Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 201750

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
150 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.


As of 150 pm: today`s pattern in our neck of the woods will be
dominated by amplification the upper ridge downstream of deepening
Great Plains upper low. This will keep rather warm mid-level temps
across the area, while surface ridge axis emanating from the
anomalously strong Bermuda high will maintain an area of relatively
low surface theta-e across the forecast area. Meanwhile, a back door
cold front was moving S/SW across the Mid-Atlantic this morning,
with the leading edge extending from northeast TN into south central
VA. There is a strong consensus in the latest guidance that this
boundary will remain north of our forecast area through the end of
the day. As such, the latest mesoscale model runs depict a
convection-free scenario through the afternoon. Other than
maintaining a slight chance across the far northern zones this
afternoon, we will generally follow suit. Temps will remain well
above climo today.

Aforementioned approaching upper low will drag a surface low and
cold front through the Plains and Lower Mississippi Valley today.
Leftover quasi-stationary boundary across the Virginias may briefly
drop into NC, but the strength of the upper ridge should prevent it
from moving too far south tonight. Did increase pops in northeastern
zones somewhat this evening as the front sags a little south, but
the bigger story will be with the approaching convection along the
front. Slug of tropical moisture will lift from the Gulf into the
Deep South in the WAA regime ahead of the front, and this combined
with the approaching LLJ running smack dab into the SW NC mountains
will provide a good setup for at least brief heavy rain overnight
tonight (toward the end of the near term). GFS has PW almost 2" late
in the period which, perfect progged, would be higher than the daily
PW max for KFFC (way higher for KGSO). NAM is a tad slower than the
GFS and has max PW values just a smidge lower, but still above the
daily max. Deep-layer shear is minimal, with deeper dynamics still
lingering to our west by the end of the period, but moderate
instability may help to enhance rainfall rates. Luckily the area
with the highest QPF is still pretty dry, with residual lingering
drought, but at this time certainly cannot rule out an isolated
flash flood potential.


As of 330 am EDT Saturday: Wedge front makes minimal progresses into
the region from the northeast by Sunday morning as major cold front
approaches from the west.  Good moisture ahead of this front and
strong forcing from synoptic upper low will lead to appreciable
rainfall and thunderstorms continuing Sunday morning and
trailing-off Monday as front makes its passage.  Current timing of
FROPA from GFS has it past the Smokies Monday morning moving
southeastward, and out of the CWA by Monday evening.  FROPA scours
moisture and advects in cooler air with dewpoints dropping 10
degrees from Monday to Tuesday and highs dropping 5 degrees.
Rainfall totals are expected to be 1 to 2 inches for the Saturday
night through Monday period with higher amounts of up to 2 to 3
inches over southwestern parts of the CWA where deep moisture will
be better.  Due to convective nature of much of the precip., local
amounts can be significantly higher, giving some concern for local
flooding with the heavier storms.

Severe potential is limited by low convective instability measures;
with LI to around -3 Saturday and Sunday afternoon, along with low
shear.  Severe chances are probably strongest ahead of the cold
front on Monday where LI may reach -7 along with improved deep layer
shear and lift as system makes its push eastward.

Precipitation with next system may begin as early as Tuesday
morning, though main brunt of that system is not expected until
Tuesday evening.


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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