Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210751

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
251 AM EST Wed Nov 21 2018

High pressure will remain across the region through Thanksgiving and
maintain dry and cool conditions. Widespread precipitation is likely
late Friday through Saturday as a low pressure system tracks
northeast from the Gulf of Mexico. Another wet frontal system will
likely arrive early next week.


As of 230 AM EST: Model profiles early this morning continue to hint
that some scattered stratocumulus may briefly fan out east of the
mountains as a thin layer of moisture around 850 mb settles under a
subsidence inversion. Even if these clouds form, 850 mb flow will
turn more westerly and then northwesterly through the later morning
hours to dissipate this layer.

Surface high pressure will weaken as it noses across the southern
Appalachians from the west today. Cool thicknesses remain in place,
but good insolation and some downsloping will allow maximum
temperatures to recover to within a category of climatology.

Meanwhile, a backdoor cold front will mass across the Virginias to
the north tonight, but generally stay north to northeast of the
region through daybreak Thursday. Very shallow northwest flow
moisture may bring a few clouds back into the northern mountains
late tonight, but with no hint of any precipitation in the
subfreezing air.


As of 205 AM EST Wednesday: Dry and cool hipres will remain in
control to begin the short term. A colder backdoor cold front will
wedge in from the northeast Thu afternoon with little fanfare
outside of CAA and drier sfc td/s. There was some possibility of
-fzdz across the BR Thu night on a previous fcst...but the moisture
looks to be much more limited by the time the colder air works in.
So...a dry fcst is in place until a moist frontal system pushes in
from the southwest.

Model dprog/dt solns over the past few runs have become drier and a
bit warmer. In fact...precip is now not expected to reach the srn
FA until late Fri afternoon. Once the precip gets here will
quickly spread as it lifts over a dome of very cold hipres thru the
overnight and into Sat. With the parent high shifting quickly
offshore Fri night...the degree of stg CAA will be limited before a
non/diurnal transition takes place arnd 06z due to llvl WAA from
the advancing low system. Still expect some -fzra across the nrn BR
and not much else besides rain as nomograms and soundings continue
to indicate either ice or rain. Raw model sfc temps generally do
good with wedge a consensus of raw output was blended
into the hr/ly temps which gave a bit warmer conds over the nrn mtns
and for a shorter period. Thus...the -fzra shud be rather limited in
scope with less than 0.1 inch fcst/d...which will transition to rain
before daybreak. Ensem probs for ice indicate arnd a 50 prob for
less than 0.1 of an inch of ice...only a 25 prob for 0.1 inches and
zero prob for 0.25 inches. This agrees well with the current fcst
thinking of advisory level a mention will be made in the
morning HWO.

The overall precip amts thru the period have come done a bit with
perhaps 1.5 inches over the srn BR over 18 hrs. So...a sigfnt hydro
threat is not very likely. The system shud take most of Sat to track
northeast of the area and expect improving conds by Sat evening. Max
temps will be pretty cold Fri with highs only in the 40s. Sat will
see a slight rebound across the srn zones to the l50s...but the NW
piedmont and nrn fthills will likely remain in the u40s.


As of 235 AM EST Wednesday: Little change was made to the ext range
fcst. The latest op models agree well with the large scale pattern
which features another h5 s/w tracking east across the srn Midwest
supporting a moist and vigorous sfc low. A cold front associated
with the low looks to cross the FA Sun night with little chance of
wintry precip outside of some back-side upslope -snsh or a -ra/sn
mix across the wrn NC mtns. Precip amts will be on low-end due to
the quickness of the system and a dry slot is progged to push in
from the southwest early Mon. Winds will be fairly stg and gusty
with this system...esp across the higher terrain where advisory
levels may be reached by Mon afternoon into the evening. The low
becomes occluded over New Eng thru the period and a secondary low
may form across the Glakes which could bring a couple rounds of
light diurnal NW flow -snsh both Mon and Tue nights. Max temps on
Sun will be near normal...but back to below normal Mon/Tue as
another Canadian sfc high builds in from the northwest.


At KCLT and elsewhere: VFR conditions will continue through the
period. There is some potential for FEW to SCT VFR clouds to form
over the Upstate and Charlotte TAF sites as a narrow zone of
moisture settles under an 850 to 800 mb subsidence inversion through
12Z this morning. However, ceilings are generally not expected and
any such clouds should dissipate quickly in developing W to NW 850
mb flow later this morning. Otherwise, the main forecast question
will center around the potential for lee troughing to permit some
WSW or SW surface winds at times east of the mountains. This looks
most likely at the Upstate sites toward 18Z, but will keep the NC
sites NW for this afternoon. Anticipate very light northerly winds
tonight along with continued clear to mostly clear skies.

Outlook: High pressure will remain over the region into Thanksgiving
Day, with light winds and VFR conditions, except for the potential
for patchy overnight fog in the southwest mountain river valleys.
Rain will return Friday into Saturday, with associated restrictions
possible across the region.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
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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