Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
753 PM EDT Wednesday Mar 12 2014
cool high pressure will build in tonight as a cold front pushes off
to the east. The airmass will slowly moderate warmer into
Saturday...before a broad and colder upper level trough builds in
Sunday through Monday.
Near term /through Thursday/...
as of 745 PM EDT...earlier convection has moved east into central NC
as the frontal boundary continues to push through the region. Cold
advection across the western zones is well underway with some mountain
sites reporting temperatures in the lower to middle 20s. Light snow
showers are indicated on kmrx radar with a few ground reports coming
in as previously forecast nwfs initiates. Winds continue to gust in
the 30-40mph range across the lower elevations with some of the
higher peaks gusting to near 60mph...especially the northern
mountains of western NC. Blended latest near term guidance in with
previous forecast which did not yield any significant changes to
snow totals overnight along the high terrain. Otherwise...made
slight tweaks to temperature trends to reflect latest observations
and left the remainder of the forecast as is.
As of 230 PM EDT...a narrow line of showers is moving across the NC
mountains at this time. The airmass has become weakly unstable ahead of the
line...and wind shear is strong with effective bulk shear values of
45 to 50 kts. The morning soundings were rather capped down at ffc
which explains why the line is struggling south of the NC line. As
the line emerges into the foothills and Piedmont it should
strengthen...mainly across the NC zones where the airmass will be a
little more unstable. Considering the strong gradient winds and
deepening mixing...areas of damaging straight line winds still look
to be the main threat. However...if the line can strengthen
sufficiently to develop deeper convection and bowing and lewping
segments...a qlcs tornado cannot be ruled out somewhere along the
Very windy conditions are expected across the region through the
evening hours. I kept the High Wind Warning confined to the northern
mountains. We are close to high wind criteria over the French Broad
valley and along the NC Blue Ridge...but the lack of deep and
gradually strengthening unidirectional flow leads ME to believe that
peak winds will top out more in the 40 to 45 knots range across these
areas. Immediately behind the line of convection gradient wind gusts
up to around 45 kts have been noted over eastern Tennessee.
1 to 3 inches of snow still looks like a good bet tonight along the
Tennessee line as strong northwest flow develops and temperatures quickly fall. In
addition to snowfall...rapidly falling temperatures should result in
widespread black ice over the higher mountain zones tonight...also
warranting a Winter Weather Advisory. I/ve extended the advisory
until 9 am Thursday as elevations above about 3kft will see wind
chill values fall to -5 to -15 which is Wind Chill Advisory
criteria. These conditions last through 9 am and it/S much better to
combine the threats in one product.
There is a fire weather concern for tomorrow...which is addressed in
the fire weather of this discussion.
Short term /Thursday night through Saturday/...
as of 230 PM Wednesday...as the upper trough moves farther offshore
Thursday night...a flat upper ridge moving east will provide for
nearly zonal flow aloft through most of the period. This will carry
high pressure across the Carolinas Thursday night and off the coast
on Friday. This will allow for temperatures to rebound nicely to something
near normal for Friday. Warm advection should kick in on Saturday
that will raise temperatures another few degrees in spite of an increase in
cloud cover and shower chances. This development will be courtesy of
a weak wave moving through the Flat Ridge on Saturday...which is by
no means certain. The GFS is most pronounced with the short wave on
Saturday but it remains disorganized. However...some upper
divergence and DPVA act on deepening moisture to develop precipitation into
the forecast area. Meanwhile...the NAM and European model (ecmwf) barely show any wave
feature and that is reflected in a lack of forcing and response as
both models keep US dry on Saturday. The forecast will keep a small
pop developing into Saturday afternoon mostly as a legacy from the
previous forecast given the new GFS which still develops precipitation. The
onset could be early enough for a brief period of high elevation
snow over the smokies and balsams...otherwise it should be rain.
This could easily be eliminated in subsequent forecasts.
Long term /Saturday night through Wednesday/...
as of 200 PM EDT Wednesday...latest global models have come into
better agreement on timing and track of a Miller-a type low moving
up along the southeast coast late sun into Monday.
