Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211451

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1051 AM EDT Mon May 21 2018

A moist air mass will remain in control of the region through at
least Tuesday, bringing a continued high chance of showers and
storms each afternoon and evening. A weak front sliding down on
Thursday may allow for some brief drying toward the end of the work
week, but another round of tropical moisture will lift across the
area for the weekend.


As of 1030 AM: Still seeing a broad area of midlevel stratus across
the middle third of the CWFA. This cloud cover is associated with
a weak boundary which is progged to begin shifting north toward
the NC/VA border this afternoon. On the whole meso models develop
most of their diurnal convection along and north of I-40, due to
the enhanced convergence near the boundary. In the central and
southern parts of the area initiation looks to occur on a more
isolated basis. Revised PoPs to these effects. Locally heavy
rain will be our main concern with any cells that develop.

This evening and overnight, we will see a steady S to SW flow
continue across the area, as 500mb heights fall slightly. Several
guidance sources develop light showers over the southern part of
the area this evening as 850mb moisture advection increases. Thus
chance PoPs have been retained over that area tonight.


As of 245 AM EDT Monday: Upper ridge will continue over the western
Atlantic and Bahamas to start the short term period, with a
subtropical weakness remaining in place across the Deep South. A
shortwave crossing the Great Lakes will push a surface trough into
and across the Appalachians as we move through the period; the
shortwave will phase with the Deep South upper weakness and attempt
to start dragging it out of the way. Expect another day of
widespread showers and thunderstorms, most numerous across the
mountains, on Tuesday, with the added enhancement of upper forcing
from the passing shortwave to the north. Lapse rates will remain
weak but a little mid-level drying might allow for entrainment of
drier air into storms that may increase the potential for gusty
winds, but as has been the case the past few days, heavy rain
remains the main concern. With the introduction of drier air aloft
this will improve PWs somewhat, but really with the warm cloud layer
so deep and the tropical profile at least in the lower layers, still
expect very efficient rainfall producers. Temps on Tuesday should be
bumped down a bit with the widespread cloud cover, a couple of
degrees below normal.

As we move into Wednesday the surface front will be pushing down the
DelMarVa as the upper trough broadens, and while pops will remain,
should start to see a decreasing trend compared to Tuesday, with the
higher pops across the Upstate and NE GA rather than the mountains.
Despite the "front" (in name only) pushing through, temps will
increase again to a few degrees above normal with additional
sunshine and the downsloping component to the NW winds.


As of 310 AM EDT Monday: We`ll begin the extended in a bit of a lull
as a result of the upper trough deepening off the Atlantic Coast,
pushing the upper ridge farther south toward Hispaniola but with a
large ridge developing over the center of the country as well. As
the upper trough deepens and pushes east, the surface front will be
forced south, and so while pops do not disappear Thursday into
Friday, they are decreased from the high chance/likely pops that
have been dominating the area for the past several days.
Temperatures will remain above normal and the diurnal nature of the
convection will, of course, result in typical "summertime"
convective threats.

Then things get complicated. No doubt anyone reading this has
watched with keen interest the evolution of global model forecasts
the past few days with respect to tropical development in the Gulf.
Certainly the details with whatever manages to develop simply cannot
be nailed down at this time, but all indications are pointing to
tropical moisture of some sort lifting into the Southeast/Lower MS
Valley on the western side of the upper ridge. Meanwhile another
incoming weak shortwave will approach the Great Lakes on Saturday,
briefly phasing with the upper tropical low before moving on its
merry way and leaving the tropical system to continue to rain out
somewhere across the South. Interactions of tropical moisture with
synoptic fronts are always problematic. Whatever happens with the
tropical system, the problem is that global models are all pointing
to a very wet period starting this weekend and going through the end
of the forecast period (and later). With fairly wet antecedent
conditions, this could be problematic.


At KCLT and elsewhere: A few light showers have persisted over the
fcst area since the last TAF issuance with most of them currently
located just south of KCLT. A few of these showers could move over
KCLT over the next hour or so, but they should not have a significant
impact on the prevailing conditions. So far this morning, most sites
have been reluctant to fall below VFR, with only KAVL going down and
staying down. The other sites could still see a combination of MVFR
and IFR restrictions over the next couple of hours, but it`s looking
less likely than previously thought. Otherwise, today looks similar
to yesterday with mountain convection beginning in the early afternoon
and possibly spreading into the foothills and piedmont. Winds should
remain light to calm thru the morning, and pick up from the SW during
the afternoon. By Tuesday morning, they should back to a more SLY

Outlook: The persistent unsettled pattern will continue through midweek,
with flight restrictions possible each day under isolated to scattered
showers and thunderstorms.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High  97%
KAVL       High 100%     High 100%     High 100%     High  82%
KHKY       High 100%     High 100%     High 100%     High  86%
KGMU       High 100%     High 100%     High 100%     Med   76%
KAND       High 100%     High 100%     High 100%     Med   79%

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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