Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 232036

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
436 PM EDT Wed May 23 2018

Unsettled weather will persist over the next week, with chances for
showers each day, along with a few thunderstorms. As low pressure
develops over the Gulf of Mexico, deep tropical moisture will
overspread the area over the weekend and persist into early next
week, making precipitation especially likely for Memorial Day.


As of 430 PM EDT Wednesday: Isolated to low end scattered showers
and thunderstorms are slowly moving south across the area this
afternoon. Best instability is to our east, but locations east of I-
77 are on the gradient of the higher CAPE values. That area is also
on the gradient of the higher DCAPE values. Cannot rule out a strong
thunderstorm with isolated heavy rain over that area. Elsewhere,
instability is weak and cells are mainly showers with little to no
lightning. Therefore, have updated the forecast to follow these
trends. Temps are running a little higher than previous forecast, so
upped a couple of degrees before diurnal cooling begins.

In the big picture, very little is expected to change over the
next 24 hours. The relatively high amplitude upper pattern remains
intact with a ridge over the center of the country and a trof
well off the east coast, and a weakness/upper low trapped over
the Deep South. The guidance drops the surface front down into
the fcst area late tonight. This could provide a focus for some
shower activity after midnight and so the precip prob actually
goes up in the pre-dawn hours across much of the region. Overnight
temps will remain seasonally mild. The boundary will not make it
through on Thursday morning and should lift back nwd or wash out
by the afternoon. Any brief northeasterly flow will be replaced by
a veering southeasterly flow at low levels on Thursday afternoon,
which should favor the development of shower activity with daytime
heating across the southern third of the fcst area and near the
Blue Ridge. Severe weather chances look remote again on Thursday
with only modest sbCAPE topping out around 1500 J/kg in a fairly
thin profile, generally weak lapse rates, and weak shear. The
dry air aloft remains as well, which should cut down on potential
precip amounts. High temps look about a category cooler on Thursday
afternoon as compared to today.


As of 200 PM Wed: Thursday night through Friday, an upper ridge
axis will extend from the southern Appalachians to the Great
Lakes. Offshore high pressure will have diminishing influence over
the Carolinas as it moves further offshore. Low-level flow will
veer to southerly, leading to better moisture flux which more or
less persists through the weekend. Accordingly, PWATs are shown
to return to anomalously high values by Friday afternoon and
remain there through Saturday. Lapse rates will be mediocre and
forcing fairly weak, with the most likely initiation mechanism
being upslope lift; PoPs generally follow a diurnal trend,
albeit still at least slight-chance nocturnally. NAM and GFS
both depict the mesolow currently over GA as drifting north into
our area early Friday, possibly bringing with it some convection
at that time. Given the abundant moisture and short hodographs,
impacts from any showers/storms will continue to be heavy rain
from slow-moving cells. Other than any areas of flash flooding,
severe weather appears unlikely. Max temps will be near normal,
with mins at least a couple categories above normal on account of
very high dewpoints.


As of 2pm EDT Wednesday:  Long term continues wet with good chances
for precipitation each day Sunday through Wednesday with fairly
monotonous humid and warm temperatures continuing with generally
cloudy skies. Most days will have a diurnal pattern to showers and
thunderstorms, with thunderstorm chances highest in the afternoons.

Latest GFS model run takes the tropical low developing over the Gulf
of Mexico further west than previous runs and more in-line with the
upper low and closer to, but still East of, the EC solution. Current
GFS path for the low is into central Georgia, while the EC has the
low into Mississippi/Louisiana; with both modeled lows into the US
South by Tuesday or Wednesday. Depending on the ultimate path of the
tropical low, the GSP area could see enhanced winds and rain for the
Monday through Wednesday period.  Impacts do not look profound at
this point, with the worse case GFS having 850mb winds of 25kts or
so early next week, and precipitation amounts only a little above
the non-tropical wave solution, with 2+inches total for the Sunday
through Wednesday period. It appears it will be fairly wet
regardless of the destiny of the Gulf low.

Wet forecast is due to persistent general southerly flow with
quiescent synoptic conditions that have no moisture scouring fronts
of any kind.  Northward moisture transport will be enhanced by
development of Gulf low, regardless of its path, and  support mid-
60s dewpoints through Wednesday.


At KCLT and elsewhere: Prevailing VFR outside of scattered
showers/thunderstorms through the evening hours. Wind should stay
light SW outside the mtns. Still hard to nail down thunderstorm
chances with guidance lacking in agreement, so only KCLT was given a
TEMPO to handle it since they are near the instability gradient. A
front should drop down into the area in the pre-dawn hours and bring
the wind around to northeast at KCLT. At some point, the boundary
should lift back northward on Thursday which may allow wind to come
around to ESE. A fog possible at all sites
around daybreak on Thursday. After the fog burns off...the rest of
the fcst looks very similar to today.

Outlook: The unsettled pattern will continue the rest of the week,
with flight restrictions possible each day under diurnal showers
and thunderstorms. Tropical moisture may increase through the late
weekend. Morning stratus/fog are possible each day - especially
following heavy rain the previous day.

Confidence Table...

            20-02Z        02-08Z        08-14Z        14-18Z
KCLT       High 100%     High 100%     High  98%     High  90%
KGSP       High 100%     High 100%     High  91%     High  86%
KAVL       High 100%     High  92%     High  88%     High  82%
KHKY       High 100%     High  91%     High  83%     High  90%
KGMU       High 100%     High 100%     High  83%     High  86%
KAND       High  98%     High 100%     High  81%     High  86%

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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