Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
203 PM EDT Sat Mar 15 2014
high pressure will weaken and shift east of the area
tonight...before a moist low pressure system develops to the west
overnight and slowly moves up and off the Carolina coastline through
Tuesday. A weak cold front is expected to cross the area Wednesday
night...with dry high pressure building back in Thursday through
Near term /through tonight/...
as of 200 PM...not many significant changes to the forecast.
Temperatures/dewpoints look to be on track for this afternoon with only cirrus seen
until late afternoon when altocu are expected to arrive from the south. 12z
models still support a peak in precipitation activity in the early afternoon
tomorrow. Did back off evening probability of precipitation with guidance not producing quantitative precipitation forecast
response until after midnight...and with isentropic upglide not
moistening up until that time. Favored raw model temperatures still are
indicative of all rain during this period. Full details shortly.
As of 145 am...tonight...a southern stream low pressure system will develop
in vicinity of the lower MS valley...as a deep upper trough digs into the Southern Plains.
The cyclone should take on a classic comma shape in precipitation
distribution and on Sat imagery overnight...with the surface center over
Arkansas by 12 Sunday. Models in good agreement on the leading edge
of isent lift and moisture entering the SW quarter of the County warning forecast area by
daybreak Sunday. I did tweak probability of precipitation with latest model
consensus...which delays the probability of precipitation in the northwest NC Piedmont...thus
producing a tighter pop gradient from NE to SW. In any case...areas
S and west of I-26 will likely see precipitation by around 12z...while areas
to the east just a little after. Temperatures will be held up initially by
the low level warm air advection and increasing clouds...but cold air damming will begin
as precipitation develops...so min temperatures are a bit tricky early Sunday.
Taking a model blend keeps temperatures in the 40s across the northern half and
upper 40s to lower 50s southern half.
Short term /Sunday through Tuesday/...
as of 245 am EDT Sat...active weather will affect the southeast U.S. For
the short range period from Sunday to Tuesday. The models feature an
initially in phase trough axis from the Midwest to Texas on Sunday.
However...the southern stream energy is expected to either cut off or
amplify over Texas/la Sunday night and then migrate to the southeast coast
through early Tuesday. Ahead of this system...Sunday definitely looks
like a wash out. Improving upper level divergence will enhance
robust isentropic lift along an 850 mb baroclinic zone stretched
across the region. Meanwhile...a surface wave initially over Arkansas early
Sunday will slide eastward along the developing wedge boundary through
the forecast area. Profiles will be warm enough throughout to
support all rain and quantitative precipitation forecast should be on the order of an inch in most
locations Sunday before the warm advection lift wanes from the S to
north through Sunday evening. A weak warm sector could be uncovered in
the far southern Piedmont/southern mountains late Sunday...where schc thunder is
possible. Unless there are heaviest shower rates in flood prone
areas...this quantitative precipitation forecast should not pose much of a Hydro threat.
As the lead surface wave along the boundary transitions to the coastal
waters...a second surface wave may develop over the northern Gulf Sun night
and then also move to the southeast coastal waters by Monday night. This will
have the effect of pulling colder profiles southward in the cad regime.
Precipitation coverage may bounce back through Monday with developing
easterly upslope focusing precipitation along the eastern slopes of the
mountains during the day...and then the best deep layer q vector
convergence with the southern tier upper wave swinging through late Monday
through Monday night. There is some potential for a wintry mix to flirt
with the northern tier during the day on Monday...but will keep surface temperatures
just above freezing most areas until later in the evening when the
southward surge of cooler near surface air looks more likely. It is
possible that a weak deformation zone could keep precipitation going
into Tuesday morning...and this would probably be the period of coolest
surface temperatures. The current severe weather potential statement mention of a wintry mix for the
northern tier looks warranted...and it might be expanded slightly across
western NC for late Monday evening through Tuesday morning. Temperatures have trended
cooler throughout the forecast.
Long term /Tuesday night through Saturday/...
as of 1030 am EDT Sunday...the current forecast was based on a blend
of the latest ECMWF/GFS/wpc. A cold air damming wedge will
persist across the area at least through Wednesday morning before eroding
by Wednesday afternoon. With persistent moist upglide flow aloft on top of
the surface wedge...latest models show very light quantitative precipitation forecast reposes across
much of the County Warning Area. Hence...have mentioned slight chance probability of precipitation through
Wednesday morning. Morning low temperatures on Wednesday appear to be too warm for
frozen precipitation at this time. By Wednesday night...an upper trough will
approach from the central Continental U.S....pushing a weak cold front into the
Appalachians in its wake. The front is expected to cross the region
through Thursday morning...with some light quantitative precipitation forecast response mainly along the
Tennessee border line per latest models. A dry high pressure will then
builds in from the west late Thursday and lingers through Friday night for
dry conditions through the period. By Sat...probability of precipitation ramp up from west to
east as another cold front approaches from the west. The old European model (ecmwf)
suggests it will be a dry frontal passage while the GFS is much wetter. Due to
the uncertainty...have mentioned slight chance probability of precipitation Sat/Sat night.
Temperatures will be near normal on Wednesday...with a few degrees above
normal Thursday through Sat.
Aviation /18z Saturday through Thursday/...
at kclt...increasing cirrus this afternoon/evening expected as upper energy
ejects out of shortwave apching from the lower Mississippi Valley.
High pressure will concurrently shift across the Great Lakes setting
the stage for cold air damming by early Sunday...later bringing low
ceilings and precipitation to the forecast area. Midlevel ceilings will arrive by daybreak
with low -ra chances. Ceilings then begin to crash as precipitation intensifies
and wedge strengthens through the morning hours. Moderately high confidence
on IFR ceilings by noon...with visibility no better than MVFR. Temperatures will
remain sufficiently warm to preclude any wintry mix...but wedge
front remains far enough S and west to also keep ts chances minimal. A
west-northwest near-crosswind this afternoon may back to west-southwest at times...with southeasterly
winds setting up overnight then becoming northeasterly Sunday within The
Elsewhere...rain chances increase from SW to NE beginning around
midnight with nearly certain -ra at all sites by late morning. Low VFR
ceilings may develop near the Blue Ridge due to upslope southeasterly flow
overnight. MVFR ceilings likely to arrive by daybreak in the upper
Savannah River valley...spreading across all the sites by middle morning
followed by IFR...with LIFR eventually possible. Visibility likely to
decrease to IFR as precipitation rates increase beginning around midday. All
precipitation expected as rain this period. Southeasterly winds overnight will veer to
northeasterly except at kavl which will remain southeasterly being near The Wedge
Outlook...cold air damming persists through Monday night. Widespread
restrictions are expected sun through Tuesday...with wintry weather possible
over the northern foothills/Piedmont Monday and Monday night. Widespread VFR
conditions may return Tuesday night or Wednesday.
18-24z 00-06z 06-12z 12-18z
kclt high 100% high 100% high 100% high 86%
kgsp high 100% high 100% high 100% high 84%
kavl high 100% high 100% high 98% high 94%
khky high 100% high 100% high 100% medium 78%
kgmu high 100% high 100% high 100% high 85%
kand high 100% high 100% high 100% high 85%
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)
another warm and dry day expected across the region. There will be
some rebound in dewpoints...as low level flow remains generally out of the
SW. Also winds should be fairly light...well under red flag. The
current relative humidity forecast has most areas east of the mountains dropping into the
25-30% range...with the NC foothills into the 20-25% range. Given
the marginal relative humidity conditions and the light winds...we do not plan to
issue any fire weather statements/hazards early this morning. But the day
shift may want to coordinate with land managers...if fire weather
conditions are trending more critical.