Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
1003 am EDT Tuesday Sep 30 2014
drier high pressure will build over the region on today and linger
through Thursday. A cold front will arrive from the northwest on
Friday with a cool and dry air mass moving back into the region over
Near term /through tonight/...
at 1000 adm EDT Tuesday...an upper trough axis was located over the
eastern Seaboard...with one shortwave leaving the coast of the
Carolinas and Georgia...and another moving into the Ohio River valley. An
upper level ridge axis Washington moving from the plains to the MS River
Valley. Surface fronts were located over the NE Gulf of Mexico...and
the Ohio River valley. Satellite imagery showed areas of fog eroding
in the Piedmont...while the NC mountain valleys showed less erosion.
The northern shortwave is expected to reach the NC mountains by late
this afternoon...but the surface front to the north is expected to
stall over WV and Kentucky. Model time heights show limited low level
moisture over our area. Some shallow instability will exist beneath
a subsidence inversion aloft...but chances for precipitation remain
quite low. High temperatures will run several degrees above normal
in under slowly rising heights aloft.
Previous discussion...a strong short wave and upper low diving down
across the Great Lakes to the upper Ohio Valley will keep an upper
trough axis along the East Coast through tonight. The pattern will
support high pressure ridging down from the northwest with dry air
gradually filtering in. For tonight...there could be more fog
development...but mainly over the mountains temperatures were kept close to an
average of the guidance...and slightly above normal.
Short term /Wednesday through Thursday/...
as of 310 am Tuesday...on Wednesday...a weak 500 mb ridge axis will
slowly ripple east across the County Warning Area. At the surface...a weak Lee trough is
forecast to develop during the afternoon...providing a weak pressure
gradient across the southern appalachian region. Winds are expected
to remain around 5 kts from the surface up to 850 mb. The combination of
middle level ridging and weak flow across the terrain should favor dry
conditions for most of the County Warning Area. However...a small pool of
instability across the western NC mountains may support isolated late
afternoon rain showers...highlighted with schc probability of precipitation. Temperatures are
forecast to range around 5 degrees above normal.
On Thursday...conditions will change very little from Wednesday.
However...the center of a low level ridge will shift along the Atlantic
coast through the daylight hours. As a result...low level winds across
the County Warning Area will strengthen slightly from the south...reaching 15kts at
850 mb. The south winds should advect Gulf moisture across the southern
Appalachians. During the late afternoon...weak surface cape and south-southeast flow
may support a few rain showers across the mountains high temperatures are
forecast to range from the u70s within the mountain valleys to low 80s
Long term /Thursday night through Monday/...
as of 330 am Tuesday...medium range models time a surface cold front and
associated l/west trough across the County Warning Area during the daylight hours Friday.
0z GFS depicts the concurrence of several ingredients on Friday to
support a pre-frontal band of convection. Between sunrise and middle
morning...a band of 300 mb jet divergence will track overhead...running
ahead of the trough axis. Veering low level flow will increase across the
region as the cold front approaches. During the late
afternoon...0-1km helicity is forecast to range from 150-200 m2/s2.
In addition...cape values may range from 500-1000 j/kg. The forecast
environment appears favorable for a band of convection to develop
along and ahead of the cold front west of the mountains early Friday
morning. The band should survive the passage across the
mountains...supported by a broad field of cape...upper forcing...and low level
shear. I will forecast probability of precipitation to range from cate to likely on Friday.
Temperatures will be limited by cloud cover and high rainfall
coverage...peaking around 5 degrees below normal.
Saturday through Monday...high pressure will track from The
Rockies...across the southern Mississippi River valley...off the
East Coast. This airmass will feature dry and cool conditions. High
pressure centered over the County Warning Area on Sunday will result in the coolest
temperatures of the period. Morning lows on Sunday are forecast to range
from the 40s across the mountains to low 50 east...highs around 70.
Aviation /14z Tuesday through Saturday/...
at kclt...conditional climatology suggests that vlifr conditions due
to dense fog will continue until about 13z...after which expect a
period of visibility improving to MVFR...then VFR by around 14z or perhaps
15z at the very latest as there is no middle/high cloud layer to
prevent a quick warmup. The rest of the day should be VFR with a few
stratocu and a light north wind. Late tonight...fog is possible once
again...so an MVFR visibility was included after 10z...but confidence not
as high as it was for fog this morning.
Elsewhere...variable visibility first thing this morning at taf
sites outside the mountains...but generally in the vlifr to IFR ranges.
Places that do not have the dense fog will probably have very low
stratus...and thus will be at least IFR as well. The fog/stratus is
expected to burn off by 14z at most places. Thereafter...VFR with a
light north wind. Expect more fog during the early morning hours on
Wednesday with most places at least MVFR. The mountain valleys have
the best chance of dropping to IFR.
Outlook...VFR conditions will prevail Wednesday through Friday morning...
with fog/visibility concerns remaining across the mountains and foothills early
each morning. A front will approach on Friday which could trigger
some showers and thunderstorms...with associated restrictions
possible. Expect VFR to return on Saturday.
14-20z 20-02z 02-08z 08-14z
kclt high 94% high 100% high 97% high 90%
kgsp high 100% high 100% high 94% high 80%
kavl high 98% high 100% high 97% high 94%
khky high 100% high 100% high 94% high 100%
kgmu high 100% high 100% high 94% high 83%
kand high 91% 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)
the planning forecast has been updated to lower transport winds
tonight. Smoke management parameters have been adjusted accordingly.