Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 201847

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
247 PM EDT Sat May 20 2017

Widespread showers and a few thunderstorms are expected later
tonight through at least Monday as a large frontal system passes
through the area.  This frontal passage will bring cooler
temperatures as well with Tuesday through Thursday highs 5 to 10
degrees below normal. Some moisture is expected to return by
Wednesday night as a strong system impacts the area with more rain,
thunderstorms, and wind.


The upper ridge axis will move to near the East Coast by 12Z Sunday,
exposing our forecast area to a prolonged fetch of deep SW flow,
anomalously high moisture (PWATs 2-3 standard devs above
climatology) and multiple rounds of convection that will continue
well into the short term. This afternoon/evening: convection will
continue to develop across the Gulf Coastal states and should make
steady progress toward the area, although it may do so in fits and
starts as it encounters the more stable air mass currently in place
across much of our area. So while chances for convection start to
ramp up this evening, likely to categorical pops are not introduced
to the western half of the area until after midnight. A cooling
boundary layer and moistening profiles should result in somewhat
modest buoyancy tonight, while moderate (at best) shear is expected,
so the severe storm potential appears minimal tonight. The high
PWATs should create a favorable situation for cells that are
efficient rain-makers, and would not rule out a localized flash
flood threat by tomorrow morning if cells can train over a
particular area. Min temps tonight will be well above normal.

For Sunday, the situation in terms of the pop/qpf/severe storm
potential is quite muddy, as forcing should remain relatively weak,
and trends will be dictated largely by buoyancy and high moisture
content. The instability forecast tomorrow is highly uncertain due
to the expected morning convection, what areas will be most impacted
by said convection, and whether any thermodynamic recovery will
occur in the afternoon, by which time a cold front should be
approaching the southern Appalachians from the TN Valley. Overall,
likely to categorical pops do appear warranted across most of the
area, but amounts and timing remain highly uncertain. As far as the
severe potential, certainly cannot rule out an isolated threat, but
shear will remain too limited and the instability picture too
muddied for there to be too much of a concern. The larger threat
should continue to be a slowly evolving hydro threat, but this
should be limited to a localized flash flood potential through the
near term. That being the case, feel the best approach is to allow a
little bit of rain to fall before any decisions are made re: hydro


As of 230 PM EDT Saturday: With upper-level troughing remaining to
the west of the forecast area Monday through Tuesday, and above
climo moisture persisting over the southeast, the weather pattern
will remain quite unsettled through Tuesday.

A deep, closed, upper-level low pressure system will wobble east
over the Great lakes Sunday night through Monday. Meanwhile, the
associated trough axis will drive an approaching surface cold front
into the NC mountains by daybreak Monday morning. Deeper moisture
and forcing just ahead of the approaching cold front will impact the
region primarily Sunday night. Rainfall totals for the Sunday night
period should close in on an inch in many locales, but the maximum
will be hard to pin down given the embedded convective rates. The
frontal system will then be very slow to clear the eastern piedmont
through Monday afternoon. Instability just ahead of the
slowly-moving front could reach 2000 to 2500 J/kg of sbCAPE during
the afternoon hours near the I-77 corridor and in locations
southeast of I-85 for an isolated severe threat to accompany a
continued isolated hydro threat where any training develops.

A piece of upper-level energy will then dive southward behind the
Great Lakes system Monday night through Tuesday and close off
another low center over the Midwest. Meanwhile, the recently
departed surface front will stall across the deep south and wave
formation is likely there. Another round of deep forcing and
moisture ahead of the redeveloping trough axis may well return in
the early morning hours Tuesday as the surface wave slides northeast
along the stalled frontal zone. Brief drying will likely wrap back
in from the southwest late Tuesday, but uncertainty remains high on
this timing, and any drying should be fairly short-lived.

Hydro problems could develop just about anywhere through Tuesday,
but we`ll need to see where the earlier period heavy rainfall lines
up before any later period Flood Watches can be considered. Also,
instability and shear could overlap in the southeast half of the
forecast area on Tuesday to provide a severe tstm/isolated tornado
threat, but the timing of this also remains uncertain. Will
advertise the heavy rain threat for both Monday and Tuesday in the


As of 145 pm Saturday:  Within the front side of a deep mid-Conus
trough, broad and moist sw flow will likely be lingering atop the
region to start off the period.  Timing details remain uncertain at
this point, but the going sensible wx fcst of numerous showers and
cooler than normal max temperatures looks to be on track for
Wednesday. Confidence is increasing that the warm conveyer belt pcpn
will have shifted downstream of the cwfa by Thursday. However, with
the passage of the upper trough axis and proximity of the eastward
wobbling of the closed low, decent shower chances remain, although
more scattered in nature and with low QPF.  The medium range model
consensus for Friday and next Saturday is featuring deamplifying and
weakening flow top the the SE conus. Will plan on a dry fcst for the
new day 6 and 7 with max temperatures returning to normal next


At KCLT and elsewhere: SW winds at 6-9 kts and FEW/SCT cumulus in
the 030-045 range are expected through the remainder of the
afternoon. Chances for convection are very low through at least mid-
evening, then flight conditions will go downhill quickly overnight,
as convection is expected to move into the area from the SW after
midnight. Categorical SHRA/VCTS is carried at all terminals except
KCLT and KHKY during the pre-dawn hours. However, can`t rule out
convection at these sites as well, although the better chances
should occur toward the end of the current TAF period, if not after.
Otherwise, another round of IFR/MVFR cigs are likely at most
terminals Sunday morning, while increasing cloud cover should limit
visibility restrictions to periods of moderate/heavy SHRA. Winds
will turn SE tonight, possibly dialing back closer to E by the end
of the period.

Outlook: Periods of showers and thunderstorms with associated
restrictions will continue off and on through the middle of the new

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High  86%     High  86%
KGSP       High 100%     High 100%     High  85%     Med   69%
KAVL       High 100%     High 100%     High  81%     High  92%
KHKY       High 100%     High 100%     High  85%     Med   72%
KGMU       High 100%     High 100%     High  82%     Med   70%
KAND       High 100%     High 100%     High  90%     High  81%

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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