Scientific Forecaster Discussion
fxus62 kffc 271125
Area forecast discussion
National Weather Service Peachtree City Georgia
725 am EDT Sat may 27 2017
the hi-res models are coming more into line with and mesoscale convective system impacting
the northern portions of the state late this afternoon/early
evening. Have raised pops before midnight in the tonight period to
reflect the current trends.
Previous discussion... /issued 356 am EDT Sat may 27 2017/
Short term /today through Sunday/...
Unsettled weather is expected across portions of northern Georgia through
the short term period.
Surface high pressure remains well to the south, while aloft the
County warning forecast area will remain on the periphery of ridge. A couple of strong upper
level disturbances are progged to move across the top of the ridge,
one today and another during the first part of Sunday. The synoptic
and hi-res models do struggle with these types of systems,
especially with timing. Both of the locally run WRF models are
similar with trends for today's system, a potential mesoscale convective system. The hrrr
looks like it may be catching on, if the last frames are any
indication. There are still some timing issues, so have taken a
blend between the two locally run wrf's and whatever frames of the
hrrr that are in.
The mesoscale convective system is currently winding up across southern MO and northern Arkansas.
The system should continue eastward in the flow for the next few
hours, then start to make a southeast dive during the late
morning/early afternoon...clipping northern Georgia. Have increased pops
to likely across those areas most likely impacted by the mesoscale convective system. Do
think areas north of the Metro will have the best chances of
With very steep mid level lapse rates, good deep level shear and
very good surface instability present...storms will have the
potential to become severe. Isolated to scattered severe
thunderstorms will be possible across the north, with the primary
severe weathers modes begin large hail and damaging wind gusts.
The models are suggesting another round of convection during the
early part of Sunday...another potential mesoscale convective system. Have taken a more
broadbrushed approach to pops for now...but it does look like areas
mainly north of Interstate 20 will be impacted. In addition, a few
of the storms could also be strong to severe.
Long term /Sunday night through Friday/...
Main forecast concern in long term period is chance for
strong/severe convection on Monday. 00z model guidance in pretty
good agreement with Monday forecast. Front progged to continue to
sag into the state Monday with above normal sfc temps and
dewpoints. SBCAPE rises to fairly high values even for late may,
GFS 2500-3500 j/kg. Deep layer shear should be sufficient for
severe convection, 0-6km bulk shear mags > 30kts over much of the
cwa, however low level shear quite weak with 0-1km bulk shear
around 5-10kts. In such environments, we typically see severe
storms with large hail and wind threats but very low risk of
tornadoes. Precipitable water will be fairly high but not too abnormally high for
this time of year (gfs values around 1.6 in). With large scale
lift not too high, quantitative precipitation forecast also less than 1 inch in most locations
Mon, so will likely see periods of heavy rain but flash flooding
risk somewhat low.
After front sags into middle or south GA, essentially washes out
Tuesday. Moisture remains plentiful but without any forcing, any
storms would be fairly isolated thru Thursday. Have kept pops
fairly low but unlikely we will see a dry period with weak swly
flow aloft. By thurs night or Friday, warm advection should kick
in with increase in moisture as well. Have bumped up pops then.
hi-res models are in somewhat better agreement with the track of
an area of shra/tsra to impact portions of northern GA late this
afternoon and evening. Latest tracks to bring the system to near
atl before it weakens. Added in vcsh around 1-3z to account for
the increased chances for shra. Not confident on the extent of
coverage for the storms, so will not mention thunder in a tempo or
prob group just yet. Another disturbance is expected to
potentially impact the area early on Sunday. Models still progging
MVFR cigs for a brief period overnight. Winds will remain on the
west side with low end gusts during prime mixing today.
//Atl confidence...12z update...
medium confidence all elements.
Preliminary point temps/pops...
Athens 88 69 89 70 / 30 30 30 20
Atlanta 87 71 87 70 / 20 30 20 40
Blairsville 82 63 82 63 / 60 60 40 50
Cartersville 86 69 88 68 / 30 30 30 50
Columbus 89 71 90 73 / 0 5 10 20
Gainesville 85 69 86 69 / 50 60 30 30
Macon 90 68 91 71 / 0 5 10 20
Rome 85 68 87 68 / 30 30 40 60
Peachtree City 87 68 88 69 / 10 20 10 30
Vidalia 91 70 94 73 / 0 0 10 10