Scientific Forecaster Discussion
fxus62 kgsp 221042
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
642 am EDT Sat Jul 22 2017
the upper level ridge that has brought such hot weather to our
region of the nation will gradually break down over the weekend as
an upper level trough develops over the eastern part of the country.
Expect temperatures to be a little cooler next week with better
chances for rain.
Near term /through tonight/...
as of 630 am EDT saturday: temps running very warm for this time
of day, which points to another hot one this afternoon.
The heat will be the main story again today as the upper anticyclone
over the plains continues to nose eastward. Think high temps will
be very similar to yesterday, if not a degree or so warmer in
some locations. Dewpoints should mix out again this afternoon,
but perhaps not as much as in recent days. The result is heat
index values that reach the 100-104 range for a few hours this
afternoon. Some locations over the wrn Piedmont and upstate will
flirt with 105, but as it stands right now the fcst does not have
enough coverage above the heat advisory threshold, so one will not
be issued at this time. We will let the fcst speak for itself. Note
that a small change in dewpoint or temp will push US over the
threshold, so an advisory might be issued later today as trends
are established. As for convection, the situation favors mostly
Ridgetop development, with a slightly better chance that we might
break through with isolated storms outside the mtns. Precip prob
is very much like climo, which is chance mtns, slight chc east of
mtns. The wildcard will be possible mesoscale convective system remnants. The GFS brings
something down from the northwest over the NC mtns late today, but this
is more of an outlier. The NAM probably has the more realistic
depiction, keeping any remnants moving past to our north. Expect
another warm night tonight. The models suggest the overnight mesoscale convective system
track will dip farther south as the upper ridge weakens and a
boundary drifts southward, so a small precip chance was retained
over the mtns and NC Piedmont after midnight, but this might be
a bit of a stretch.
Short term /Sunday through Monday night/...
as of 300 am saturday: an upper level trough will swing into the
Great Lakes on Sunday, bringing some height falls across the region.
An associated cold front will push into the Ohio Valley and guidance
is in decent agreement on axis of best moisture/instability and
convection coverage just to our north and west. Still expect the
southern Appalachians to see solid chc to likely coverage Sunday
afternoon. The Piedmont will be more isolated to scattered. Steering
flow should be out of the WNW, so that may help convection survive
somewhat off the Blue Ridge. Temps will continue to be above normal
with high humidity. Heat index values should be generally a couple
degrees cooler than Saturday, but still in the 100-104 range across
the Piedmont. Convection may linger well into the night Sunday
night, as the front approaches from the northwest. So I have pops taper
down slower than usual diurnal trends. Lows will be above normal. As
for severe threat, the new day 2 convective outlook has a slgt risk
across much of the Ohio Valley to the mid-Atlantic, with a
surrounding area of mrgl risk that includes all our NC zones.
Monday, the front is progged to push into NC by Monday afternoon.
Meanwhile, the axis of the upper trough will slowly cross the
Appalachians. So I expect decent coverage of diurnal convection
again. Temps should start a slight cooling trend with more clouds
and earlier convective initiation. Heat index values should also be
a little lower than past few days. The severe threat looks a little
lower due to less instability and the main trough starting to exit
to the east.
Long term /Tuesday through Friday/...
as of 315 am saturday: an upper ridge will try to reassert itself
somewhat across the center of the country, but troffiness will
linger across the Great Lakes to the New England. A cold front will
stall either across the southern tier of the County warning forecast area or just to the
south, while a weak transient sfc high drifts by to our north.
Another cold front will dive south into the Ohio Valley Thursday and
push into our area by Friday. Overall, the latest medium range
guidance doesn't give our area much of a break in convective
chances, as by the time the first front dissipates, the next one
pushes into the area. So pops will continue to be above climo, with
scattered to numerous aftn/eve showers and storms each day. The
quietest convective day looks to be Thursday. Max temps will start
out slightly above normal Tuesday, then slightly below normal
Wednesday, warming back to slightly above normal by Friday. Min
temps will be at or slightly above normal thru the period.
Aviation /12z Saturday through Wednesday/...
at kclt and elsewhere: VFR through the period, with the exception of
kavl where fog lurks about the French Broad valley. Another round of
high based cu expected again today after we warm up the boundary
layer starting in the late morning. Light wind should become SW around the
time that cu start to develop. At this point, think only kavl and
khky have enough of a chance of storms Worth mentioning. Coverage is
spotty on the recent mesoscale model guidance, so this was kept at a
prob30. Wind should stay SW into Saturday evening. Expect another
round of Mountain Valley fog again early Sunday.
Outlook: increasing chances of convection will continue into early
next week ahead of an approaching cold front on Monday. Overnight
restrictions will continue in the mountain valleys with chance
10-16z 16-22z 22-04z 04-06z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl Med 61% high 100% high 100% high 100%
khky high 83% high 100% high 100% high 100%
kgmu high 100% high 100% high 100% high 100%
kand high 83% 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: