Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Peachtree City Georgia
740 am EDT Friday Aug 29 2014
Previous discussion... /issued 400 am EDT Friday Aug 29 2014/
Short term /today through Saturday/...
Guidance in decent agreement with at least the main synoptic
features in the short term and continues to support the abnormally
strong upper ridge shifting east with the return of Gulf moisture by
Saturday. Today through most of tonight looks to be dry with a
gradual increase in cloud coverage from the SW. The upper shortwave
trough prognosticated to approach the western Tennessee and Ohio valleys by
Saturday looks to dampen and lift northeastward keeping most of the
wave energy northwest of the state. There does not look to be much in the
way of positive vorticity advection for enhanced precipitation chances and likely more of a
diurnally driven or orographically enhanced component to convection.
Guidance is consistent with abnormally high precipitable waters advecting into the
County Warning Area by Saturday afternoon near 2.0 or slightly higher which is
between the 75th-99th climatology percentile. While activity should have
high precipitation efficiency...there is not high confidence in
shower/storm coverage being very great. Hi-res solutions suggest
fairly limited coverage and more of a Middle-Range chance probability of precipitation by 18z
Saturday. Given the enhanced moisture field and the chance for a
weak perturbation traveling along the trough to the northwest...have kept
some likely probability of precipitation in the far northwest but chance elsewhere for Saturday
afternoon. MLCAPES look to be greatest SW near 1500 j/kg so not
anticipating much intensity to development. Quantitative precipitation forecast varies quite a bit
among the guidance members so went with general blend and about a
quarter to third of an inch possible Saturday /slightly lower than
before/ though of course higher values possible in local areas given
the aforementioned high precipitable waters .
For temperatures...general blend of model solutions have verified well
recently which gives highs today about 5 degrees above normal...then
shift to near normal for Saturday with more typical moist airmass.
Long term /Saturday night through Thursday/...
Models in a little better agreement as the long term period begins
with vorticity maximum lingering over the Tennessee Valley as upper ridge
holds over the southern part of the state. This vorticity maximum is more
strung out providing little lift on the European model (ecmwf) but more potent with
distinct trough noted on latest GFS. Regardless...precipitable
water values will be high...on the order of 1.8 to 2.2 inches
which should be sufficient for likely probability of precipitation to the north where
shortwave energy resides and Middle Range chance to the south. There
is a distinct area of middle level dry air noted on the European model (ecmwf) which
could lead to reduced probability of precipitation for the northern half of the forecast
area despite aforementioned energy but will need a few more runs
to confirm before getting fully onboard with this solution.
Into early and middle week...the Euro is definitely more aggressive in
build middle and upper ridge back over the area and limiting probability of precipitation.
For now have gone with a blend of this drier solution and the
slightly wetter GFS which suppresses ridge to the south while
strung out vorticity maximum provides focus for convection. This yields
generally low to Middle Range chance probability of precipitation for the remainder of the
extended portion. With upper ridge in place...should see some
increase in maximum temperatures although guidance not especially bullish on
this aspect. Will need to monitor to see if they trend upward with
future runs but would not be surprised to see some middle 90s by Tuesday
VFR conditions expected through period. Initial calm to light vrb
winds becoming southerly with some possible waivering from south-southwest to
south-southeast today under 7 kts. Looking like katl will have south-southwest before 19z
then shift south-southeast and mostly stay on east side through the period.
Some few to scattered coverage near 6-7 kft along with
cirrus...otherwise increased coverage near end of forecast period
along with increasing shower/storm chance. Have started prob30 for
-shra after 15z Saturday for katl but greater chance just beyond
//Atl confidence...12z update...
medium on wind shift.
High all else.
Preliminary point temps/pops...
Athens 95 67 92 70 / 10 10 40 20
Atlanta 92 72 89 72 / 10 10 50 20
Blairsville 87 62 84 65 / 10 10 50 30
Cartersville 93 67 90 69 / 10 10 50 20
Columbus 93 73 92 74 / 10 10 50 10
Gainesville 91 69 88 71 / 10 10 50 30
Macon 95 70 93 71 / 10 10 40 30
Rome 94 66 91 70 / 10 20 70 20
Peachtree City 92 67 89 70 / 10 10 50 20
Vidalia 95 72 93 73 / 10 10 30 10