Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
655 am EDT sun Apr 26 2015
an area of low pressure will move off the Carolina coast this
morning...with a trailing cold front pushing through the forecast
area. Cool and dry high pressure drops down from central Canada
later today and will remain the dominant feature for the beginning
of the work week. Another area of low pressure will move east from
Texas toward the middle of the week and bring more rain Wednesday
and Thursday...with a drying trend for the weekend.
Near term /through tonight/...
as of 650 am...The Wedge boundary has slide SW along I-85 over the
past the couple hours. The placement of the boundary will likely
play a key role in surface convergence and convection early this
afternoon. Therefore...I will update the forecast to shift chance probability of precipitation
SW...displaced from the recent runs of the cams. Otherwise...I will
update sky and temperatures to align with the latest observations.
As of 330 am...satellite and radar images continues to show the old
mesoscale convective system ashing out across the western Carolinas. Latest run of the cams
indicate the coverage will spread east...gradually fading to chance to
schc. Is was interesting to see a splitting cell across Chesterfield
Colorado at 730z. At the surface...The Wedge front runs from the northern tip
of Lake Wateree...west across the gsp metropolitan area...north along the
east facing slopes across the NC foothills. North of the
boundary...low clouds and fog will persist through sunrise. Low
clouds and fog will dissipate from the south to north during the
early daylight hours. By middle day...the center of low pressure is
expected to pass across the upstate of SC...tracking east. A weak
backdoor cold front will sweep south across the region during the
afternoon. As the low passes...the environment between I-85 to I-77
could support scattered thunderstorms and rain during the middle to late afternoon...I will
forecast chance probability of precipitation. High temperatures are forecast to range from
around 80 across the upper Savannah River valley to the middle 60s
across I-40 to the mountains
Tonight...in the wake of the cold front...high pressure will build
across the southern appalachian region. Conditions across the region
should become clear to mostly clear...winds below 5 kts...with
dewpoint depression greater than 5 degrees. Low temperatures should
fall close to normal...from the low 40s within the mountain valleys to
low 50s across the upper Savannah River valley.
Short term /Monday through Tuesday night/...
as of 300 am Sunday...short term begins with large upper closed low
spinning over New England and southeast Canada...with a reinforcing
shortwave pushing around the western periphery of the trough at the
beginning of the period. Surface high pressure centered over south
central Canada will be ridging into the southeast. Expect a clearing
trend during the day on Monday in the wake of the dry upper trough
axis pushing through with highs below normal under weak deep layer
cold air advection. Continued below-normal temperatures Monday night and will have
to keep an eye on lows at the higher elevations in case frost might
be a concern...especially as winds die off toward sunrise Tuesday.
The cool temperatures continue on Tuesday even with midlevel ridging
beginning to build in across the area.
A cutoff low over the Southern Plains will induce cyclogenesis
across the northern Gulf Monday night and early Tuesday...spreading
moisture into the southeast Tuesday afternoon and evening. Have
continued trend of ramping up probability of precipitation late in the day on Tuesday as
isentropic upglide increases across the area. European model (ecmwf) is beginning to
trend a little farther north with the surface low...more into
southern and even central Georgia rather than remaining along the
Florida Panhandle...which might mean an increase in quantitative precipitation forecast. Overall
mass fields not too dissimilar but enough differences in the
details...especially timing and placement of quantitative precipitation forecast...to continue
blanketing the area with high chance to low likely probability of precipitation at best as
we transition into the extended.
Long term /Wednesday through Saturday/...
as of 315 am Sunday...chance to likely probability of precipitation /especially for southern
zones/ begin the period as the aforementioned surface low works its
way east across southern Georgia. With widespread cloudiness and
rain...not to mention decreasing thicknesses plus just being on the
cold side of the surface low...we continue to advertise highs on
Wednesday well below seasonal normals...in some locations 10 degrees
Model inconsistencies and run-to-run discontinuity continue with
this system as the 00z guidance has once again switched...with the
GFS exiting the precipitation from the area faster than the European model (ecmwf).
Continued the compromise trend...pulling back on probability of precipitation during the day
on Thursday. European model (ecmwf) wants to bring another shortwave around the base
of the elongated trough Thursday night into Friday but have not
reflected this in the grids at this time...preferring instead the
drier solutions...however as we move forward this may need to be
reevaluated. We should see a drying trend into the weekend with
temperatures trending more towards climatology but confidence in any
one solution not particularly high at this point.
Aviation /12z Sunday through Thursday/...
at kclt and elsewhere...latest surface analysis and satellite images
indicated that The Wedge boundary was curved from Lake
Wateree...northeast kand...then north along the east facing slopes
across the NC foothills. Areas north of the boundary were observing
LIFR to vlifr ceilings with pockets of LIFR visible.
Kclt...khky...kgsp...and kgmu will remain within the corridor of low
ceilings and visible through the early daylight hours...then conditions
should improve to MVFR by middle to late morning...then VFR by middle day.
Kavl and kand will remain removed from the wedged air
mass...combined with winds above 5 kts...conditions are generally
expected to remain MVFR or higher this morning. The rest of the day
should feature broken low to middle clouds...thicker and more persistent
across khky and kclt. All terminals should see winds turn from the
north today following a cold front.
Outlook...a cold front will sweep across the region on Sunday then
followed by dry high pressure through Tuesday. Another low pressure
system may track across the deep south Wednesday and Thursday.
11-17z 17-23z 23-05z 05-06z
kclt medium 78% high 81% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 100% high 100% high 100% high 100%
khky medium 78% medium 75% 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)