Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 181432

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1032 AM EDT Wed Apr 18 2018

After a warm day today, another cold front will move through the
area tonight, bringing a chance of rain to the mountains. Dry and
seasonal conditions are expected for the end of the week. Another
system will move into the area early next week, bringing a chance of
rain back to the region.


As of 1020 AM: With essentially no support for precip in the latest
suite of models, PoPs have been dropped below slight chance for
the whole CWFA thru 00z. Temps looked to be warming a bit faster
than previously fcst, but max temps generally appear to be on
track. Thus minor adjustments to the hrly trends have been made thru
peak heating. This does still permit RH to dip below 25 percent in
the most mixed spots of WNC. Henceforth the Fire Danger Statement
will remain in effect for all NC counties within the CWFA for today,
with low humidity and breezy winds providing increased fire danger.

Otherwise, heights will slowly rise across the region as the
broad and deep upper trof continues to lift farther NE. An upper
shortwave will makes its way across the Northern Plains and towards
the Southern Great Lakes later today and tonight, however we will
remain under relatively flat upper ridging thru tonight. As we
move into Thurs, the upper shortwave will dive SE and towards the
fcst area. At the sfc, broad high pressure centered to our south
will drift off the Florida Coast later today as another cold front
approaches the fcst area. This front is expected to be mostly dry
and move thru the CWFA early Thurs with broad Canadian high pressure
spreading back over the area in its wake. As for the sensible wx,
850mb flow will continue to remain fairly robust this morning before
increasing again this afternoon as the upstream low approaches
the region. The front will likely bring some scattered light
precipitation to the mountains late tonight into early tomorrow,
however no thunder is expected. Lows early tomorrow look to be
above climatology.


As of 200 AM EDT Wednesday: Cold front will be pushing through the
area at the start of the period, with some lingering shower activity
in the mountains just after daybreak. Temperatures will be cold
enough at higher elevations for some of this to fall as snow, but
moisture will dry up pretty quickly as the shortwave lifts out of
the area, leaving mean troughing in place down the East Coast. Cool
high pressure centered over the Plains and Upper Midwest, underneath
an upper ridge dominating the center of the country, will begin to
build in, but winds will stay breezy with the strong surface
pressure gradient remaining in place. Highs on Thursday will drop
back below normals as the roller coaster continues; the CAA behind
the front will compete with the downsloping from the NW winds across
the Piedmont which will keep temperatures warmer than they would
otherwise be with just the airmass change, with lows Thursday night
a good 10 degrees blow normal. Will likely have to consider a freeze
product in the NC mountains that are turned on, but right now winds
look a little strong for much in the way of frost concerns. Very
little change to temperatures for Friday highs though winds will
drop somewhat as a mesohigh is progged to develop across the
Piedmont. Still well below normal for Friday night lows but the
airmass will begin modifying somewhat so not quite as cold as
Thursday night.


As of 230 AM EDT Wednesday: A lovely weekend is shaping up across
the area with high pressure in place, weak northwest flow aloft, and
temperatures just slightly below seasonal levels. The next weather-
maker in the form of a closed upper low will push off the Rockies
into the Southern/Central Plains on Saturday, with surface low
formation over TX. Moisture will advect into the Lower MS Valley
from the Gulf through the weekend. Meanwhile the upper ridge over
the MS Valley will get squeezed between the incoming low and the one
over eastern Canada, and the confluent flow aloft over the Great
Lakes and northern Mid-Atlantic will lock the surface high in place,
allowing a diabatically-enhanced classical CAD event to shape up
Sunday night into Monday. Isentropic upglide atop the cold (cool)
dome will help to reinforce the damming, and though the surface high
will push offshore Monday into Tuesday, the residual cool pool will
remain locked up against the Southern Appalachians leading to a
fairly persistent cool and wet pattern for the end of the period.

12z guidance yesterday and the 00z GFS are wet, though the GFS is a
little faster with the development of precip Sunday night. The new
00z ECMWF is trending a little farther north with the mid-level low
and thus with the precip as well, lining up more with the GFS and
other guidance than 00z yesterday`s dry run, though the ECMWF`s run-
to-run continuity definitely leaves something to be desired. QPF
could become a bit of a problem; while any 6-hr period isn`t all
that concerning, the global models all indicate a good 3-4" soaking
across the favored upslope areas of the SW mountains, though the
ECMWF is considerably drier over the Piedmont than the GFS. WPC
guidance is a reasonable compromise. For now, with the strong
damming, any severe threat is nil.


At KCLT and elsewhere: expect VFR conditions to prevail thru the 12z
taf period. Skies will remain mostly clear with a few high clouds
moving across the area thru the morning and into the afternoon, and
sct low to mid level clouds expected over KAVL by the end of the period
early Thursday. Otherwise, winds will remain brisk and out of the SW
to WSW thru the taf period. Speeds will pick back up again this afternoon
with sustained values in the 12 to 16kt range with gusts in the low to
mid 20s. Winds should taper off a bit later tonight and into tomorrow,
although they will likely remain at or above 10kts thru 12z Thursday.

Outlook: A mostly dry cold front will arrive from the west Thursday
morning, with some scattered showers and brief restrictions possible
over the NC mountains. Winds will remain gusty through Thursday. Broad
high pressure will overspread the region on Friday keeping things dry
and pleasant into the weekend.

Confidence Table...

            14-20Z        20-02Z        02-08Z        08-12Z
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  81%

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:


Warm temperatures, well above-normal, with a dry airmass in
place will lead to RH values hovering generally in the 25 to 35
percent range this afternoon, with SW winds of 15-20 mph across
the area. This will lead to high fire danger conditions across NC,
and possibly across SC and GA as well. A nearly dry front will push
through tonight, with cooler but much drier conditions expected
through the end of the work week. RH values will slowly drop
Thursday to between 25-30 percent, and Friday possibly dropping
below 25 percent. Strong NW winds Thursday will combine with the
drying airmass and may result in high fire danger across the area
again. Winds will decrease on Friday below critical thresholds
but will have to be monitored.


NC...Fire Danger Statement until 7 PM EDT this evening for NCZ033-


NEAR TERM...JPT/Wimberley
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