Scientific Forecaster Discussion
fxus62 kgsp 250114
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
914 PM EDT Wed may 24 2017
an area of low pressure to our west will push a warm front northward
through the area tonight. Unstable conditions south of the front
will lead to showers and thunderstorms producing heavy rain. A few
more showers may occur Thursday before drier and warmer conditions
return to end the work week.
Near term /through Thursday/...
as of 900 PM edt: the weak convective line moving into the I-77
corridor of the Piedmont this evening is moving into a stable region
worked over by earlier convection. The Tornado Watch has been
allowed to expire. The Flash Flood Watch has also been pared away
across GA, most of SC and the NC mountains, with just the NC
foothills and the I-77 Piedmont area remaining in the Flash Flood
Watch until expiration time at 10 PM.
Plenty of shower activity is evident on upstream radars across Tennessee
and Alabama this evening and some of this could survive into the forecast
area overnight. However, coverage should diminish farther east
overnight and any precip rates with this activity should be fairly
Otherwise, the deep upper trough moving to the southern Appalachians
overnight and then across the region tomorrow will produce
additional scattered showers. Thunderstorms will probably be
confined to the Carolina Piedmont. Temps should be below normal.
Short term /Thursday night through Saturday/...
as of 230 PM EDT wednesday: the sharp trough, and associated upper
level low pressure system, we have been dealing with will shift to
the east of our forecast area (fa) Thursday evening. The last of the
convection should be ending Thursday evening in the NC mountains.
As the upper low lifts northeast into New England we will remain in
a broad northwest flow aloft. Meanwhile high pressure from the Gulf
of Mexico will build north into the Tennessee Valley. Moisture will
be on a downward swing, and with the high pressure system close
enough, we should have a dry Friday.
The upper flow will deamplify Friday night and Saturday, with
ripples of energy riding east/southeast from the Mississippi Valley
into parts of the southeast. At the same time, a weak frontal
boundary will try to drop south (from Kentucky and virginia), but
will encounter the southern end of the westerlies and halt.
Nevertheless there appears to be a boundary of sorts (depending on
model preference) which allows some moisture to pool and
precipitation to develop. The GFS is the fastest, with the NAM and
European models slower. Since the Gulf of Mexico is cutoff from this
scenario, we have sided with the slower precipitation guide Friday
night, limited late in the NC mountains. We will have chance, or
small chance, pops Saturday, perhaps reaching down to near the SC/NC
state line and NE Georgia.
Interestingly the sb CAPES from both the GFS and NAM are hefty for
Saturday. However, a Stout cap is in place. Therefore without a
strong trigger it might be difficult to realize the instability.
In the temperature department our thermal profile signals a warming
trend into Saturday, which we will follow.
Long term /Saturday night through Wednesday/...
as of 215 PM EDT wednesday: the extended fcst picks up at 00z on
Sunday with flattening upper ridging over the southeast and another
broad upper trof digging down over the western Great Lakes. The long
range models still have some lobes of mid to upper lvl shortwave
energy passing just to our north on Sunday, however the main trof
will be slow to progress eastward and isn't expected to move over
the fcst area until late tues into early Wed. At the sfc, a cold
front will approach the County warning forecast area early Sunday but is not expected to
move thru the County warning forecast area until Monday. It appears to become stationary
just to our southeast by early tues and lingers over the area until another,
more robust cold front pushes thru the County warning forecast area on Wed into early thurs.
As for the sensible fcst, no significant changes were made. We still
have solid chances for convection on Sunday and Monday, and slight
to solid chances on Tuesday and Wednesday with the highest pops
generally over the higher terrain. Temps will remain above climatology
thru the period with a slight cooling trend towards the end of the
Aviation /01z Thursday through Monday/...
at kclt: a pre-frontal line of weak convection will likely cross the
Piedmont through the late evening hours, but thunder chances look a
bit limited as this region has already been worked over by earlier
convection. Will keep a tempo window for rain showers for this through 04z.
Southerly flow will turn more SW over time. Low clouds could develop
overnight, but mixing should battle this and permit any MVFR cigs to
scatter out at times. Will also feature prob30 for rain showers Thursday
with the passing upper trough. Winds will be a bit more gusty as
well with mixing.
Elsewhere: convective chances have diminished from the west.
Ceilings may briefly settle down to MVFR for a bit before mixing and
drying permits scattering to VFR overnight. Additional scattered
showers will swing through the region late tonight and Thursday with
the passing upper trough axis. Winds will gust from the southwest,
except becoming northwest at kavl.
Outlook: drying is expected to finally occur Thursday night through
Saturday. Moisture will return ahead of the next Sunday starting
01-07z 07-13z 13-19z 19-00z
kclt Med 74% high 82% high 100% high 100%
kgsp high 96% high 100% high 100% high 100%
kavl high 89% Med 64% high 100% high 100%
khky Med 78% high 93% high 100% high 100%
kgmu high 95% 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:
NC...Flash Flood Watch until 10 PM EDT this evening for ncz035>037-
SC...Flash Flood Watch until 10 PM EDT this evening for scz008-009.