Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 271733

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
133 PM EDT Thu Apr 27 2017

A dissipating cold front will cross our region today, following by
warm and humid high pressure building back in for Friday and
Saturday. Another front approaches from the west on Sunday, and
crosses the area on Monday.


As of 11am EDT Thursday:  Main issue for near term continues to be a
round of thunderstorms expected this afternoon and tonight.  CAM
guidance is fairly consistent and shows development of stronger
storms beginning near peak heating after 4pm over the Southern
Appalachians with ENE movement of convective elements spreading
across most of the CWA with the heaviest activity ending diurnally after 8pm.
Current radar shows some widely scattered light showers approaching
the mountains from the west and scattered over the CWA.  This
activity will advance and increase this afternoon with some light
showers persisting through midnight after BL cooling ends the threat for

Current extensive cloud cover will hold temperatures to the mid-70s,
but low 60s dewpoints should allow MUCAPE to exceed 1000 j/kg this
afternoon, which will combine with effective bulk shear of up to
50kts to produce the chance for severe.  SPC guidance has a 2%
probability for a tornado with the main threat being the chance for
severe wind gusts.

Main front with this system makes only gradual progress
southeastward across the CWA and front largely dissipates Friday
afternoon with little in the way of dry or cold advection in the
wake of the system.  Consequently, Friday`s temperatures will warm
into the 80s with more sensible humidity than we have seen in recent
months. Wet ground and any new precipitation along with elevated
dewpoints will give a good chance for fog Friday morning at some
level, especially in sheltered areas with reduced wind.


As of 300 AM Thursday: An upper ridge will build along the East
Coast, while a deep upper low closes off into a large low over the
Four Corners region during the Short Term. At the surface a cold
front will wash out over the Carolinas, leaving just a tongue of
higher surface dewpts just to our south. Return flow will start to
advect this moisture back north, but the building ridge should keep
a strong capping inversion just above the mixed layer. So I will go
with a completely dry forecast for Friday. Temps will rebound back
to above normal readings with low to mid 80s in the Piedmont.

On Saturday, the dewpts should increase and strong diurnal heating
will result in stronger instability and weaker cap. So isolated to
widely scattered deep convection is expected, especially along the
Blue Ridge where southerly upslope flow enhances lift. Shear will be
weak, so pulse storm mode is expected, a few of which may produce
large hail and/or microbursts. It will feel like early summer, with
highs in the upper 80s to near 90 across the Piedmont with dewpts in
the mid to upper 60s. Diurnal convection should wind down Saturday
night, but it will remain muggy with lows mainly in the upper 60s.


As of 330 AM EDT Thursday...An upper low will lift northeast across
the Central Plains to the Upper Midwest Sunday thru Monday.
Meanwhile, the large upper ridge near the East Coast will drift
east, allowing decent height falls atop the CWFA. An associated low
pressure system will track northeast across the Midwest and drag a
cold front across the Mid-South on Sunday. The 00z suite of medium
range guidance shows good agreement on the timing of the front,
pushing it across the CWFA on Monday. The front should be
accompanied by a solid band of precip, likely strong storms Sunday,
weakening by the time it reaches the CWFA on Monday. Given the
expected timing, the line may re-intensify Monday afternoon,
especially across the eastern half of the area, with daytime heating
to the east. The GFS and ECMWF show about 800-1200 J/kg of sbCAPE
with 0-6 km bulk shear ramping up to 35-55 kt in the vicinity of the
line. A severe threat seems possible, but confidence is probably not
high enough this far out to get a Day 5 outlook area by SPC. Anyway,
PoPs ramp up to likely by Monday. The models still seem overdone on
PoPs for Sunday within the moist return flow ahead of the front. but
will go with a CHC PoP, favoring the Blue Ridge. Temps will be a
little cooler with more clouds and precip potential on Sunday, and
even cooler on Monday with the fropa.

Behind the front, expect conditions to dry out and temps to return
to near or even slightly below normal for Tuesday. A broad upper
trough remains anchored over the western CONUS, with more energy
bringing another low pressure system toward the area by Wednesday.
So PoPs start to increase into the CHC range with temps warming back
above normal.


At KCLT and elsewhere:  Main issue will be progress of a round of
thunderstorms this afternoon and evening.  Progress of system and
approach of front from the west has reduced CIGs to as low as 20kft
in areas without precip., and down below 10kft in areas with.
Current widely scattered showers will give way to some -TSRA this afternoon
with the heaviest activity expected from 20Z to 24Z and lingering
scattered showers after that.  Front with this system is quite weak,
and winds outside of areas with convection will be weak, with light
and variable winds on Friday.  Continued moist low-levels is
expected to lead to fog in the morning, especially in areas with
less wind.

Outlook: High pressure on Friday should lead to widespread VFR with
more diurnally-based showers and thunderstorms possible Saturday and

Confidence Table...

            17-23Z        23-05Z        05-11Z        11-12Z
KCLT       High 100%     High  95%     Med   60%     Low   29%
KGSP       High 100%     Med   75%     Med   60%     Med   75%
KAVL       Med   62%     Med   60%     High 100%     High  87%
KHKY       High  81%     Low   58%     Med   75%     High  81%
KGMU       High 100%     Med   70%     Low   55%     Med   75%
KAND       High  87%     Med   75%     Med   60%     Med   65%

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:




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