Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 271844

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
244 PM EDT Mon Mar 27 2017

Moist southerly flow will linger over the southeast through Tuesday,
with several rounds of showers and thunderstorms possible. Drier air
will briefly return by the middle of the week as a cold front
settles south of the area. Rain chances return to end the week as a
low pressure system approaches from the west and interacts with the
stalled front.


As of 240 PM EDT: Isolated to scattered convection will move east
across the area this afternoon and early evening ahead of a lead
short wave moving ahead of an upper trough. An isolated severe storm
cannot be ruled out given the moderate instability and relatively
steep lapse rates, mainly along and south of I-85. Organization is
not expected this afternoon as the best shear will be over the
mountains and doesn`t overlap the best instability.

That may change this evening and early tonight as relatively strong
short wave swings across the area just ahead of the approaching
upper trough. This forcing will combine with good upper jet
divergence and deep layer Q vector convergence. Good shear remains
across the area, but elevated instability, while continuing, will be
on the decline. All this means a good chance of convection during
the evening and early overnight across the mountains then spreading
east but diminishing as it moves across the area. Severe storms
cannot be ruled out, especially across the mountains this evening.
The question remains whether organized MCS type convection can get
going across TN. If so, then this may be enough to increase severe
chances if it can hold together as it moves into the mountains. Lows
will be around 15 degrees above normal.

The upper trough swings across the area Tuesday but the surface cold
front moves more slowly. Convective coverage overall will be greater
in the morning as that`s when the deepest moisture and forcing will
be over the area. Coverage will be better along and east of the I-77
corridor during the afternoon. The best overlap of instability and
shear will be to our north and east Tuesday. However, there could be
enough overlap during the afternoon along and east of the I-77
corridor for an isolated severe storm. The air mass behind the front
is not very different, and with the slow movement, highs will be 10
to 15 degrees above normal.


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: MVFR cigs have scattered out. However, low VFR and the
potential for convection return during the afternoon. Guidance has
backed off on the potential for TSRA at the airfield, but chance is
high enough to change the TEMPO to VCTS. Decreasing nocturnal
instability should lead to mainly showers overnight. Guidance has
been consistent bringing in MVFR VSBY and IFR cigs overnight, so
have added these to the TAF. Should be a quick VSBY recover Tuesday
but a slow cig recovery. Winds will be S to SSW through the period.

Elsewhere: Similar conditions to KCLT except with an earlier onset
of convection. Wind will shift after daybreak to NW at KAVL and will
go more westerly elsewhere.

Outlook: Another round of thunderstorms will be possible on Tuesday
afternoon, especially near KCLT. Continued low level moisture will
provide a good chances of restrictions throughout. Drier conditions
should briefly return during mid-week before another storm system
begins affecting the region Thursday and Friday.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High  90%     High  92%     Low   59%     Med   75%
KGSP       High  97%     High 100%     Med   60%     Med   74%
KAVL       High 100%     High 100%     High  83%     High  84%
KHKY       Med   75%     Med   78%     Med   62%     Low   58%
KGMU       High 100%     High 100%     Med   64%     Med   75%
KAND       High  93%     High 100%     Med   77%     Med   77%

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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