Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
354 am EST Friday Dec 19 2014
weak high pressure over the region today will give way to a brief
increase in moisture along with chilly temperatures on Saturday.
Drier high pressure then returns for Sunday before an area of low
pressure brings rain chances back for Monday.
Near term /through tonight/...
at 330 am EST Friday...zonal upper level flow is in place over
the southeast this morning. Channeled vorticity over Tennessee and NC is
expected to depart to the east this morning...while and upper
ridge amplifies from the Gulf to the the Ohio River valley.The
models show a rather noise pattern of vorticity being advected
from the Gulf states toward our area this afternoon into
tonight...while a weak surface wave developed in the northwest Gulf.
A surface ridge now extending from the Great Lakes to the middle MS
River Valley will moves slowly east and over the Appalachians by
tonight. With limited cloud cover today in the wake of the departing
middle level energy...and slightly above normal heights aloft...maximum
temperatures should run near normal...despite weak cold advection in
NE surface winds. The models show only weak isentropic upglide
setting up over the surface ridge late tonight...supportive of only
light precipitation despite substantial cloud cover. Minimum
temperatures will run slightly above normal under the clouds...but
still near or below freezing in much of the mountains.
Model soundings...while cold enough for ice nucleation at the upper
end of the moist layer...have either a warm nose aloft or warm
surface temperatures by late tonight...depending on the model and
the location. Examining both a top down approach and the bourgoiun
method...and blending the NAM and GFS...would yield a light freezing
rain event in much of the NC mountains...before a transition to
sleet and snow...and then snow.
Ice accumulations would be quite light...but would still warrant a
Winter Weather Advisory should confidence be great enough. With
model soundings so marginal for ice versus snow...confidence is too
limited for a second period advisory at this time. A mention of a
wintry mix will be made in the hazardous weather outlook however.
Short term /Saturday through Sunday night/...
as of 320 am EST Friday...the latest 00z short range models have
trended drier with the passage of upper shortwave on Sat.
Therefore...have made reasonable changes to the probability of precipitation accordingly.
A weak southern stream upper shortwave is forecast to pass overhead
on Sat...shearing out as it pushes off the Carolinas coast by Sat
night. 500 mb heights will rise over the southeast by sun as weak upper ridge
builds in from the west.
Surface pattern and sensible weather...a weak cad will set up over
the County Warning Area on Sat as high pressure centered over the Ohio Valley shifts
east. Meanwhile...a cyclogenesis will occur off the SC coast and
this will aid in moisture advection to our southeast zones of the County Warning Area. The
latest NAM and GFS have backed off on the quantitative precipitation forecast response...especially
over the SC Piedmont as deeper moisture will remain south while
better upper forcing passes north of the area. This upper forcing
will provide better lift over the northern half of the County Warning Area...with
models showing light quantitative precipitation forecast response less than a tenth of an inch.
Thermal profile is supportive of a brief period of snow over the NC
mountains/valleys on Sat morning...transitioning to rain by late Sat
morning. After coordinating with neighboring offices...have
mentioned low end likely probability of precipitation over the NC mountains and portions of the
NC Piedmont with 40/50s elsewhere on Sat morning...ramping down into
the chance range by Sat 18z. Conditions should dry out by Sat
evening as the shortwave pushes offshore and moisture associated
with the low off the Carolinas coast pulls farther NE.
Sun...NE flow will continue in weak cad regime over the region as
1028 mb high becomes centered over the New England. Have gone with
mostly dry forecast (slight chance probability of precipitation at best) under the weak
ridging aloft during the day on sun...with probability of precipitation ramping up from west
to east into the solid chance/likley range by late Sun night as
moist isentropic lift increases and better upper diffluent southwesterly flow
sets up. No frozen precipitation is expected on Sun night given warmer
thermalprofile. Temperatures on Sat will run 5-7 degrees below
normal...with a few degrees below normal on sun.
Long term /Monday through Thursday/...
as of 230 am EST Friday...the latest 00z global models are in
good agreement with regard to the overall synoptic pattern and
thermal through the medium range period.
Southwesterly upper diffluent flow will set up by Monday as a low
amplitude upper ridge axis pushes off the East Coast while an upper
trough digs into the upper mid-west. A low in this upper trough will
close off as it digs into the Mississippi Valley through Tuesday
night...becoming negatively tiled as it swings across the southeast
by Wednesday. Northwesterly flow aloft will set up by Thursday as the upper trough
pushes off the Atlantic coast.
At the surface...1030mb high centered over the NE will be
responsible for cad over the County Warning Area on Monday...with a cyclogenesis
occurring off the Carolinas coast at the same time. Given the
combination of strong moist isentropic lift...upper diffluent flow
and good moisture advection in the wake of the cyclogenesis...precipitation
will be ongoing across the County Warning Area. Despite the cad with decent warm
nose aloft...surface temperatures will run above freezing...suggesting all
liquid precipitation att. With decent quantitative precipitation forecast response (0.25-0.75")per 00z GFS
and European model (ecmwf)...have raised probability of precipitation into the low likely range during the
day on Monday...ramping down into the low end chance range through Monday
night as the low off the NC coast pulls farther NE away and the cad
Tuesday...models agree that in situ cad and moist isentropic
lift regime will support low end chance for precipitation through the
afternoon before probability of precipitation ramp up from west to east into the likely
range as a surface low and associated cold front approach from the
west. The low is expected to lift NE across the Tennessee Valley and into
Kentucky Tuesday night into Wednesday morning...pushing a strong cold front through
the forecast area. Although there will be strong low level wind
shear/upper forcing...severe weather chnaces look minimal given zero
SBCAPE. The main issue will be strong cold air advection flow behind the
front...supporting rain showers transitioning to snow by Tuesday night
over the NC mountains with wrap around moisture and prevailing cold air advection northwest
flow...snow will be ongoing over the Tennessee border counties through Wednesday
night. Elsewhere...conditions should dry out by Wednesday afternoon as the
front and associated moisture move off to the east and dry high
pressure builds in from the west. Temperatures will be 5-7 degrees below
normal on Monday...near normal Tuesday...and 2-5 degrees below normal
Aviation /09z Friday through Tuesday/...
at kclt...VFR. Model time heights suggest that low VFR clouds will
erode around daybreak as low level flow veers from north to NE and dry
high pressure builds into the area. High level cirrus will then be
the dominate cloud group until Thursday evening...when low level
moisture spreads in from the SW ahead of a Gulf of Mexico surface
low. Low VFR ceilings and precipitation will probably not arrive until
after this forecast package however.
Elsewhere...VFR. The models suggest that low VFR clouds will diminish
around daybreak as dry high pressure arrives and winds veer from north
to NE. Cirrus will then prevail until Thursday evening when low level
moisture arrives in advance of low pressure in the Gulf of Mexico.
Low VFR ceilings are expected late Thursday evening at kavl and the SC
sites...with precipitation as well. Rainfall could mix with snow at
kavl at that time.
Outlook...a more organized system will approach from the west early
Saturday bringing elevated precipitation/restriction chances. Some wintry
precipitation is possible across the NC mountains and foothills affecting
kavl/khky Saturday morning. Wintry precipitation is not expected at this
time for any other taf sites. A wetter system will track into the
area for Monday likely bringing some restrictions.
09-15z 15-21z 21-03z 03-06z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 100% 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)