Models agree that a wave of low pressure will develop along a
surface boundary over the central Gulf Coast early sun as a northern
stream upper energy digs into the lower Mississippi Valley. This low
will track eastward across southern Alabama/Georgia Sun night and moves off
the southeast coasts by 12z Monday. Probability of precipitation ramp up from west to east through sun
as deep moisture flux increases ahead of the approaching low. The
best chance of precipitation will occur Sun night into early Monday as the low
moves southeast of the area. Latest models indicate that thermal profile
will cool off considerably by Sun night as 1030+mb Canadian high
pressure builds to our north...leading to some p-type issue mainly
across the northern zones of the County Warning Area. Using a top-down method...I/ve
mentioned wintry mix over the northern NC mountains and portions of the
NC Piedmont north of I-40 Sun night into Monday morning. Elsewhere will
see a solid rain event. Probability of precipitation taper off by Monday afternoon as the low
pulls further NE away from the region. Expect dry weather with
moderating temperatures Monday through Wednesday as high pressure centered off the East
Coast brings moist return flow to the region.
Aviation /00z Thursday through Monday/...
at kclt...VFR conditions will prevail through the period as cold
advection dominates behind passing frontal boundary. Main aviation
concern through this taf cycle will be gusty winds. Strong low
level flow around 4-5kft will mix down through the first few hours
of the taf cycle. Thus...prevailed sustained northwest winds around
23kts with gusts at approximately 35kts. Winds will relax somewhat
overnight as the mix layer lowers...however gusts in the 25kts
range remain possible. Finally...winds will pickup yet again late
Thursday morning across the region as the mix layers deepens leading
to slightly higher surface winds of around 10-12kts
sustained...gusting to near 22kts. Wind directions will remain
northwesterly...however a weak Lee trough to the west could yield
slightly backed west-northwest flow by midday.
At kavl...low VFR conditions will prevail through the overnight
hours as northwest flow snowfall along the Tennessee border spreads
low ceilings over the airfield. Winds will remain gusty overnight as
previously forecast due to strong synoptic scale flow in the low
levels mixing down and channeling in the French Broad River valley.
Sustained surface winds of 25kts with gusts nearing 40kts are expected.
Otherwise...snow showers should stay west of the airfield but would
not be surprised to see a flurry or two reach the ground in the next
few hours before the activity becomes more confined to the Tennessee
Elsewhere...VFR conditions will prevail with strong northwesterly
winds being the main aviation concern through the overnight hours.
Sustained flow in the upper teens with gusts nearing 25-30kts will
continue into middle morning. At that time...the SC sites will likely
see winds back westerly due to weak Lee trough and decrease to around
10kts with little to no gusts forecast. Winds at khky will remain
northwesterly through the period.
Outlook...dry high pressure will remain over the western Carolinas
through the remainder of the work week.
00-06z 06-12z 12-18z 18-24z
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)
although temperatures will be well below normal on Thursday...the air mass
moving in behind the front tonight will be very dry...such that relative humidity
should drop well down into the teens east of the mountains in the
afternoon. Precipitation today will probably not amount to enough to
wet down the fuels. There should be enough residual wind gustiness
to raise the possibility of dangerous fire conditions Thursday
afternoon. After consultation with land managers...a Fire Weather
Watch will be issued for NC east of the mountains temperatures will be cooler
over the NC mountains and some locations near the Tennessee border might be snow
covered...so no watch there. Elsewhere...wind will be lighter...but
additional coordination will be needed across South Carolina and NE
Georgia...for a possible Fire Danger Statement.
Georgia...Wind Advisory until midnight EDT tonight for gaz010-017-018-
NC...Fire Weather Watch from Thursday morning through Thursday
afternoon for ncz035>037-056-057-068>072-082-501>510.
Wind Advisory until 6 am EDT Thursday for ncz048-051>053-058-
Wind Advisory until midnight EDT tonight for ncz035>037-056-
Winter Weather Advisory until 9 am EDT Thursday for ncz033-
Wind Chill Advisory until 9 am EDT Thursday for ncz033-048>052-
High Wind Warning until 6 am EDT Thursday for ncz033-049-050.
SC...Wind Advisory until midnight EDT tonight for scz001>014-019.