Flash Flooding in North Carolina, June 20, 2023

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
738 PM EDT Mon Jun 19 2023

.SYNOPSIS...
A low-pressure system developing over the Ohio Valley will
usher in a prolonged period of wet and stormy weather throughout the
week as it drifts slowly over the Deep South. Numerous rounds of
showers and thunderstorms will develop east of this system over the
western Carolinas and northeast Georgia continuing throughout the
week, with locally heavy rainfall expected. Shower and thunderstorm
coverage will diminish slowly over the weekend as the low pressure
system lifts east.

&&

.NEAR TERM /THROUGH TUESDAY/...
600 PM Update...Ongoing convec has remained low-topped with little
lightning potential outside of a cell or two as overall instability
remains weak. A persistent mesolow from earlier this afternoon has
spawned periodic funnel clouds east of Davidson, NC and the radar
continues to show good tornadic structure and interesting SRM
velocities. The current TOR expires at 615 PM and another TOR may be
needed on this storm as it pushes across srn Rowan Co.

As of 243 PM EDT Monday: The initial band of thunderstorms and
heavier rain has moved off to the east, leaving behind scattered
showers and low cloudiness across the fcst area as a weak mesolow
lifts northeastward across the western Carolinas. As the mesolow
pulls away, we should start to get more holes in the overcast that
will allow for some better destabilization. The latest mesoanalysis
shows areas just to the west of the fcst area with sfc-based CAPE
upwards of 2500 J/kg but weak shear. It stands to reason that the
environment to the west will creep into at least the western part
of the fcst area thru the early evening, with improving dCAPE above
800 J/kg as well. This would support the possibility of a few severe
storms with possible wind damage thru sunset or thereabouts. Temps
will be modulated by the location/movement of storms.

Over the next 24 hours, our potential to produce additional rain
will improve as a stacked low pressure system drops southward across
the TN Valley region tonight and sets up shop to our W/SW over AL
or western GA on Tuesday. Model guidance is not overly impressive
with the mid/upper forcing. However, Precipitable Water is already
above 1.5 inches and may approach the climatological max eventually,
and moisture is deep. As the low moves into position, the low level
flow will back from SW this afternoon to SE Tuesday morning and
then easterly by late in the day, so at least the low level forcing
will be excellent. This will shift the focus for precip production
to the upslope areas along/near the Blue Ridge Escarpment starting
late tonight and then even more so by Tuesday afternoon. Precip
probs ramp up to likely then categorical thru the period, with
fairly high confidence. It seems inevitable that a Flood Watch
will have to be put into effect eventually with this pattern, but
it may take a period of time to wet things down because of the dry
antecedent conditions. The model guidance at least in the very near
term seems to focus the heaviest precip potential to the north of
the forecast area. However, with the strengthening easterly flow,
our potential for flooding really starts to improve after daybreak
on Tuesday. Things being what they are, think it prudent to begin
a Flood Watch at 6 am Tuesday along/near the escarpment in NC as
a starting point and we can modify from there. Temps will remain
relatively cool with low diurnal ranges. Wind will not be a problem
through Tuesday.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
As of 200 pm EDT Monday: Short term model guidance generally agrees
that the upper low...which is expected to be centered near the
"confluence" of TN/GA/AL at the start of the period...will begin to
slowly wobble/retrograde N/NW through the short term. This would
tend to re-focus the wave train of vorticity rotating around the low
center more over our area (as opposed to east of our area). As the
same time, strengthening surface high pressure over New England is
expected to sharpen the inverted ridge/E-NE flow down the eastern
seaboard, which is expected to advect cooler/more stable air through
a good chunk of the forecast area for Tue night->Wed night. There
will still be plenty of easterly upslope flow, which combined with
the upper air support will result in plenty of showers and a few
storms during this time frame, but the more stable conditions should
limit the rainfall rate potential. Nevertheless, a threat for
excessive rainfall will continue...if nothing else due to the
deteriorating antecedent conditions.

The relatively stable conditions are expected to be short-lived, as
the retrograding upper low...along with New England surface high
moving out into the Atlantic is expected to result in the return of
southerly flow and a warm sector-ish air mass to the CWA on
Thursday. Higher instability is therefore expected Thu afternoon,
which should allow for another round of widespread showers and
storms, with said storms creating higher potential for excessive
rainfall rates...and perhaps a low-end severe storm threat.
Additional rainfall in the short term is expected to be in the 2-4"
range, with locally higher amounts possible, especially along the
eastern Blue Ridge escarpment. This being the expected 4th
consecutive day of multiple rounds of numerous/widespread
convection, Thursday should see another uptick in the flash flood
threat, as well as an increasing threat for flooding on larger
streams. With widespread clouds and precip, max temps are expected
to be around 10 degrees below climo, and min temps a little above
normal through the period.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
As of 220 pm EDT Monday: The upper low...which is expected to be
somewhere in the Ohio/TN Valley region at the start of the period...
is expected to continue plaguing the area into the early part of the
extended period. However, the global models generally agree that the
stubborn/blocking ridge west->north of the upper low will begin to
break down late in the week, as a substantial short wave ejects from
the northern Rockies into the northern High Plains. This is expected
to allow the upper low to begin opening up/nudging its way to the
east over the weekend. In fact, there's an emerging consensus that
the trough axis will finally be east of the CWA by Sunday. In the
interim, we'll probably have to deal with one more day of numerous/
widespread showers/storms...with a continued ticking threat of
excessive rainfall...on Friday. Beyond Friday, convection should
trend toward a more diurnally-driven cycle, and coverage should
become less widespread...although expected moisture and instability
should support above-climo PoPs Saturday through Monday. Although
this will be more of a typical early summer pattern, poor antecedent
conditions are expected to result in a higher-than-usual threat for
locally excessive rainfall. Temperatures are expected to return to
near-normal levels for much of the period.

&&

.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
At KCLT and elsewhere: An active TAF period in store across all
terminals. Ongoing convec will affect the Upstate sites and KCLT
early on with lowering CIGS and VSBY into the MVFR/IFR range thru
the evening. There may be a lull in activity aft 23z or so for a few
hrs, but expect an increase in precip and storms thru the overnight
hrs as a main line looks to pivot across the I-77 cooridor. Lowering
CIGS/VSBY overnight to IFR or lower can be expected across all
sites, possibly lingering into the early afteroon period. More
thunderstorms are progged Tue afternoon and all sites have a PROB30
for -TSRA thru the end of their periods. Winds will maintain a
general se/ly direction with periodic low-end gusts probable during
the afternoon.

Outlook: Occasional showers and thunderstorms will result in
periods of flight restrictions from MVFR to LIFR through the
remainder of the week as the upper low stalls west of the area
and becomes stagnant over the Southeast.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...Flood Watch from 6 AM EDT Tuesday through Thursday afternoon
     for NCZ033-035-049-050-053-065-501>510.
SC...None.

&&

SYNOPSIS...DEO
NEAR TERM...PM/SBK
SHORT TERM...JDL
LONG TERM...JDL
AVIATION...SBK