Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Peachtree City Georgia
329 PM EDT Sat may 30 2015
Short term /tonight through Sunday night/...
A quiet weather pattern will continue through this evening as drier
air has spread in over the state and the active region of an upper
trough remains mainly focused over the lower MS valley region. Best
chances for convection this late afternoon and into the evening will
be across parts of far north Georgia where orographic lift will help
enhance the chance for mainly showers... although cannot rule out an
isolated thunderstorm there this evening. Regional radars show sea
breezed induced isolated to scattered convection spreading into the
far southeast zones around Vidalia... and this will likely persist
through early to middle evening. However... no severe storms are
expected north or south through tonight.
The short term models show good agreement with the upper trough...
currently along and just west of the lower MS valley region...
continuing to deepen and drift slightly eastward through Sunday
night. This... coupled with a deepening moist southerly flow... will
warrant an increase in rain chances Sunday and Sunday night. At this
time... the greater rain chances will be across west central and
north Georgia... generally along and north of the I-85 corridor...
where models show subtle disturbances embedded in the southwest flow
aloft interacting with 1.5 precipitable waters and 2000-3000+ MUCAPE by Sunday
afternoon and evening. This coupled with added forcing from a weak
cold front pushing into western Tennessee Sunday afternoon has prompted Storm Prediction Center
to place much of the forecast area in a marginal risk for severe
storms for mainly Sunday afternoon and evening. Although cannot rule
out a strong to severe storm just about anywhere on Sunday... still
think the greater strong/severe storm threat will be along and north
of the I-85 corridor on Sunday.
Expect convection to weaken by midnight Sunday... although isolated
to scattered showers will likely linger across the area Sunday night
as the front and upper Trough Draw closer.
A model blend for temperatures looked reasonable through the short term.
Long term /Monday through Saturday/...
Still significant differences between the GFS and European model (ecmwf) through the
extended forecast period concerning the evolution of the closed
upper low over the northern Gulf of Mexico and subsequent surface
features. Current extended forecast grids were based largely on the
GFS and this continues to be the most consistent solution so I have
not made significant changes to the long term forecast grids at this
time. Please see the previous long term forecast discussion below.
Previous long term forecast discussion /Sunday night through
Synopsis...00z GFS and European model (ecmwf) begin the period in good agreement
but once again diverge on the evolution of a cutoff low over the
Gulf of Mexico/Gulf Coast states by middle week. Differences in the
position of the cutoff low make the forecast confidence fairly low
for days 5 through 7 especially. Good news is both models are
consistent in developing much needed ridging over Texas and the
central U.S. Where an Arkansas is the preferred transportation method
Interesting to note with the current mesoscale convective system over Texas at this time and the
associated vorticity maximum are what ultimately lead to the eventual cut
off low and our potential week long weather maker. However...the
GFS is currently doing a bit better with the position and track of
that system which lends itself to a potential better solution
downstream with the cut off low. Too early to tell at this point
but certainly something to watch.
Sunday night through Tuesday...fairly decent coverage of storms
expected Sunday night to start the period as a weak shortwave and
surface front move into the region. By Monday the mean 500mb trough
axis is centered over US with a weak surface front located across
central Georgia providing a focus for afternoon storms. With the lack of any
real suppression...afternoon storms will be prevalent each day. At
the same time per the GFS and GFS ensemble...cut off low taking
shape anywhere from central Gulf of Mexico to south Mississippi to
Florida Panhandle. European model (ecmwf) has trended a tad west from previous runs but
is still on the eastern edge of the model suite.
Wednesday through Friday...the cutoff low...where ever it ends
up...begins to tap into significant amounts of deep tropical
moisture from the northwest Carribean. Regardless of whether you
go with the European model (ecmwf) or GFS...they both agree on the deep moisture
tap by the low and begin to bring that moisture north on the east
side of the low...which is why it will be important to know where
the low eventually ends up. A western position will mean higher
probability of precipitation...more rain...and increased flood potential for US Wednesday
through Friday while a further east location will still mean a chance
of rain...but much lower overall chances and minimal flood threat.
Jury is still out on the eventual evolution of the low...but am
still leaning toward a more western position given the GFS
ensembles still favoring that solution...while not as far west as
runs 24 hours ago. Still maintaining afternoon probability of precipitation in the
forecast but one can expect these to change with time as we
resolve the upper low feature. At the surface...all models
including many ensemble members also hinting at a weak low within
the deep tropical plume that surges north. Again...upper low
position will determine the fate of such a feature if it occurs at
all. Given the upper low and its influence...any such feature
would be some sort of hybrid tropical system likely to remain
weak. Again...stay tuned to the forecast next week as its likely
to change and evolve with time.
VFR conditions expected until MVFR ceilings spread into most taf sites
tonight around 08-10z Sun. Morning MVFR ceilings will likely persist
through the morning... then lift into the middle level range by the
afternoon. Still expect isolated convection this afternoon to remain
west and north of the airports today. This changes on sun...as
expect scattered coverage Sun afternoon around all taf sites. Will
continue to show prob30 thunderstorms and rain all taf sites starting at 17-18z sun.
Winds will be east-southeast through tonight... then become south-southwest by 16-18z sun.
Speeds 7kts or less.
//Atl confidence...18z update...
medium on MVFR ceilings late tonight-Sunday morning.
Medium on timing of convection Sunday.
High on remaining elements.
Preliminary point temps/pops...
Athens 66 85 67 85 / 20 40 40 60
Atlanta 69 84 69 83 / 20 40 50 60
Blairsville 62 78 62 78 / 30 60 50 60
Cartersville 66 83 66 82 / 30 60 50 60
Columbus 69 87 69 88 / 20 50 40 50
Gainesville 66 82 67 82 / 20 50 50 60
Macon 67 88 67 89 / 20 50 40 60
Rome 65 84 66 82 / 30 60 50 60
Peachtree City 66 85 66 85 / 20 40 40 60
Vidalia 69 88 69 90 / 20 40 30 50