NWS Greenville-Spartanburg, SC wrote its last area forecast discussion before the first event at May 20, 9:38 PM EST, 1 hour 22 minutes earlier.
Area forecast discussion, May 20, 9:38 PM EST, 1 hour 22 minutes before
Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1038 PM EDT Wed May 20 2020
.SYNOPSIS...
A large upper low will linger over the Tennessee Valley Region
into Thursday as it weakens while generating several rounds of
moderate to heavy rain across much of our region. This pattern has
the potential to produce more significant rainfall and flooding over
parts of the region. The low will finally start to move away by
late Thursday and Thursday night. On Friday, one more disturbance
will move across the region. We should see a return to more typical
late spring weather over the weekend.
&&
.NEAR TERM /THROUGH THURSDAY/...
As of 1035 PM EDT Wednesday: Forecast is on track. Still watching
the feature discussed below which may be a bit less of a concern
based on latest radar/model trends. Will continue to monitor.
Otherwise, one feature to watch is a mesoscale convective feature
across southern reaches of coastal South Carolina that is being
triggered by low level convergence between onshore flow and a nearly
stationary front. Outflow from this feature combined with moist
southeasterly flow may bring the next round of heavier rain to the
Upstate of South Carolina after midnight and to portions of the the
mountains before sunrise. The HRRR (which currently matches this
feature best) shows upscale growth with this feature intensifying as
it crosses I-85 into the mountains. Deep layer moisture to 500 mb
and perhaps some elevated instability would support heavy rainfall
rates of a half of an inch per hour, locally up to an inch per hour
and could put down a quick inch or so of rain within a couple hours.
This could easily aggravate ongoing flooding as well as introduce
new flash flooding to the region should these heavier rates
materialize. We will watch this potential closely as the evening
wears on.
Expect periods of showers to move northward across the area as
moisture continues to infiltrate in. Per latest guidance, the
notable upper closed low is progged to meander over TN through
tonight before slowly retreating to the NW on Thursday with the sfc
low remaining to the south and the sfc wedge slowly retreating on
Thursday. Could not entirely rule out a few rumbles of thunder the
rest of today, but confidence is low attm. Min temperatures will
remain just below normal.
On Thursday, CAMs continue to depict scattered showers across
western NC and into the Midlands throughout the rest of the period.
Despite rainfall becoming isolated in nature across portions of the
area, with runoff from rainfall over the past few days as well as
ongoing flooding, expect flooding concerns will continue. The Flash
Flood Watch currently in effect may need to be extended past
Thursday morning. Max temperatures on Thursday will be warmer than
today with wedge erosion, but still just shy of normal.
&&
.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
As 215 PM EDT Wednesday: The short term begins Thursday night with
the upper low still hovering over the southern Appalachians, though
the main area of focused heavy rainfall associated with it will have
moved out of the area. Friday will present a bit of a lull in
rainfall during the day, though an upper shortwave will move through
in the afternoon and with plenty of moisture still in place over the
area, some enhanced diurnal convection will likely break out across
the area. PWATs will still be on the order of 1.5-1.6" and any
storms that do develop will likely be accompanied by heavy rainfall
rates, potentially exacerbating any lingering flooding issues. The
other factor to consider Friday afternoon is severe potential with
any storms that do develop - deep layer speed shear still looks like
it will be on the order of around 40kt with forcing provided by an
upper shortwave moving across the area. The question that remains is
instability - guidance has been inconsistent with the extent
spatially and quantitatively of the instability that may develop,
ranging from almost none to close to 2000 J/kg in the southern
Upstate. The extent of the severe weather threat will probably have
to be more of a day-of assessment, as cloud cover during the day on
Friday will make all the difference in the development of instability.
The upper low will finally shift just a little bit eastward on
Saturday, enough to remove the large-scale forcing for ascent
driving the widespread heavy rainfall. The tail of a surface cold
front will remain over the area, though, and scattered diurnal
convection is expected.
As of 321 AM EDT Wednesday: The model guidance shows the cutoff
low finally but slowly on the move away from the fcst area, altho
we will remain under the influence of its cyclonic flow. More
importantly, at daybreak on Thursday, the plume of tropical moisture
will also be on the move across the western Piedmont/foothills of
NC. Unfortunately, this might be too late, and one might anticipate
the Flash Flood Watch to be extended over parts of the fcst area
through the daylight hours on Thursday to account for what looks
like an ongoing threat at least over the area east of Highway
321 in the morning. Very high precip probs will be maintained in
that area, and across the rest of the mtns as well as wraparound
moisture/precip remain across that region well into the day. We
should finally see the threat wane late Thursday as the mid/upper
level dry air wraps in across the rest of the fcst area while the
upper low moves toward the OH Valley. Temps should rebound but
remain below normal. On Thursday night, we might finally get a
break with the forcing having exited to the northeast, so precip
probs tail off considerably, but not entirely. Friday looks like
yet another busy day, what with so much lingering moisture to work
with. The guidance shows a short wave coming through an upstream
ridge Thursday night and then getting entrained into the cyclonic
flow around the departing upper low on Friday. That scenario brings
this forcing overhead during peak heating, which would support
numerous showers/storms across the mtns/foothills and a chance
everywhere else. Fortunately, buoyancy appears to be rather limited,
so we do NOT appear to have an appreciable chance at strong/severe
storms. Temps should climb back to normal. Finally, in the wake of
this short wave Friday night and the upper low having moved even
farther away, dare we say that we might start to briefly dry out
after midnight? One can hope at that point.
&&
.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
As of 145 PM EDT Wednesday: Long-overdue pattern change is on tap
for the extended forecast. The upper low finally moving out of the
area will give way to upper ridging by Saturday night/Sunday, though
at the surface a trailing front will get hung up over the area, and
surface moisture lingering over the area will keep conditions
favorable for what will likely be a more diurnal pattern of showers
and thunderstorms. PWATs will be a bit lower than they have been the
past week, though a generally muggy airmass will still be in place
through the balance of the extended. The only thing lacking will be
a focused forcing mechanism, though with at least 1000+ J/kg of
instability each afternoon, convection will be widespread in the
mountains and scattered in all other areas and will support decent
localized rain rates. Temps will generally be around 5 degrees above
climo through the middle of next week.
&&
.AVIATION /03Z THURSDAY THROUGH MONDAY/...
At KCLT and Elsewhere: Below normal confidence in 00Z TAFs related
to timing of cigs/vsbys and decreasing shower activity. Enough
elevated and weak instability may develop for some areas between 04Z
and 10Z and linger into Thursday for isolated TSRA. Not high enough
confidence for TAF inclusion at this time. A diurnally driven
easterly llvl jet may support weak to moderate LLWS at times through
12Z.
Outlook: A building ridge of high pressure will support a trend
towards diurnally driven SHRA/TSRA heading into this weekend.
Confidence Table...
02-08Z 08-14Z 14-20Z 20-00Z
KCLT High 94% Med 79% Med 77% Med 79%
KGSP High 92% High 94% High 94% High 80%
KAVL High 87% High 82% High 90% Med 76%
KHKY High 98% High 94% High 90% Med 78%
KGMU High 92% High 90% High 93% High 84%
KAND Med 69% High 89% High 95% 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 are available at
the following link:
www.weather.gov/gsp/aviation
&&
.HYDROLOGY...
Very wet near-term antecedent conditions are setting the stage
for a hydrologically active Thursday with widespread forecasted
rainfall of 2-4+" expected over saturated upper soils following
several rounds of heavy rain since Monday. The heavy rainfall
already observed, exceeding 5-6 inches in some locations along and
near the Blue Ridge Escarpment and Upstate SC, has caused
significant rises on area streams. While many small streams have
crested and are beginning to recede, flows remain elevated heading
into Thursday. Though much of the area was dry over the two weeks
leading up to this event, the entire region has seen a wet winter
and early spring with above-normal rainfall.
Given these antecedent conditions and forecasted rainfall, WPC's
Moderate Risk for excessive rainfall again tomorrow is well placed
and warranted.
With a lower threshold for excessive runoff and extremely low flash
flood guidance, new rapid stream rises and higher crests will be
easier to achieve with any heavy rainfall that occurs tonight into
tomorrow morning. Minor flooding along the Blue Ridge Escarpment and
adjacent foothills of North and South Carolina has already occurred
today, and is anticipated closer to the Charlotte Metro area later
tonight into Thursday morning. Isolated to scattered areas of more
significant (Moderate) flooding potentially impacting several homes
and businesses are possible in areas that have already seen
excessive rainfall, especially along and adjacent to the Escarpment
from Henderson and Polk counties north to Avery and Caldwell
counties, but such impacts cannot be ruled out further east and
south into the Piedmont, especially near urban areas, including
Charlotte.
As always, it will take many hours for streams to rise, crest, and
recede so flooding could persist well after the rainfall ends.
Larger streams will be the last to respond and recede from
excessive runoff; therefore, rivers like the Broad, Catawba,
and SF Catawba may see flooding persist well into Thursday.
Moderate flooding along these mainstem rivers, which includes
structural flooding, is possible so it will be important to monitor
forecasts (https:/water.weather.gov/ahps2/forecasts.php?wfo=gsp)
and Warnings for the latest information if conditions deteriorate.
Given the potential for high rainfall rates and event-total (since
Monday) rainfall amounts exceeding 10 inches in isolated locations
along and adjacent to the Escarpment, the threat for landslides is
also increasing. Residents in areas prone to landslides are urged
to review the GSP Public Information Statement for preparedness
information and be prepared to act if a Flash Flood Warning is
issued for your area.
&&
.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...Flash Flood Watch until 8 AM EDT Thursday for NCZ033-035>037-
049-050-053-056-057-064-065-068>072-082-501>510.
SC...Flash Flood Watch until 8 AM EDT Thursday for SCZ002-003-
005>010-013-014.
&&
SYNOPSIS...DEO
NEAR TERM...65/12
SHORT TERM...17
LONG TERM...17
AVIATION...65
HYDROLOGY...