Scientific Forecaster Discussion
fxus62 kgsp 220313
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
1013 PM EST Tue Feb 21 2017
a frontal boundary will push across the area tonight and Wednesday,
bringing a good chance for some modest rainfall. This will be
followed by brief drying, before another frontal system brings
additional chances for rain and thunderstorms late Friday or
Saturday. Overall, daytime temperatures will remain well above
normal through the weekend.
Near term /through Wednesday/...
as of 1010 PM EST tuesday: little change across the region since
the last update. However, its seems the planetary boundary layer is moistening as a
few observations across the region are indicative of precipitation
making it to the ground amidst pockets of upglide induced dz/ra.
Thus, will opt for no changes to pops with this update. Did tweak
T/TD through the overnight as latest cons guidance favors slightly
warmer min temps given ovc skies. That said, still expecting some
evaporative cooling effects, therefore didn't feel comfortable
raising mins much further than a degree. Otherwise, no sig changes
needed/made with this update.
As of 200 PM EST...an upper low will drift from the arklamiss to
near Tampa, Florida by the end of the day on Wednesday. This will place
our area under a col in the upper flow with the northern jet stream
staying well to the north over the Great Lakes. In the low levels,
south/southeasterly flow will continue to gradually advect more
moisture into the area. Upslope lift along the south and east-facing
Escarpment should begin to produce increasing showers this evening
thru the overnight. Overall forcing will remain weak, and guidance
in good agreement on low quantitative precipitation forecast in this flow regime. The moisture
will produce widespread stratus tonight, which will keep temps
elevated well above normal with lows in the upper 40s to mid-50s.
On Wednesday, the upslope flow and deep moisture should keep at
least isolated to scattered light showers along the Escarpment
thru most of the day, while the rest of the area should remain
fairly socked in with low clouds. A 1028 mb surface high will drift
off the mid-Atlantic coast and weaken. So it's tough to say how
much of an in-situ wedge may linger across the Piedmont thru the
afternoon. In any case, the clouds should keep a lid on temps,
and highs are expected to "only" warm into the low to mid-60s
across most of the area. If the low clouds manage to erode along
the south and east fringe of the cwfa, temps may get into the low
70s per the met MOS. This may also unveil a little SBCAPE in the
southern upstate. Lapse rates look rather weak in the soundings, so
only a slight chance of thunder will carried in the southern fringe.
Short term /Wednesday night through Friday/...
as of 215 PM EST tuesday: the closed upper level low pressure system
near the West Coast of Florida crosses the Florida Peninsula through Thursday.
North of this system, 850 mb southerly flow will continue but slowly
weaken through this time. With copious low level moisture in place,
the upglide and upslope flow will lead to isolated to scattered
showers across the area Wednesday night and Thursday. Weak
instability will lead to isolated thunderstorms and rain Thursday afternoon. With the
low level inversion eroding and the less widespread and showery
nature of the precipitation, highs and lows will increase to 15 to
20 degrees above normal.
Mid and upper southwesterly flow develops on Friday between the
weakening upper low off the southeast Atlantic coast and a deepening upper
low and trough over the upper Midwest. The weak surface ridge over
our area erodes as a cold front moves into the MS valley. Southerly
850 mb flow lingers through the period along with low level moisture.
Expect isolated to scattered convection to develop across the
mountains and foothills of the Carolinas and NE Georgia. Highs and lows
will remain nearly steady from 15 to 20 degrees above normal.
Long term /Friday night through Tuesday/...
of 220 PM tuesday: a strong, but weakening short wave trough will
lift from the miss valley through the Ohio Valley/mid-Atlantic and
off the northeast coast early in the medium range period. This will
give way to the return of a progressive/quasi-zonal regime early
next week, before the flow begins to amplify again late in the
period as the next western storm system approaches The Rockies.
Early in the period, a cold front associated with aforementioned
strong short wave will sweep across the Tennessee Valley. However with much
of the deep-layer forcing expected to lift west and north of the
southern Appalachians, an expected weakening frontal circulation,
and the likelihood that the meager pre-frontal surface-based
instability will deplete further as it approaches western NC, it
continues to appear as if any convective band accompanying the front
will lose considerable steam as it moves into the forecast area late
Fri night/early Saturday, with the bulk of significant shower
activity likely passing north of the area. Pops during this time
will range from likely across the western-most NC mtns, to only a
low chance across much of the Piedmont.
Conditions will dry out considerably behind the front on Saturday,
although Max temps will once again range from 10-20 degrees above
climo. A brief period of northwest flow rain and snow showers will be
possible Sat night, but chances will be slight at best.
The remainder of the period will be generally dry and unseasonably
warm (although 5-10 degrees cooler than what has been seen for much
of this week.) A couple of short wave troughs may pass near the
region early next week, but their potency will be weak in the fast
zonal flow, so any chances for showers will be low. More substantial
chances for precipitation will likely hold off until after the end
of this forecast period.
Aviation /03z Wednesday through Sunday/...
at kclt and elsewhere: a mixture of VFR/MVFR and IFR expected
through this fcst cycle as moisture continues to increase ahead of a
stacked low pressure system centered over the MS valley this
evening. Expecting low VFR cigs to prevail initially as upglide
-shra/-dz spreads across the region from the south. With that, cigs
will gradually lower from the southwest to MVFR then IFR, possibly
to LIFR/vlifr by early morning which is featured in taf via
prob30s. Expecting multiple waves of -shra/dz to spread atop the
area during this time as well, therefore opted to prevail vcsh
initially, leading into tempos for shra at all sites after midnight,
before prevailing shra at all sites aside for kclt given its
proximity to the better moisture advection and waves of upper
divergence associated with the approaching upper low. Precipitation
will taper into late morning, before increasing yet again into late
afternoon. As such, restrictive visb/cigs will recover as well to
MVFR and/or low VFR to round out the period. Winds through the
night will be a bit tricky as current swly flow backs enely thanks
to precip induced strengthening of the insitu wedge across the
western Carolinas, therefore did carry enely flow through midday
before flow veers sly yet again as the wedged high retreats for
Outlook: moist southerly to easterly low-level flow will persist
across the area until a cold front pushes thru on Saturday. This
will keep high chances of morning stratus and possibly fog each day
thru Saturday morning.
03-09z 09-15z 15-21z 21-00z
kclt high 100% Med 74% high 96% high 80%
kgsp high 91% high 80% high 100% high 94%
kavl Med 74% high 93% high 93% high 92%
khky high 98% high 98% Med 79% high 88%
kgmu Med 78% Med 78% high 98% high 86%
kand high 95% high 80% high 81% high 84%
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: