Scientific Forecaster Discussion
fxus62 kgsp 260721
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
321 am EDT sun Mar 26 2017
low pressure lifting from the Midwest to the Great Lakes will push a
band of moisture into the southern Appalachians today. Deeper
moisture and persistent southwesterly flow will then linger over the
southeast through Tuesday, with several rounds of showers and
thunderstorms possible. Drier air should briefly return by the
middle of the week as a cold front settles south of the region.
Another plains low pressure system will bring moisture back to the
southeast Thursday through Friday as weak cold air damming develops
east of the mountains.
Near term /through tonight/...
the occluded frontal system continues to advance towards the area
this morning with a decent stream of precip developing across the NE
Georgia. Expect the mech lift showers to increasing in coverage thru
daybreak across the farther wrn zones.
The op models have been having a hard time with the eastward
progression of the precip shield across the forecast area over the daytime
period today. The more conservative NAM has performed better in
this regard the past few runs...so will give more weight to it's
Expect the -shra to spread across the wrn zones rather quickly thru
15z as a well defined S/W embedded in upper low swings thru
enhancing coverage somewhat. After the S/W passes the line of -shra
shud become more disorganized and weaken thru noon...before
increasing instability generates sct/num tstms this afternoon.
Soundings are showing weak/thin SBCAPE during the afternoon and deep
layered shear will remain modest at best...so expect the Mode to
remain general pulse with perhaps loose organization across the NC
fthills and upstate where shear/cape interact the best. With loss of
instability thunder will become isol to nil overnight with isol/sct
-shra continuing thru daybreak...mainly across the br in mech lift.
Max temps will reach a couple degrees above normal...with some areas
likely higher in diff heating. Good moisture overnight will hold
mins about 10 degrees above normal.
Short term /Monday through Tuesday night/...
as of 245 am EDT sunday: a briefly closed 500 mb low pressure system
crossing the Southern Plains Sunday evening will open up over
Missouri on Monday and ride up the Ohio Valley through Monday night.
Deep layer southwest flow will keep fairly abundant moisture in
place on Monday ahead of this feature, but triggers for convection
will be hard to find. Still, SBCAPE values in excess of 1000 j/kg
will support scattered convection in the southwesterly flow. The
best upper support ahead of the system will cross the southern
Appalachians Monday night, but with weakening convective trends
given the loss of daytime instability.
The best Omega with the passing upper support will move east of the
area by midday Tuesday. Meanwhile, a surface cold front will
approach the Appalachians from the northwest on Tuesday and settle
southeast across the region through Tuesday night. Anticipate the
best combination of lingering Tuesday PM instability and forcing to
occur across eastern sections, generally east of I-77. Some Piedmont
locations could push 80 degrees for maxes Tuesday just ahead of the
Long term /Wednesday through Saturday/...
as of 255 am EDT sunday: strong upper level ridging will cover the
eastern Continental U.S. Through Wednesday as yet another closed low pressure
system moves from The Four Corners region to the Southern Plains.
Meanwhile, surface high pressure will build southward from the Great
Lakes as a passing, weak cold front settles south of the forecast
area. Temperatures will remain warm on Wednesday, with plenty of
lower 80s in southern sections, as the thermal contrast across the
boundary is weak and insolation will be good.
The upper ridge will migrate across the forecast area on Thursday
and off the southeast coast Thursday night. At low levels, an
Atlantic moisture return will develop on Thursday and cold air
damming may onset as the returning moist upglide encounters high
pressure extending southward from a 1030 mb high center east of the
appalachian mountain chain. Deeper upglide and moisture will then
arrive Thursday night through Friday as the upstream surface low
moves out across the mid/deep south early Friday and then crosses
the region around the retreating wedge, in Miller b fashion, through
late Friday. Instability should be uncovered from the south and
southeast on Friday as The Wedge boundary retreats northward. This
could lead to a severe thunderstorm threat in southern sections if
the instability and shear appropriately line up. Will lean toward
the cooler side of the guidance envelope for Thursday and Friday
Model differences increase Friday night into Saturday, but with a
consensus blend bringing a trailing surface cold front through the
area from the northwest after 12z Saturday - ushering in drying.
Temperatures should rebound about a category Saturday as downsloping
flow develops east of the mountains.
Aviation /08z Sunday through Thursday/...
at kclt: VFR conds thru the first period...then lowering ceilings/visibility
aft 11z to mfvr as prefrontal trof and precip approaches from the
west. This line of showers will affect the airfield aft 18z with
little in the way of forcing or instability to produce tstms...thus
prob30s will mention -shra. Expect an improvement to VFR arnd 23z
with linger -shra thru 02z. Se/ly winds will veer during the
afternoon as the p/grad re-aligns with low-end gust possible.
Elsewhere: taf sites will remain VFR over most of the overnight
period. Prefrontal/upslope -shra will begin to affect kavl arnd
daybreak. This line of showers will move slowly east thru most of
the taf period and affect all sites with MVFR/IFR cigs/vsby.
Thunderstorms will likely develop in the afternoon across and in the
vicinity of all sites expect khky where instability will remain low.
An improvement to VFR conds all sites within the last 6 hrs of the
period with lingering vcsh or -shra. Winds will remain aligned
generally S/ly to SSE/ly with low-end gusts possible outside of
stronger tstm gusts.
Outlook: areas of precip will continue into Sunday evening
likely creating additional flight restrictions across the area. An
unsettled and moist pattern will persist thru next week and maintain
chances for low ceilings and visibility.
07-13z 13-19z 19-01z 01-06z
kclt high 90% Med 73% high 100% high 100%
kgsp Med 75% Med 74% high 92% high 100%
kavl high 85% high 88% high 93% high 84%
khky Med 64% Med 73% high 86% high 91%
kgmu Med 77% Med 75% high 95% high 100%
kand Med 74% Med 74% high 93% high 94%
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: