Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Peachtree City Georgia
155 am EDT Sat Apr 25 2015
Previous discussion... /issued 742 PM EDT Friday Apr 24 2015/
.Significant severe storms and tornadoes possible Sat
current forecast looks on track with mainly some patches of light
rain possible during the evening... then a better chance for showers
after midnight. No thunder is expected tonight...but could hear some
embedded rumbles of thunder with the heavier showers expected after
6-7 am in the morning.
Short term /tonight through Saturday night/...
starting to hone in on severe event Sat. Potential for elevated
strong or severe storms overnight or Sat morning appear very limited.
Dry low level air and overnight cooling will greatly limit
instability. As S to southeast flow above surface increase and slight cooling
aloft...instability will rapidly *increase* in the afternoon. Forecast
afternoon lapse rates fairly impressive.
12z model guidance still indicating isolated to scattered
significant severe storms Sat 3pm-midnight behind more widespread
area of rain showers and elevated thunderstorms and rain. Prognosticated stp values the highest we
can remember in the last 2-3 years and has worked well with
previous high end events. 12z GFS has stp 4-6 from southern Alabama to northwest
Georgia to eastern Tennessee at 00z sun. Very impressive deep shear with this
event and even though directional shear not that great...0-1km bulk
shear also impressive at 30-40kts. Thompson et Alabama 2003 had shown
previously that 0-1km bulk shear just as helpful as 0-1km srh when
predicting tornadic supercells. If even more favorable shear
develops on storm scale and boundary with wedge front interacts
with any supercell...moderate to long track tornadoes cant be
Although not one of the top 15 cips analogs /which majority were
associated with severe reports including tornadoes/...this event
reminds ME of 15 March 2008 when warm front help focus period of
prolific supercells over north Georgia and several tornadoes...most
notable was Polk/Bartow County EF3. Large scale models show scattered
convection while hires window and both of our arw local WRF models
all show vigorous convection initiating behind area of rain showers and
elevated thunderstorms and rain around 22z.
Heavy rain likely Sat morning as well but based on latest guidance
only 0.75 to 1.5 inches expected through sun...which even with recent
rainfall should not result in widespread flooding.
As front sags south into middle and south Georgia Sat night and
sun...upper forcing lifts will away. Models very reticent on
producing any quantitative precipitation forecast Sunday...but would not be surprised to see thin
line of isolated thunderstorms and rain Sunday afternoon. Should be south of County Warning Area but
added small probability of precipitation for far southern areas Sun afternoon.
Long term /Sunday through Friday/...
made a few tweaks to the timing of the precipitation for the
middle week system and have increased probability of precipitation where the models are showing
consistency. See previous discussion below...
Long term /Saturday night through Thursday/...
/issued at 324 am EDT Friday Apr 24 2015/
weak surface ridging will allow for a short dry period. Rain returns
for Tuesday night into Wednesday as a shortwave tracks across the
southeast with the surface low sliding south of the County Warning Area along the Gulf
Coast. Slight differences between the GFS and European model (ecmwf) with how
quickly this system exits with the GFS on the faster slide
clearing things out by Thursday...while the European model (ecmwf) clears out the
system Thursday night.
relative humidity values will approach values of less than 25 percent in the far
eastern County warning forecast area later this afternoon. At this time...the duration of
low humidities will not meet criteria at this time for a Fire
Mesoscale convective system is expected to impact the County warning forecast area later this morning...impacting
ceilings and visibilities this morning. A second round of convection is
possible later in the afternoon. Have handled this with a prob30
group. Will probably have to refine timing in later versions of
the taf. Winds will flirt with due south through the morning...but
should switch to the SW side by early afternoon.
//Atl confidence...06z update...
low to medium confidence on late afternoon convection timing.
Otherwise...medium to high confidence remaining elements.
Preliminary point temps/pops...
Athens 70 63 82 55 / 90 50 20 5
Atlanta 76 65 80 56 / 100 30 20 10
Blairsville 67 59 72 46 / 100 50 20 10
Cartersville 76 63 78 51 / 100 30 20 10
Columbus 80 67 85 60 / 90 30 30 5
Gainesville 68 62 77 53 / 100 40 20 5
Macon 79 67 85 59 / 90 50 30 5
Rome 77 62 78 50 / 100 30 20 10
Peachtree City 78 64 82 55 / 100 30 20 5
Vidalia 82 70 86 62 / 90 50 30 5