Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
140 PM EDT Wednesday Mar 12 2014
a cold front will push through the forecast area today with cool
high pressure building in on Thursday. The airmass will slowly
moderate warmer into Saturday...before a broad and colder upper
level trough builds in Sunday through Monday.
Near term /through tonight/...
as of 130 PM EDT...a line of showers is developing as expected over
eastern Tennessee. Cut back shower activity considerably ahead of the line over
the western Carolinas and NE Georgia this afternoon...but have still
maintained likely probability of precipitation along the probable mesoscale convective system track which will
primarily be across North Carolina.
As of 1110 am...low clouds are holding on a little longer than
expected over the foothills and Piedmont of the western Carolinas and NE
Georgia this morning. And the latest NAM has come in with it/S greatest
instability just a little east of the forecast area around 21 UTC
this afternoon. There is still a line fine over eastern Tennessee which represents
the surface cold front. Convection is slowly strengthening along the northern
portion of the line where there is greater forcing...but the
southern end shouldn/T develop into deep convection until it gets
east of the mountains. The thinking now is that it will steadily
strengthen once east of the mountains...but that the greatest severe
potential will be a little east of the forecast area. Still...the
i77 corridor is the area most under the gun this afternoon in our forecast area.
Also...winds in the boundary layer just behind the line are quite
strong...and even a relatively weak line of showers has the
potential to produce damaging winds. Over the upstate and NE Georgia the
atmosphere looks more capped...though dewpoints will be a little
higher. Overall...the hrrr trend of the line of convection not
developing very far into the upstate looks good and has been
reflected in the grids. But even here 40 knots gradient wind gusts are
possible and the Wind Advisory still looks valid.
As of 630 am...I will update the forecast with the recent
observations across the forecast area. In addition...I will increase
probability of precipitation early this morning...based off of the recent radar data. In
addition...I will add timing and placement details for a line of
thunderstorms and rain to sweep across the northern I-77 corridor this afternoon.
As of 415 am...a very complicated weather event in the offing. The
latest radar images indicated that widespread light rain was
spreading north across NE Georgia and the western Carolinas. NAM
indicates that 305-315k isentropic lift will increase through
daybreak...then becomes scattered during the rest of the day. However...a
field of middle level q-vector convergence will slide across the
forecast area from middle day through sunset. I will increase probability of precipitation to
cate across the mountains and foothills through middle day. A strong cold
front is expected to sweep west to east between 21-00z. The
strong...cold...moist northwest 850 mb winds will remain across the NC mountains
Southwest winds will increase across the County Warning Area ahead of the
approaching cold front. This afternoon...winds across the foothills
and Piedmont will increase to 20 to 25kts with gusts above 40 kts
during the late afternoon and evening. Across the mountains...winds will
also increase sharply during the middle day. However...both the NAM and
GFS indicate that 850 mb winds will reach 50-55 kts between 0-6z. I
will highlight the foothills and Piedmont with a Wind Advisory from
noon through midnight. Across the mountains...the advisory will highlight
the noon to 6 am period. Northern mountain areas will see gusts around
50kts...highlight with a High Wind Warning from noon to 6 am.
The environment along and ahead of the cold front will be a high
shear and Low Cape environment. The 0-1 km helicity ahead of the
front are forecast to range from 250-300 m2/s2. Forecast soundings
indicate a thin layer of cape between 250-300 j/kg. The greatest 0-3
km ehi...approaching .8 m2/s2...is expected to occur along and east
of the I-77 corridor during the late afternoon. In addition...sherb
values within the I-77 corridor is forecast to exceed 1 around 21z.
It appears that a qlcs severe wind event may occur across the I-77
corridor late this afternoon. The severe thunderstorms and rain threat will be
highlighted in the severe weather potential statement.
Tonight...850 mb temperatures will rapidly cool to -10 to -14 c across
the Tennessee border counties. The combination of strong northwest winds and deep
moisture should support shsn along the western slopes of the
southern Appalachians through tonight. In addition...weak
instability may linger within the low level northwest flow during the middle
evening...supporting moderate shsn. Min temperatures are forecast to
range from the teens to the l20s across the mountains...near 30 east. The
cold temperatures should yield snow ratios at least 20 to 1. Event
snowfall across the Tennessee border counties is forecast to range from 2-3
inches...will highlight with a Winter Weather Advisory.
