Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
308 am EDT Monday Apr 27 2015
a large cool and dry air mass will dominate the weather across the
southern appalachian region through Tuesday. A well organized low
pressure system will lift across the deep south...tracking across
the Atlantic coast during the middle week. Another Canadian air mass
will settle back over the region during the late week and will
remain into the weekend.
Near term /through tonight/...
as of 230 am Monday...the near term is overall fairly quiet. Large
upper low spinning off New England keeps US in northwest flow in the
wake of the cold front that pushed through yesterday. Another
shortwave spins around the western periphery of the low...pushing
through the southern Appalachians tonight. Surface high pressure
locked across the Great Lakes under the confluent flow tonight will
continue to ridge toward the southeast...keeping the sensible
weather dry through the near term. Next weather-maker is a cutoff
low over the Southern Plains that will bring a surface low from the
Gulf into the lower Mississippi Valley and southeast...but with no
impacts until the short term period. The few clouds out there at afd
time will continue to dissipate through the morning hours...though
bufr soundings do indicate enough relative humidity at the top of the mixed layer
that we could see some fair-weather cumulus developing this
The primary concern through the near-term will be temperatures
overnight tonight...which are prognosticated to be 5 or so degrees below
normal for most of the upstate/Piedmont/foothills. The problem might
be in the mountains with deep layer /albeit weak/ cold air advection through the
column. This combined with the decreasing surface pressure gradient
as the high continues to ridge in...and winds will drop off tonight.
Based on temperatures...dewpoint depressions...and winds...expect to see
some frost developing just before sunrise at the higher elevations
of the North Carolina mountains. However...areal extent of currently
forecast frost coverage is not enough to warrant an advisory at this
time. Overnight temperatures/dewpoints/winds will have to be closely
reevaluated through the day with updated guidance in case an
advisory becomes warranted.
Short term /Tuesday through Thursday/...
as of 230 am Monday...short range models agree that Tuesday will see
dry conditions under short wave ridging. Forecast soundings indicate that
moisture will increase steadily from the top-down during the
daylight hours...deepest across the SW. A consensus of the
temperature guidance support high temperatures ranging from the middle
60s within the mountain valleys to the upper 60s east...or 5 to 7 degrees
On Wednesday...short range models remain in general agreement...all
mainstream solutions indicate that the surface low will lift north
across the Florida Panhandle early Wednesday...reaching the Carolina
coast by Wednesday evening. At the middle levels...a deep trough will
slide east...reaching the southern Appalachians by Wednesday
afternoon. A wide field of moderate q-vector convergence is forecast
to sweep across the Carolinas Wednesday afternoon. At the low
levels...the low circulation will result in east winds at
850 mb...providing upslope flow during the morning hours. The
combination of deep moisture...deep synoptic scale forcing...and
low level upslope flow should result in rapid increase in rainfall
coverage on Wednesday. Temperatures may begin the day around
50...rising to around 60 during the afternoon. The expanding
rainfall and thick cloud cover should limit daytime temperatures to
nearly 20 degrees below normal.
Thursday...the European model (ecmwf) indicate that a broad middle level trough will swing
across the region...the GFS indicates that the core of a 540 closed
low will track over the NC mountains by 0z Friday. I will use a blend of the
two solutions...which will yield more clouds and precipitation than the
inherited forecast. I will increase sky to mostly cloudy through
middle day...then indicate cloud cover eroding within the downslope
areas Thursday afternoon. In addition...I will keep chance probability of precipitation through
the daylight hours. Light rain to rain showers should occur both
Wednesday and Thursday...model instability is very weak.
Temperatures are forecast to warm above Wednesday/S values by 5 to 8
degrees. Quantitative precipitation forecast on Thursday should remain less than a quarter of an
Long term /Thursday night through Sunday/...
as of 300 am Monday...through the extended...the coastal surface low
will likely continue to track northeast and intensify. Dry Canadian
high pressure will sweep across the southern appalachian region
during the late week and remain into the weekend. At the middle
levels...a broad trough will slowly give way to rising heights from
the west. Friday-Sunday should see moderating
temperatures...beginning below normal and ending with temperatures
very close to normals. Overall...conditions will remain dry with
partly cloudy sky cover.
Aviation /07z Monday through Friday/...
at kclt...expect VFR through the period. Clouds at taf time
range from scattered to broken 6-8kft and these will clear out through the
morning as the front continues pushing south. Cannot rule out some
brief few-scattered 5-6kft cumulus this afternoon. Otherwise just high clouds
through the period. Only caveat to prevailing VFR conditions remains
that some guidance...namely the RUC...does still want to develop IFR
to LIFR fog across the NC Piedmont this morning including at kclt.
Based on dewpoint depressions and condensation pressure deficit...
not inclined to believe this outlier solution but cannot rule out
some patchy MVFR fog. Northwest winds generally through the period but
picking up to between 5-10kt this afternoon...with potential for a
brief very low-end gusts.
Elsewhere...very similar trend to clt. Some MVFR ceilings still hanging
out across the upstate...but ceilings should lift and begin to scatter
in the next couple of hours. Generally north-northwest to briefly north-northeast winds
everywhere between 5-10kt this afternoon...but will probably see
slightly stronger winds at kavl...10kt or just above with gusting
potential to 20kt or so. Otherwise just high clouds through the
Outlook...dry high pressure will continue through Tuesday. Another
low pressure system will track across the deep south Wednesday and
Thursday...with ceiling and visibility restrictions possible.
07-13z 13-19z 19-01z 01-06z
kclt high 100% high 100% high 100% high 100%
kgsp high 91% high 100% high 100% high 100%
kavl high 94% 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)