Area Forecast Discussion
Issued by NWS Greer, SC

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS62 KGSP 280613

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
213 AM EDT Tue Mar 28 2017

A cold front will approach from the northwest today and settle
southeast of the area tonight into Wednesday. Drier weather will
develop as high pressure builds southward along the eastern seaboard
from eastern Canada. Moisture will gradually return on Thursday,
with abundant moisture setting up by Friday as a strong low pressure
system moves east from the Mississippi River Valley. Drier weather
will return for the weekend.


As of 145 AM EDT: The lead convective band ahead of the better upper
support is driving quickly through the western Carolinas early this
morning. Strong convection remains possible out across the I-77
corridor as mid level lapse rates steepen up and low level flow
backs slightly. The secondary band of convection driving southward
from central and eastern Tennessee continues to push mainly across
northern AL and northern GA into the more unstable air. Western
sections should see mainly rain/showers through the early morning
hours as thunder chances erode.

Otherwise, water vapor imagery shows the upper level wave lifting
northeast over the Ohio Valley, with the trailing trough axis
rapidly approaching the southern Appalachians early this morning.
The latest HRRR has the lead convection transiting the I-40 corridor
through 08Z. The upper trough axis will then cross the northern tier
this moving and be generally east of the area by 18Z. Mid level
drying will also work in quickly from the west through the later
morning hours. However, the surface cold frontal passage will be a
bit delayed and scattered shower/thunderstorm redevelopment along
the passing front is expected this afternoon, especially along and
east of I-77. Westerly downsloping flow will work against coverage
in most other locations. Upper ridging will develop Tuesday night as
the surface cold front gradually settles off to the southeast by
daybreak Wednesday.


As of 215 PM EDT Monday: In the Short Term, an upper ridge will
cross the Eastern CONUS, while a deep upper low ejects out of the
Four Corners region into the Southern Plains. At the surface, a weak
cold front will slide east, as high pressure builds into the the
Great Lakes. Low-level CAA will be weak behind the front, so well
above normal temps are expected Tuesday night and Wednesday.
Conditions should be dry under partly cloudy skies.

Wednesday night and Thursday, as a vigorous low pressure system
tracks across the Plains, low-level flow will turn out of the south
atop the CWFA, with WAA and upglide supporting the development of
clouds and spotty light showers. With an ~1032 mb surface high
centered over central Quebec, hybrid cold air damming should set up
by daybreak Thursday. This will make for a tricky temp forecast, as
precip will be forced mainly by isentropic lift and weak southerly
upslope flow. So mainly drizzle and sprinkles with a few light
showers are expected. PoPs still ramp up to likely in the southwest
corner of the CWFA by the end of the day on Thursday. If the models
prove to be overdone on precip, temps may end up being a couple
categories warmer than the going forecast. Both the NAM and GFS show
elevated CAPE above the wedge in the upper Savannah Valley. So will
keep a chance of thunder mention thursday afternoon there. But
severe threat should remain well to the west closer to the
approaching cold front.


As of 230 PM EDT Monday:  The medium range fcst period kicks off
on Thursday evening amidst a highly amplified upper pattern that
is highlighted by a ridge axis along the east coast, as well as
a closed and robust H5 cyclone embedded in a longwave trof atop
the Plains and MS Valley regions.  At the surface, the pattern
is just as complex thanks to an entrenched hybrid CAD centered
off the Jersey shore, while cyclogenesis dominates the field
across Dixie Alley, eastward into the Southern Appalachians by
way of an approaching zone of baroclynicity.  With that, the
cold front associated with said surface cyclone will be well to
west by fcst initialization, likely yet to cross the MS river,
however a warm front will have laid out across the I75 corridor
region in GA.  Moisture advection tied to this pattern still looks
as if it will lift isentropically atop the wedge airmass leading
to increasing chances for stratiform precipitation into the early
overnight hours.  At that point the fcst becomes quite interesting
given the approaching upper wave to provide synoptic scale lift,
as well as cooling the mid levels and thus increasing lapse rates.
Meanwhile, the CAD will begin to retreat to the east leading to
intrusion of the warm front, and thus providing focus for llv srh
enhancement given remnant backed surface flow.  Eventually the
flow will veer leading to straightening of hodographs, at least in
the 0-1km layer, however improved mid level lapse rates will yield
increasing instability while the primary shortwave passes overhead.
Given the fcst range these details are still a bit uncertain,
yet it still looks as if the pattern could be rather convectively
active Friday morning/afternoon before the cold front sweeps in
Friday evening.

Moving on, remnant northwest flow showers are progged behind the
fropa on Saturday while the remainder of the fcst area tends to dry
out as deep layer ridging builds in for Sunday. This deep ridging
looks dominant through Monday as well, however with sly flow to be
reestablished as the surface ridge shifts over the western Atlantic.
Meanwhile another deep upper system will be building across the
Plains, before sharply ejecting northeast into the OH/TN valleys
toward periods end, which will lead to another round of wet weather
across northeast GA and the western Carolinas into midweek.

As for the fcst itself, pops will ramp up sharply on Thursday night,
holding at widespread high chance to likely levels through much of
the day Friday.  Said pops will taper into the overnight of Saturday
morning given the fropa, thus no mentionable pops are featured by
12z regionwide with the exception of the TN line upslope regions.
By midday Saturday, those pops are removed leading to dry conditions
through Sunday and into Monday when pops increase slowly from
the west as the next system ejects from the southern Plains.
Temperatures through the period will be a bit variable, however
above normal.


At KCLT: Convection working east from the NC foothills into the
piedmont will require a TEMPO for TSRA as thunderstorms strengthen
after 07Z under the steepening lapse rates aloft. Anticipate a
period of moderate rain behind the convection before the showers
gradually taper off from the west through 11Z. Ceilings should
gradually settle down in the MVFR range with the steadier rain. IFR
is possible but not likely at this point. Anticipate some recovery
to lower end VFR by mid to late morning, with the trailing surface
cold front then approaching from the northwest for the afternoon
hours. Will time the PROB30 for TSRA along the front to mainly the
17Z to 21Z period. Surface winds will be southwesterly with possible
low end gusts this afternoon, turning WNW with fropa during the
evening hours.

Elsewhere: Thunderstorm chances will be best around KHKY through the
early morning hours, with mainly light to moderate rain across the
Upstate TAF sites and KAVL as the ongoing convection moves east.
Gusts are likely in the stratiform rain zone before winds toggle
back toward southwesterly ahead of the cold front approaching from
the northwest. Coverage along the approaching boundary this
afternoon should be less than at KCLT as drier mid level air works
in from the west. Anticipate mainly MVFR cigs this morning,
recovering to VFR by late morning, and with tempo MVFR for any isold
showers/thunderstorms that might form. Southwest winds will be gusty
at times ahead of the front, turning northwest with fropa later this
evening. Winds will remain NW at KAVL throughout.

Outlook: Drier conditions should briefly return Wednesday before
another storm system begins affecting the region Thursday through
Friday. Drier conditions will return again for the weekend.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High  85%     Med   79%     High 100%     High 100%
KGSP       High  99%     High  97%     High 100%     High 100%
KAVL       Low   51%     High  88%     High 100%     Med   79%
KHKY       Med   62%     Med   73%     High 100%     High 100%
KGMU       High 100%     High  90%     High 100%     High 100%
KAND       High  93%     Med   77%     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:




AVIATION...HG is the U.S. government's official web portal to all federal, state and local government web resources and services.