Short term /Thursday through Friday night/...
as of 330 am Wednesday...strong channeled short wave crosses the
area Thursday as an upper trough moves east of the area. Any lingering
mountain snow showers will dissipate quickly Thursday morning as low level
dries up. Tight pressure gradient will relax through the day as high
pressure builds into the area. However...gusty winds could linger
through the morning. Very low relative humidity is likely as the air mass moving
into the area is very dry. There is some concern on Thursday that
portions of the forecast area will flirt with red flag fire conditions
because of the very low relative humidity in the afternoon and potentially
lingering wind gusts over the mountains expect highs 10 to 15 degrees
Zonal flow develops Thursday night and continues Friday and Friday night. Some
weak short waves move across the area through the zonal flow. At the
surface...the high pressure center moves over the area Thursday night then off
shore Friday. A weak frontal system drops into the Ohio Valley Friday night
as a low pressure center moves into the Southern Plains. Moisture returns
across the area in the developing southerly flow as the high moves off
shore. Expect mainly clouds from this return moisture. Precipitation should
hold off until after Friday night. Highs Friday will rise to near normal.
Lows Thursday night around 10 degrees below normal will rise a little
above normal Friday night.
Long term /Saturday through Tuesday/...
as of 145 am EDT Wednesday...there is still much uncertainty in the ext
range forecast. The ulvl pattern/energy and mass fields are not well
agreed upon which leads to varying surface sensible weather solutions by the way the
main op models. The incoming cold front Sat is more pronounced and
has greater gom moisture flux than the cwc/ECMWF. However...there is
decent agreement with the low level thermal fields and will count on
mainly all rain in isent warm sector lift. There could be brief
-snsh across the highest NC mountain terrain with no sigfnt accums
through 14z Sat.
The bigger difference in the models occurs sun through Tuesday with the
evolution of the aforementioned surface boundary. While the GFS is
maintaining nearly uniform flow through a deep layer and thus a
nearly stationary coastal surface front...the European model (ecmwf) and CMC develop a
strong ulvl trough and coastal Miller/a cyclogenesis Sun night. This
latter scenario would bring in much greater moisture and along with
some measure of lowering partial thicknesses and diurnal
cooling...snow or a rain/snow mix sun and Monday nights. This idea was given
a little more weight than the totally suppressed and dry GFS solution.
The going middle chance probability of precipitation still look good Sun night through Monday
night with snow accums across the NC mountains early Monday and perhaps low
end accumulating snow into the NC foothills and Piedmont Monday night.
Temperatures will begin the period right around normal and likely drop to
8-10 degrees below normal by Monday with winds shifting NE/ly as a
Canadian high builds to the north and a broad ulvl trough develops
over the eastern Continental U.S..
Aviation /18z Wednesday through Monday/...
at kclt...the center of 990 mb low will track from the Ohio River valley
to the New England coast through the taf period. Low level lift and deep
moisture will sweep north ahead of approaching cold front this
morning and afternoon. -Shra with IFR ceilings is expected during
the early morning daylight hours. Frontal passage is expected between 22-02z
across the terminal. As the front approaches...SW will strengthen
through the afternoon...with gusts near 40 kts. In the wake of the
front...wind will veer from the northwest with gusts in the middle to upper 30
Elsewhere...IFR ceilings and -shra will increase ahead of an
approaching cold front this morning. By middle day...VFR conditions
with SW winds strengthening significantly...with gusts in the middle
to upper 30 kts. In the wake of the front...wind will veer from the
northwest with gusts in the middle to upper 30 kts.
Outlook...dry high pressure will remain over the western Carolinas
through the remainder of the work week.
17-23z 23-05z 05-11z 11-17z
kclt high 84% high 100% high 100% high 100%
kgsp high 96% high 100% high 100% high 100%
kavl medium 61% high 100% high 100% high 100%
khky high 87% high 100% high 100% high 100%
kgmu high 94% high 100% high 100% high 100%
kand high 86% 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)
Georgia...Wind Advisory until midnight EDT tonight for gaz010-017-018-
NC...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 from 6 PM this evening to 6 am EDT
Thursday for ncz033-048>052-058.
High Wind Warning until 6 am EDT Thursday for ncz033-049-050.
SC...Wind Advisory until midnight EDT tonight for scz001>014-019.