Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
651 PM EST sun Nov 29 2015
a frontal boundary will sink southward into the area tonight...with
a cool and wet air mass settling in Monday through Tuesday. Another cold
front is likely to bring additional rain Wednesday...before dry high
pressure arrives Wednesday night.
Near term /through Monday/...
as of 630 PM...the radar mosaic shows a broad area of stratiform
rain entering the NC mountains in association with a rer jet divergence
maximum. At the surface...a backdoor cold front was analyzed a couple hours
ago just pushing into northern NC. This front should make progress through the
County warning forecast area tonight...help enhance low level isent lift atop the County warning forecast area. So with
all that said...the current pop trends in the grids look good.
Overall...forecast is on track.
As of 215 PM EST...regional radars continue to show rain spreading
into the area from the west this afternoon. Measurable precipitation
is most likely across the mountains and western foothills through
late afternoon...with mostly virga from higher bases or trace
surface amounts farther east into the Piedmont. The coverage of the
precipitation should expand tonight as lobes of weak upper forcing
move over the region. The area remains on the southern periphery of a
broad 90 to 100 knots upper jet core stretching eastward from the broad
western trough...and this will provide occasional upper divergence.
In addition...weak 500 mb vorticity lobes will provide several
rounds of weak DPVA through the near term period. Moist upglide will
also become established at lower levels tonight and then persist
through Monday as the 850 mb flow slowly veers SW.
At the surface...the frontal boundary to the north of the area will sag southward
into western NC overnight and then make slow progress to the SW through
Monday...as a surface low along the boundary to the west lifts from
central Tennessee into Kentucky. All told...the deeper moisture and forcing
should focus primarily on the northern and western parts of the forecast area
through Monday. Cold air damming conditions also look most likely
across the northern half...but with increasing uncertainty over how far
SW across upstate SC and NE Georgia the cad layer will penetrate. This
will largely depend on how much precipitation falls into the
developing wedge. Will feature a fairly sharp north to S gradient on
temperatures and probability of precipitation through Monday. Hydro problems do not look especially
likely through Monday with rainfall totals of about 1.5 to 2.0
inches in the wettest SW NC mountain areas...but with more to come later.
Short term /Monday night through Wednesday/...
as of 220 PM Sunday...as the center of high pressure slides over the
northern Atlantic...cad pattern will gradually weaken Monday night
across the western Carolinas. The loss of the low level wedged cold air
will result in the weakening of low level lift. I will taper probability of precipitation from
cate to chance during the overnight hours. After sunrise...a cold front
will slide across eastern KY/TN...reaching the NC mountains late Tuesday
afternoon. Forecast profiles indicate that cloud cover across the
County Warning Area will remain at least mostly cloudy through the day. Surface winds
may veer briefly from the south during the afternoon. Using a blend
of preferred MOS...high temperatures may range within a degree or
two of normals. Given the slow approach of the cold front...I will
forecast likely probability of precipitation across the mountains with chance probability of precipitation east.
On Wednesday...short range guidance indicates that the surface cold
front will continue to slowly slide east...limited by nearly
parallel middle level flow. At 500 mb...a positively tilted trough
associated with a closed low over the Great Lakes will ripple east
during the daylight hours. Middle level q-vector convergence will
increase across the Appalachians...especially north of the County Warning Area.
However...short range guidance indicates that a frontal wave will
develop along the cold front Wednesday morning...then race quickly to the
NE. In addition...both the NAM and GFS indicate a field 200-800 j/kg
of cape will gather along and east of the cold front Wednesday afternoon.
The combination of the middle level forcing...surface wave/cold front...and
weak instability should support at least likely probability of precipitation across the mountains
and foothills on Wednesday. Using wpc quantitative precipitation forecast...an additional 1-1.5
inches of rain is expected across the mountains temperatures are
forecast to range a few degrees warmer than Tuesday.
Long term /Wednesday night through Sunday/...
of 220 PM Sunday...the global models are in generally good
agreement regarding the evolution of the large scale pattern late in
the week...although some of the details remain sketchy...which is
unsurprising due to the relative complexity of the pattern. The
upshot is that an evolution toward a split flow regime should be
completed early in the period...as northeast Continental U.S. Vortex shifts
progressively off the coast...while vorticity across Texas holds up
the progression of the pattern across the southeast Continental U.S.. although
the European model (ecmwf) and GFS take different paths to get there...they arrive at
the same conclusion regarding establishment of an upper low across
the southeast by early in the weekend. They also agree that any
surface development will likely be displaced far enough from the
forecast area to preclude any significant periods of forcing and/or
moisture advection into the area.
What this means in terms of sensible weather is decreasing probability of precipitation Wednesday
night...as frontal moisture shifts toward the southeast coast. This
will be followed by at least a couple of days of precipitation-free
weather...although continues height falls to our west could result
in continued periods of cloud cover. Probability of precipitation ramp back up to a token
slight chance over the weekend as the upper low gets its act
together. A combination of clouds and below-to-near normal upper
heights should result in temperatures a little below-to-near normal through
Aviation /00z Monday through Friday/...
at kclt...not much change in thinking from previous tafs. Starting
out VFR with period -ra this evening...then lowering ceilings and
reduced visibility overnight...as a round of moderate rain moves in from
the west. A backdoor cold front will slide through by daybreak
Monday...keeping the winds out of the NE...and allowing ceilings to
become locked in around 500-800 feet for the day on Monday.
Elsewhere...area of light to moderate rain working into the NC mountains
at time of taf issuance. Expect this rain to overspread all taf
sites through the evening hours and continue through the overnight. So
ceilings should lower to IFR at all sites...with visibility ranging from IFR to
VFR...depending on rain intensity. On Monday...cold air damming should
set up...and generally lock in IFR conditions through the day. Periods
of light rain will also continue...especially through midday.
Outlook...unsettled conditions and associated restrictions will
continue Tuesday into Wednesday before a cold front crosses the area
midweek to bring drying.
00-06z 06-12z 12-18z 18-00z
kclt high 84% low 49% medium 68% high 80%
kgsp high 87% high 80% high 89% high 83%
kavl high 82% high 81% medium 78% high 90%
khky high 84% high 86% high 84% high 84%
kgmu medium 72% medium 73% high 89% high 83%
kand high 83% low 45% medium 75% medium 74%
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)