Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Peachtree City Georgia
735 am EDT Sat Apr 18 2015
Short term /today through Sunday/...
Although medium range and hi-res models are pointing to a relative
minimum for precipitation today as weak upper ridging takes place...
ample moisture remains in place over the region. Better rain has
moved off to the east...but scattered showers and areas of drizzle
persist early this morning. Will keep scattered showers and isolated
thunderstorms in the forecast during the day today. Instability is
marginal through the daytime period and severe threat is minimal.
Much better rain chances spread across tonight through Sunday as the
closed upper low over the southern rockies finally opening up and
kicking east. Instability tonight remains fairly low...but shear is
increasing...may see some marginal severe threat into the western
portion of the forecast area after midnight. Moderate instability...
shear and dynamics Sunday will up the severe threat to at least
marginal...approaching slight. Area soils are at or near saturation
and precipitable water values remain well above seasonal normals.
Forecast rainfall amounts may produce some isolated minor flooding
but coverage is expected to remain a bit limited for a Flood Watch
at this time.
Long term /Sunday night through Friday/...
Long term period begins with a lull in activity as shortwave moves
east of the area but stronger shortwave and attendant cold front
remain well west of the area as 00z Monday. Translational speed of
these features however will be very rapid to the east and expect
showers and thunderstorms to move into the northwest zones prior
to 06z. Into Sunday night...anticipate high shear environment with
50 knots 850 jet and 400 m2/s2 low level helicity. Instability will
be a little harder to come by and most unstable air will remain
outside of those areas with the highest shear. Despite
this...anticipate enough energy to allow for strong to severe
thunderstorm development across the northern tier with winds the
main concern but isolated tornadoes not out of the question
Interesting spike in instability over southeast corner of the
forecast area Monday afternoon with still enough low level winds
and shear around to warrant concern over additional strong to
severe storms. Cape values maximum out in the 2000 to as high as 2500
j/kg range from Columbus to Macon to Athens shifting quickly out
of the area by 00z. Nam12 stp values pick up in this with values
close to 5 during the peak time Monday afternoon.
Finally some drying out Monday night and especially into Tuesday
as high pressure builds in west of the area. Should be largely a
good day on Wednesday as well although the GFS is more aggressive
in bringing moisture associated with embedded shortwaves in west-northwest
flow aloft than the European model (ecmwf). Went with a compromise for now which
indicates low end chance probability of precipitation for the extreme north Wednesday
afternoon. Next major system looks to be late next Friday into
Saturday with decent agreement this far out between the GFS and
Euro and will indicate high end chance probability of precipitation for now.
Most areas IFR or lower range at this time and I expect IFR or lower
conditions to predominate through 16z. Most areas should improve to
MVFR conditions by 18z with areas of VFR conditions possible between
20z and 02z. Most areas will be back down to IFR or lower conditions
by 06z. Scattered areas of light to occasionally moderate rain this
morning should exit by 16z. Patchy areas of light rain or drizzle
are expected are expected to be more widespread through 16z.
Scattered showers and isolated thunderstorms are possible across the
area this afternoon...generally between 18z and 00z. A large area of
widespread rain and showers will spread across the area between 00z
and 06z...continuing through 12z. Winds will be northeast to
southeast through 14z Sunday...generally between 4-8kts. Winds will
become southwest after 14z Sunday.
//Atl confidence...12z update...
Medium to high.
Preliminary point temps/pops...
Athens 77 63 73 63 / 40 80 80 60
Atlanta 76 64 74 64 / 50 80 80 50
Blairsville 74 59 69 58 / 40 80 80 60
Cartersville 76 63 75 62 / 50 80 80 50
Columbus 78 66 76 64 / 50 80 80 40
Gainesville 75 62 71 62 / 40 80 80 60
Macon 78 65 76 63 / 40 80 80 50
Rome 77 63 74 62 / 50 80 80 50
Peachtree City 76 64 74 63 / 50 80 80 40
Vidalia 79 67 77 64 / 40 70 80 50