Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
302 am EDT Sat Aug 29 2015

Synopsis...
weak surface high pressure will linger over the region through the
weekend...but with moisture gradually increasing from the southwest.
Tropical system Erika is expected to move slowly northward through
the eastern Gulf of Mexico on Monday and Tuesday. The fate of Erika
will become much more uncertain from middle week Onward...but tropical
moisture could gradually reach the western Carolinas and northeast
Georgia through the upcoming work week.

&&

Near term /through tonight/...
as of 3 am...the center of weak surface high pressure will remain north
of the region today...slight ridging will likely continue across the
Carolinas and NE Georgia. Surface winds are forecast to remain light from the
east-northeast. Latest water vapor images show the center of a middle
level low over the Mississippi River Delta...with ridging over the
southeast Continental U.S.. the circulation around the low will result in
moisture across the forecast area to increase from the top-down
through tonight. I expect that cirrus and condensation trails will become
thicker through this afternoon...the middle and high clouds should
sweep north across the region tonight. Forecast soundings indicate
that the low level environment will remain rather dry through this
afternoon. The combination of dry low levels...a significant
inversion around 675 mb...mild temperatures will result in little to
no cape today. I will keep probability of precipitation limited to single digits east of the
mountains...and schc across the western mountains high temperatures are
forecast to range from 2 to 3 degrees below normal.

Tonight...the middle level low is forecast to slow lift north over the
deep south...but remaining south of I-20 through 12z sun. A plume of
moisture will likely lift across the southern appalachian region. It
appears that a few showers will develop over the eastern upslope
region first...then increase across the western mountains through
sunrise. The GFS is several hours faster than the NAM with the
arrival of measurable rainfall Sun morning. I will side with the
NAM since models have generally been trending slower with the middle
level low. I will keep probability of precipitation limited to a chance across the western
tier...and schc west of Highway 321. I will forecast low temperatures
from the low 60s across the mountains to near 70 around kand.

&&

Short term /Sunday through Monday night/...
as of 230 am EDT Saturday...a 250 mb jetlet will move northward over the
forecast area on Sunday as a middle/upper level shortwave phases back
into the northern stream flow and lifts NE across the southern Appalachians.
This improved forcing will be coincident with deeper moisture
arriving in profiles from the SW to warrant solid chance probability of precipitation for
showers in all but the northwest NC Piedmont through Sunday. The associated
cloudiness will limit instability and the diurnal temperature range
on Sunday...with some weak in situ wedging possible if showers
become a bit more widespread. Despite the clouds...briefly steeper
lapse rates aloft under the passing trough will warrant an isolated to
scattered thunder mention for Sunday afternoon...especially in southern sections.

An upper ridge will then build over the southeast Monday through
Monday night as the shortwave departs to the NE and the lingering
southern tier trough retrogrades to East Texas. Temperatures should rebound
closer to climatology for Monday...with diurnal isolated to scattered
showers/thunderstorms expected with weak instability but with developing southeast
upslope flow. Precipitation chances may be slow to wane Monday night
if any tropical moisture from Erika starts to enter the picture.

&&

Long term /Tuesday through Friday/...
as of 230 am EDT Saturday...the eventual track of Erika and the
associated moisture will provide plenty of uncertainty in the
current medium range forecast. However...the surrounding synoptic
features look a little better resolved. The upper level trough axis
near the MS River Valley will strengthen a 250 mb jetlet from the
Gulf Coast to just west of the Appalachians Tuesday through Thursday. This
upper level feature may become coincident with a low level 925 to
850 mb ridge of equivalent potential temperature stretched through
the Piedmont of the Carolinas. There is slowly growing concern that
this might provide a favorable setup for a predecessor rainfall
event...pre...somewhere over the region by middle week ahead of the
tropical system expected to move slowly northward through the
eastern Gulf of Mexico. One limiting factor may be the low level
blocking ridge over the middle Atlantic which could inhibit moisture
transport northward. The combination of any pre along with east to southeast
upslope flow into the higher terrain of the southern Appalachians
could create the potential for heavy rainfall through the period.
This might be beneficial given the current drought status...but it
could also become excessive by middle to late week.

Given the uncertainty...will simply feature above climatology probability of precipitation for the
Tuesday through Friday period as the Erika remnants meander northward. Will
give the pop trends a slightly diurnal character with an isolated to
scattered afternoon thunder mention...but will keep some coverage going through
the overnight hours. And...will shade the diurnal temperature
changes to a smaller range given the potential for more
clouds/precip. Will hold off on any severe weather potential statement mention of heavy rainfall
until the tropical moisture/pre potential is better resolved.

&&

Aviation /07z Saturday through Wednesday/...
at kclt...VFR. Latest west/v images indicated the center of a middle level
low over the northern Gulf of Mexico...lifting moisture north across
the southeast Continental U.S.. forecast soundings indicate that moisture will
increase from the top down through tonight. Cirrus and condensation trails
should increase through the day...possibly a few cumulus. A significant
inversion around 675 mb should keep the environment capped through
this afternoon. Winds will remain light through the
period...favoring a east-southeast directions with speeds 6kts or
less.

Elsewhere...Mountain Valley fog is possible at kavl around dawn.
Otherwise...terminals should observe VFR conditions through the taf
period. Models indicate that a strong 675 mb inversion and a wide
low level dewpoint depression will keep instability limited today. As
mentioned above...a middle level low will advect high level moisture
across the region through tonight....resulting in an increase in
high clouds and condensation trails. Winds will remain light through the
period...favoring a east-southeast directions with speeds 6kts
or less.

Outlook...the southeast region is forecast to remain under a plume
of Gulf and Atlantic moisture through the middle of next week. In
addition...moisture associated with the remnants of Erika could
reach the southern Appalachians during the middle week. Restrictions
are possible during periods of rain and within the mountain valleys
around dawn.

Confidence table...

07-13z 13-19z 19-01z 01-06z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 90% high 100% high 100% high 100%
khky high 100% high 100% high 100% high 100%
kgmu high 100% high 100% high 100% high 100%
kand high 100% high 100% high 100% high 100%

The percentage reflects the number of guidance members agreeing
with the scheduled taf issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link: (must be lower case)

Www.Weather.Gov/gsp/aviation

&&

Gsp watches/warnings/advisories...
Georgia...none.
NC...none.
SC...none.

&&

$$
Synopsis...hg
near term...Ned
short term...hg
long term...hg
aviation...Ned

National Weather Service Glossary of Abbreviations