Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 191133

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
733 AM EDT Fri May 19 2017

Another summerlike day is expected today thanks to high pressure
near Bermuda. Saturday could be nearly identical as it looks more
like a cold front will stall to the north of our area. However, a
stronger system will move in from the west on Sunday and Sunday
night, bringing an excellent chance of rain. A cold front should
move east across the region on Monday. Unsettled weather will become
the rule for next week as several disturbances move through.


As of 700 AM EDT Friday: Only changes to the near term forecast are
to update trends, plus changes for aviation-related parameters.
Temperature and dewpoint trends generally running okay; made some
minor adjustments to lower pops just a tad but nothing significant.

Otherwise, upper ridge continues to stretch from the Gulf of Mexico
to the southwest Atlantic, with northwest flow aloft over the
Southern Appalachians. Deep upper low over the Rockies is
responsible for all the havoc in the Plains, but in itself will not
affect us till the short term. Weak front draped from the Southern
Plains surface low across the Ohio Valley and up the East Coast to a
low over Newfoundland. This boundary will provide focus for
convective development this afternoon and indeed SPC has continued
the Marginal Risk along it, but this stays to our north in VA. We`ll
continue to be solidly in the warm sector today with southerly to
southwesterly low level flow. On top of that, a weak shortwave will
ride up the southwest flow and round the top of the ridge today,
providing a little extra oomph for convection as well. Both the GFS
and the NAM have some portions of the Blue Ridge and Piedmont with
sbCAPEs rising to over 2000J/kg, but with a very tight CAPE gradient
to the southeast where modified airmass from the Bermuda high will
lead to lower dewpoints not supportive of convection. The shortwave
isn`t quite enough for shear to organize the storms, but with the
additional lift plus the convective instability, certainly an
enhancement to typical pulse activity can be expected. Couldn`t
totally rule out a brief strong to low-end severe storm, especially
over the mountains. Across the Piedmont, warm air just below 600mb
should keep a bit of a cap in place, and not sure if convective temp
alone will be enough to initiate convection. Have brought slight
chance pops down as far southeast as GSP/CLT but confidence is low.

Behind the shortwave passage tonight, the ridge will briefly pop
back up with a slightly higher amplitude, quickly helping to shut
off convection tonight except for maybe some isolated activity
lingering over the mountains. Temperatures continue 5-8 degrees
above seasonal normals for both highs and lows.


As of 250 AM Friday: Been looking at this for the last five days
and still not entirely sure what to make of Saturday. The models
have been troubled with how to handle a back-door cold front,
which represents the edge of a cooler and drier air mass centered
over ern Canada. The trend in the guidance for the last 24-36 hrs
has been to hold the cold front to the N/NE of the fcst area, and
now the guidance barely has the boundary making down into the NW
Piedmont. Problem is, in many situations, the models do not handle
well how far to the S/SW these boundaries may ooze, particularly
when precip generated near the boundary helps to reinforce the
cooler air on the N side. The latest model guidance keeps almost
the entire fcst area dry through Saturday, but am not ready to
give up on our fcst that has a chance of precip mainly over NC,
because of the possibility that the boundary may ultimately make it
farther S before stalling/washing out. High temps will be highly
dependent on the boundary position. Have nudged the NW Piedmont
up a few degrees as a hedge against the boundary not making
it. Even if the boundary does, it should wash out Saturday night
and a developing S/SE low level flow should help to force precip
development in the upslope areas near the Blue Ridge Escarpment.

Another trend in the new model guidance is to bring more mid/upper
forcing in from the W/SW earlier on Sunday to act on deepening
Gulf moisture. The new ECMWF is in general agreement that more
widespread precip is likely to develop W to E across the fcst area
on Sunday, such that by the end of the day we have a gradient from
categorical over the wrn part to chance in the I-77 corridor. The
combination of 1000-1500 J/kg of CAPE and shear on the order of
20-25 kt suggests some threat of severe storms, but this is far
from certain. The best forcing will move east across the fcst
area during the overnight hours as a jet streak passes to the
north and the main short wave lifts out of the upper trof still to
our west. Precip prob has been raised into the categorical range
across the entire fcst area. Shear will maximize overnight just
high enough for concern, while CAPE remains fairly low.

Which brings us to Monday. Although the guidance has come into some
agreement with the timing of the passage of the cold front earlier
in the day, there is more indication that precip will linger on
the cooler side of the front longer into the day. As a result,
higher precip chances have been extended into the afternoon farther
to the west. Temps are a bit tricky again as we keep them close
to normal with lingering precip and cloud cover, but an earlier
departure might allow for more recovery if that happens.


As of 330 AM Friday...any confidence in the passage of a cold
front to our east Monday night, allowing for some drying as weak
high pressure moves in, is quickly dashed by Tuesday as the
models continue to be at odds with how far south the boundary
will make it before stalling and how soon a wave will move along
the front. At least the guidance is consistent from run-to-run,
with the GFS still showing a fairly dry Tuesday while the ECMWF
brings the precip right back in from the SW with a wave during the
day. Will not attempt to reconcile that difference right now, as the
ongoing fcst has increasing precip chances during the day. However,
there is agreement that we will maintain deep moisture and some
forcing through Tuesday night and Wednesday as the next upper trof
swings across the TN valley and southeast. So, instead of showing
a brief lull in activity, it appears more likely that some chance
of precip will continue through the middle part of the week and
the fcst will be adjusted accordingly. The mid- to late-week part
of the fcst looks like it will be dominated by a long-wave upper
trof over the east, which should spell temps being held close to
normal. Few changes were made to the late part of the week.


At KCLT and elsewhere: Midlevel deck moving across the area has
prevented much in the way of fog formation. Convection should
increase across the area late this morning and early afternoon,
beginning in the mountains and spreading into portions of the
Piedmont later in the day. KAVL and KHKY carry VCTS this afternoon,
and introduced VCSH at KCLT to account for low-end pops as well as
the HRRR occasionally progging convection to the NW of the airport.
Winds will be S to SW through the period, 5-10kt this afternoon and
diminishing again overnight. Increasing moisture should lead to an
increased chance of fog Saturday morning across the mountains, with
another slug of moisture moving into the Upstate but for now only
introduced MVFR at KAVL.

Outlook: Diurnal convection will increase again on Saturday ahead of
a backdoor cold front that approaches the forecast area from the
north over the weekend. Another front is expected to bring more
significant chances of showers/storms/restrictions early next week.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     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:




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