Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 041851

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
151 PM EST Sun Dec 4 2016

Increased moisture will cross our region from the west today and
linger into Tuesday. A strong cold front will arrive Wednesday night
bringing some of the coldest air of the season so far at the end of
the week.


As of 145 PM EST Sunday:  A deep/closed upper low atop the Gulf
Of California continues to advect Pacific moisture across Mexico
into the central/southern CONUS.  Meanwhile, a northern stream H5
shortwave is diving through the plains states, while a broad area
of surface high pressure is entrenched along the east coast.  With
that, hybrid cold air damming is underway with the aforementioned
moisture lifting isentropically atop the CAD wedge periphery.
This upglide regime is enhanced by the northern steam shortwave,
which once it passes, will yield a lull in the widespread light
to moderate rain that is underway across northeast GA and the
western Carolinas.  Fortunately, llv warm advection has allowed for
temperatures to moderate to levels not supportive of any wintry
ptypes, thus this event looks to be all rain from here on out.
As stated above, the continued upglide will warrant categorical pops
across the region tonight, amidst a non diurnal temperature trend.
Once the H5 shortwave passes, along with the H85 front, the llv
flow will veer w/nw into mid/late morning leading to eventual
dissipation of widespread rain as the parent CAD surface high
erodes/ejects, leaving only weak upsloping along the TN line as
the primary forcing mechanism.  Therefore, by daybreak Monday pops
will be lowered to slight chances north of I85, with chances south.

By that point, model pattern evolution indicates movement of the
deep upper low into south TX, while another transient surface
ridge slides through the OH valley and begins wedging down the lee
side of the Appalachians.  Given that the moisture stream from the
southwest will still exist, am expecting light upglide induced rain
to move in from the southwest by Monday afternoon, thus pops will
comply with likely levels west of I26 by periods end.  As already
stated, temperatures will favor a non diurnal trend given continued
southerly llv flow tonight, then trending more diurnal into the
later part of the fcst period.  All in all, expecting lows to be
at or just above normal, with highs at or just below normal.


As of 245 AM EST Sunday: We begin the short term with a shortwave
trough pushing through the Great Lakes with its attendant front
across the eastern Carolinas, surface high pressure over the Ohio
Valley, and a slowly filling upper low beginning to push northeast
from Mexico into TX, meanwhile pulling Pacific and Gulf moisture
into the coastal states as surface cyclogenesis is induced along the
TX Gulf Coast. Behind the initial front, we`ll be in a bit of a lull
though with the subtropical jet right over us, will keep the trend
of low chance pops south with at best slight chance across the
northern tier. We might see some breaks in the clouds as well with
highs only a couple of degrees below seasonal normals on Monday.

Meanwhile the surface high will cross the Appalachians during the
day and begin to ridge down the Eastern Seaboard in a diabatically
enhanced classical CAD setup. The Gulf Coast low will lift northeast
in a Miller B scenario late Monday into early Tuesday, with
widespread moisture overtopping the cold dome. PW values between 1-
1.5" can be expected by Tuesday morning across the area, the upper
end of which is pretty darn close to record PW values. This is
fairly short-lived with the fast-moving system but could bring some
hefty rainfall amounts to portions of the Piedmont as the southern
of the two Miller B lows pushes northeast in the predawn hours. 6-hr
QPF values are 0.75-0.9" across the GA/Carolinas Piedmonts in our
area from 06z-12z Tuesday so this would be the period of greatest
concern for impacts from heavy rainfall. Storm total QPF ranges from
about 1.5" across the northern tier to almost 3" across the southern
tier. Additionally, with the damming in place and strong
ageostrophic adjustment, winds will pick up out of the NE Monday
night, with sustained winds 10-15kt and gusts up to 20kt. Certainly
nothing close to advisory criteria or anything like that but worth

Precipitation will exit quickly to the east during the day on
Tuesday, but with lingering moisture across the mountains with NW
surface flow and and SW flow aloft. Highs Tuesday will be highly
dependent on if the damming is able to erode earlier than
anticipated, but since usually guidance is too fast at eroding the
wedge, continued to lean slightly on the cool side.


As of 145 AM EST Sunday:  The strong cold front will arrive from the
west by late Wednesday. Ahead of the front Max temps will be about a
category above normal which will make Wednesday the mildest day of
the whole week. There will be no inflow from the Gulf as low level
wind will be from a general west direction Wednesday into Thursday
morning shifting to NW by PM Thursday into Friday.  The moisture
starved environment associated with the cold front will yield very
little precip. Based on the latest run of the GFS, only precip will
be in the mountains being light snow behind the front with falling
thickness values. All that shrivels away by early Friday. A little
more precip on the ECMWF but that also ends abruptly early Friday
with much drier and colder air moving into the area.  Expect cold
air advection Friday and then light to no wind on Saturday as the
center of the cold surface high will be over the Carolinas and VA
early Saturday. Both the GFS and ECMWF have a wintry precip producer
arriving to our region later on Sunday lingering into next Monday.
That is after the current forecast so stay tuned for how this
evolves in the days ahead.

Temps a little above normal Wednesday then falling to 15 degrees or
more below normal for Friday and Saturday. Some moderation in Min
Temps Saturday night as cloudiness increases ahead of the next


At KCLT and Elsewhere: A mixture of MVFR/IFR and LIFR conditions
expected at all sites through this taf cycle as moist upglide atop
a CAD wedge prevails.  Initialized all tafs with either IFR/LIFR
cigs/visb per obs and near term guidance, with further deterioration
expected over the next few hours as the next wave of light/moderate
ra moves into the region from the southwest.  Therefore, late this
afternoon into tonight conditions are lowered to LIFR at all sites,
remaining at those levels through the overnight.  Expecting ra
to taper through early morning as the northern stream upper wave
passes and the 850mb front moves east.  Thus all sites indicate
such by way of no prevailing ra however with prob30s for ra, timing
based upon location.  Lastly, expecting cigs to recover to at
least mvfr by late in the period at all sites, with the exception
of KAND where impacts of the next round of precip being to occur.
Winds through the period will remain light northerly at all sites,
except for at KAVL where flow prevails southeasterly down the French
Broad valley, but should eventually veer northwesterly by daybreak.

Outlook: Another round of heavier rain will move into the region
Monday night into Tuesday leading to widespread restrictions
. Brief dry weather is expected on Wednesday, with another front
approaching from the west.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High  80%     Med   65%     Med   66%     Med   67%
KGSP       High  84%     Med   73%     Med   65%     Low   57%
KAVL       Med   75%     Med   77%     Med   65%     Low   46%
KHKY       High  85%     Med   63%     Med   65%     Med   64%
KGMU       High  84%     Med   70%     Med   65%     Med   63%
KAND       Med   79%     Med   69%     Med   64%     Med   62%

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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