Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
958 am EDT Thursday Oct 30 2014
a cool and dry airmass will remain across the southern appalachian
region through Friday. A strong middle level area of low pressure is
forecast to track across the region Friday night into Saturday. A
strong surface low pressure system will develop off the Carolina
coast on Saturday...then will track quickly north by Saturday night.
Circulation around the coastal low will bring a Canadian air mass to
the region for the first half of next week.
Near term /through tonight/...
at 1000 am EDT Thursday...an upper trough was over eastern North
America... with an upper ridge over the western portion of the
continent. A pair of shortwave were moving through hte base of the
trough...one over California...and the other over the Gulf states...while
additional shortwave energy was noted over thje northern plains.
At the surface...a cold front was located off the Carolina and Georgia
coasts...and extended to the northern Gulf of Mexico. A surface high
was centered over the Ohio River valley. The pressure gradient between
this high and the coastal front will keep winds up and gusty from
the north today. Model time heights show very limited low level
moisture today...and upper level moisture associated with a surface
low entering the middle MS River Valley only reaches the mountains
toward sunset. Temperatures will run around 5 degrees or so below
normal in cold advection behind the front.
Tonight...the axis of a deep l/west trough will reach the southern
Appalachians by 12z Friday. Heights will fall across the County Warning Area
tonight...however...h85 temperatures will not cool by more than two
degrees. As the trough approaches...cirrus level moisture will begin
to build across the western Carolinas. Lower moisture/clouds are
forecast to arrive across the west facing mountain slopes late
tonight...supporting increasing upslope cloud cover. Min
temperatures should range from middle to upper 30s across the mountains to
low to middle 40s east.
Short term /Friday through Saturday night/...
as of 315 am Thursday...the 00z guidance (except the cmc)...have
come into better agreement on a very potent compact vortmax/closed
500 mb low...diving south out of Canada Friday...and rounding the
base of a sharp/deep trough over the County warning forecast area on Saturday. The energy of
the system will likely induce cyclogenesis east of the Outer Banks
during the day on Saturday...with a deformation zone setting up across
central VA/NC. A surge of very cold air will also accompany the
system...with 850 mb temperatures plummeting to -6 to -8c along the Tennessee line
and generally 0 to -4c across the Piedmont. The low level winds will be
around 40-50 kts out of the northwest...as the middle-upper low swings by...with
strong cold air advection...should result in probably wind advection level winds in the mountains
(even higher at the highest peaks). Breezy to windy conditions will
then spread across the Piedmont during the day on Saturday...as the
low level system deepens near the Outer Banks...and continued northerly cold air advection
flow persists. This event is only 48-60 hours away...and yet
confidence is low on the details on sensible weather. If the 00z GFS
verifies...it/S 500 mb heights will be about 5.5 Standard deviations
below normal (or about a 50-year return interval). However...one
thing seems almost a slam dunk...and that is accumulating snow along
the Tennessee border...as a very good northwest flow snow set up should occur.
Friday should see increasing clouds...as the system approaches from
the northwest. Showers may start as early as middle afternoon in the northern
mountains...then spread across the much of the area...as deep layer relative humidity will
accompany very strong qg forcing. Models in decent agreement on snow
levels falling to the valley floors by daybreak Saturday...with
perhaps a lull in shower activity outside the usual northwest flow areas.
Highs will be 1-2 categories below normal on Friday...then drop with
onset of cold air advection...especially in the high terrain Friday night...with lows in
the 20s to middle 30s mountains...and upper 30s to lower 40s Piedmont.
Elevations above 3500 feet along the Tennessee border may see a few inches of
snow...depending on how robust the shower activity is.
Saturday...taking a GFS/European model (ecmwf) blend...it looks like there should be a
period of wrap-around rain/shower activity across the I-77
corridor...while the northwest flow snow showers continue along the Tennessee
border. Temperatures/probability of precipitation for the day are low confidence...given the
dynamic nature of the system. Snow levels may rise back up to about
3500 feet...as the compact middle level low exits to the east. Some
portions of the NC foothills/Piedmont may see a few snow flakes
early Saturday morning...but it looks like the deformation zone
precipitation should remain liquid...as the surface layer is just too warm. Even
if some snow falls outside the mountains on Saturday...any significant
accums looks highly unlikely...as soil temperatures are running middle 50s to
middle 60s ahead of the system. Charlotte has not had measurable snow
earlier than Nov 11th going back to 1878. I blended in The Straight
GFS temperatures to account for the anomalously low thicknesses and
lingering deformation zone in the east. Highs in the mountains mainly in
the 30s to lower 40s...and middle 40s east to middle 50s west in the
Saturday night...snow shower activity should subside...and guidance
agrees on probability of precipitation tapering off by 12z Sunday. Min temperatures will be
tricky...as low pressure exits to the east...allowing skies to clear and
winds to diminish...but to what extent? So depending on how good the
radiational cooling conditions can get...may a light freeze across
much of the Piedmont. The going forecast is for mainly lower to middle 30s
across the Piedmont. Mountains will be mainly in the 20s.
Long term /Sunday through Wednesday/...
as of 345 am Thursday...given the forecast concerns in the short
term...there was no time to make any changes to the extended this
morning. Dry Canadian high pressure will build in for Sunday and
Monday...with generally clear skies and well below normal temperatures. A
warming trend is expected early to middle of next week as the high
slides east and an upper ridge builds in from the west.
At 230 PM EDT Wednesday...on Saturday evening...a strong...anomalous
upper low will be located off the NC coast...while an upper ridge
will be over the plains. The upper low will weaken and fill as it
lifts NE off the New England coast on Sunday. By Monday the upper
ridge reached the Ohio and Tennessee River valleys...and on Tuesday this
ridge crosses the East Coast. An upper trough progresses from the
Great Lakes to the southern and central Appalachians by Wednesday in
the GFS...but is less progressive in the European model (ecmwf).
Dry high pressure will build southeast across our area from Saturday
night through Monday. As a result...very tight pressure gradient
will gradually relax on Saturday night and Sunday. By late
Tuesday...a light moist upslope flow on the back side of the
departing surface high will reach our area...starting in the
mountains...while a cold front moves into the Ohio and middle MS River
Valley. This cold front is expected to stall to our west by
Wednesday morning. Probability of precipitation will be allowed to ramp up late Tuesday into
Wednesday... however precipitation accumulations will be light...and
mainly limited to the mountains.
Temperatures will start out around 15 degrees or so below normal in
a cold air mass behind the departing upper low and an associated
cold front. Temperatures will moderate to near normal values by
midweek ahead of the next front.
Aviation /14z Thursday through Monday/...
at kclt and elsewhere...VFR conditions expected through the taf
period across the western Carolinas. Broad high pressure will build
across the region...resulting in light north-northeast winds and dry conditions.
Cirrus level moisture will begin to increase late this evening and
overnight as middle level trough amplifies across the region.
Outlook...another cold front approaches on Friday afternoon bringing
clouds and a chance for showers as it moves through by Friday
evening. Some restrictions are possible...especially in the
mountains where even some snow showers are expected Friday night and
NC...Winter Storm Watch from Friday evening through Saturday
evening for ncz033-048>053-058-059.