Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Peachtree City Georgia
332 am EDT Sat Aug 29 2015
Short term /today through Sunday/...
Upper low pressure along the central Gulf Coast bringing deep moisture
north across the area today as a short wave in the upper flow enhances
shower chances. Surface high pressure nosing down the eastern Appalachians
will aid in a sharp gradient of precipitation chances over the far eastern
counties...making it a difficult pop forecast. Forecast instability
rather low so thunderstorms chances are rather minimal with the greatest
chances over the western and southern areas.
The upper low is forecast to move to the southern Appalachians for Sunday.
This will shift the greatest probability of precipitation to the northern and eastern areas.
Just a little more instability forecast as the upper low moves to the north-northeast.
Forecast high temperatures running 5-10 degrees below normal today and
4-8 degrees below normal for Sunday.
Overall confidence is medium.
Long term /Sunday night through Friday/...
Extended portion continues to be fraught with uncertainty due to
the precarious status of Tropical Storm Erika and the resultant
impact on model guidance solutions. Latest official forecast from
the hurricane center has Erika tracking just to the west of the
Florida Peninsula and winding up as a depression over south
Georgia Wednesday evening. This solution continues to have support from
from the latest round of operational models including the GFS and
to a large extent the previous run of the European model (ecmwf). The big questions
then become...does Erika actually follow this path or does the
trend westward continue and if it does follow this path...what
happens after it makes its appearance over south Georgia.
For this forecast...will of course go with the NHC forecast
through Wednesday evening and then lean heavily on the GFS for pop
distribution but subdue the really high values based on the
continued uncertainty. So for the forecast...Monday could be the
first day that we really see the tropical moisture make it into
the southern sections of the forecast area. Sharp gradient though
to deep moisture with only 1.30 inch precipitable water values
from the Atlanta metropolitan northward. Will go around likely from csg
to mcn and points southward for this time period.
Main slug of tropical moisture looks to slowly creep northward
through middle week with 2 inch precipitable water values encroaching upon Atlanta
by late Wednesday and tropical moisture engulfing most all of the area
by Friday. Again...in normal circumstances with the values
present...likely probability of precipitation would be a given if not higher. But with the
amount of uncertainty present and the distinct possibility that
probability of precipitation could actually be below climatology if Erika tracks further
west...will limit probability of precipitation to high end chance for now. Likewise...will
not go as cool on afternoon highs as models suggest on the chance
that we see a drier week than the GFS is advertising.
00z update... initial VFR ceilings should lower to IFR between 06z
and 09z tonight then gradually improve to MVFR after 14z...though
may struggle to get into VFR tomorrow afternoon as rain showers chances
increase. MVFR visibilities possible in early morning and with any
precipitation. Held off on thunderstorms and rain mention Saturday due to uncertainty in
best timing and how late ceilings have been able to scatter the last
several days. Winds staying east-southeast generally 7-10 kts near katl with
some lower magnitudes at other sites.
//Atl confidence...00z update...
medium to low on thunderstorms and rain chance/timing.
High on all else.
Preliminary point temps/pops...
Athens 86 70 84 69 / 50 40 40 20
Atlanta 85 70 84 70 / 60 50 40 20
Blairsville 81 64 78 63 / 50 30 50 20
Cartersville 85 68 84 67 / 60 50 40 20
Columbus 86 71 87 71 / 70 60 40 20
Gainesville 83 70 81 68 / 50 40 40 20
Macon 87 71 87 70 / 60 60 50 30
Rome 85 69 84 67 / 60 50 50 20
Peachtree City 85 69 84 68 / 60 50 40 20
Vidalia 89 73 88 72 / 60 60 60 30