light to moderate rain along and ahead of this front...some
of which has been affecting far north Georgia for most of the day already.
Radar trends also showing precipitation filling in across Alabama...as
constant flux of tropical Pacific moisture aids in isentropic
upglide along the boundary. With increasing jet support and middle
level height falls...rain will be able to sustain itself through the
period. Chances will gradually increase from northwest to southeast this afternoon
and evening with one more night of mild low temperatures. Minimal
instability will keep thunder out of the forecast.
Cold front/rain shield will be slow to move south overnight tonight
into Wednesday. Highest rain chances overnight will be across north
Georgia...including the atl metropolitan. Models indicate a weak surface low and
enhanced middle level moisture plume developing in the northern Gulf on
Wednesday...sending a secondary surge of precipitation across the state.
This will likely affect the central portion of the County Warning Area where it will
merge with the cold front.
Rain will end from northwest to southeast during the day Wednesday and should be
out of our hair by 06z Thursday. Surface high pressure building in behind
the front will quickly clear things out and bring pleasant...albeit
cooler... weather to the area going into the long term.
Long term /Thursday through Tuesday/...
only tweaks to the long term based on the lastest guidance.
There are significant differences between the GFS and European
after Sunday with the GFS much wetter.
precipitation Wednesday evening will be confined to the southeastern
third of the forecast area...and should be ending fairly quickly even
there. Primary medium range models have continued the dry trend from
Thursday through the majority of this extended forecast period...and
I have withdrawn the low-scale probability of precipitation we have been carrying Thursday
through Sunday night. GFS and European model (ecmwf) remain fairly similar concerning
the surface pattern through this forecast with a large ridge of high
pressure developing over and dominating the eastern third of the
country. The larger scale nature of the upper pattern is similar
between them as well...but the finer details remain chaotic. Both are
indicating that enough moisture returns to the region as the surface
ridge weakens early next week...allowing low probability of precipitation to creep back into
the main weather focus will be the slow moving cold
front moving through north Georgia tonight. A series of upper level
disturbances will pass over and parallel to the front. This pattern
combined with orographic lift and training echoes will produce
periods of heavy rain over portions of north Georgia through Wednesday
Wpc quantitative precipitation forecast around two inches tends to blend the major synoptic and
hi- res model 24hr quantitative precipitation forecast through 00z Thursday. Isolated higher amounts
near three inches will be possible in the ridgelines of north Georgia.
Have accepted the compromise of amounts and heavy rain locations
as far south as north Fulton and DeKalb counties. Still looking
for around 1.5 inches of rain to cause the Big Creek to reach
flood stage from Cumming to Alpharetta. Continue to expect
localized street flooding due to clogged drains from Leaf debris.
Several other northwest Georgia creek and river forecast points will
reach or exceed flood stage by a foot or two.
wedge has begun to build SW this evening. Current observation with IFR
ceilings are right on the doorstep of the airfield. Once ceilings go
IFR...they should stay that way until The Wedge clears and/or the
front moves through. The main change to the previous taf was to
bring ceilings down earlier. Rainfall timing still looks good. Have
also held onto the southeast winds a little longer than previously
//Atl confidence...00z update...
medium confidence all elements.
Preliminary point temps/pops...
Athens 58 63 40 56 / 70 70 10 0
Atlanta 57 61 39 54 / 80 80 10 0
Blairsville 53 56 33 51 / 100 90 5 0
Cartersville 55 58 36 52 / 90 90 5 0
Columbus 61 66 43 58 / 40 60 10 0
Gainesville 55 60 39 53 / 90 90 10 0
Macon 60 67 43 59 / 30 60 20 0
Rome 54 59 36 52 / 100 90 5 0
Peachtree City 58 62 39 55 / 60 60 10 0
Vidalia 63 72 47 62 / 10 60 30 0
Flood Watch through Wednesday evening for the following zones: