NWS Greenville-Spartanburg, SC wrote its last area forecast discussion before the first event at May 16, 5:53 AM EST, 2 hours 47 minutes earlier.
Area forecast discussion, May 16, 5:53 AM EST, 2 hours 47 minutes before
Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
653 AM EDT Wed May 16 2018
.SYNOPSIS...
Expect numerous showers and a few thunderstorms for the rest of the
week as a slow moving low pressure system impacts our region.
Locally heavy rainfall is possible each day. High temperatures will
be near normal and low temperatures about ten degrees above normal.
&&
.NEAR TERM /THROUGH TONIGHT/...
As of 640 AM: The CWFA continues to see the effects of a moist
southerly flow, forced by the Bermuda high offshore and a weak
sfc/upper low now near the Florida Panhandle. Widespread showers
extend across the NW half of the area, being maintained by upslope
forcing. Notably, sfc based instability has remained higher than
depicted by the HRRR/RAP overnight, and regeneration of showers
has occurred over the Savannah River Valley, at least moreso than
the HRRR progged. A lull still is expected in the late morning
over the Piedmont, though not as significantly as previously fcst,
so PoPs were updated thru early aftn.
Diurnal destabilization is expected to lead to another uptick
in coverage this afternoon, so all areas return to likely or
categorical PoPs late in the day. Over the course of the day, the
upper low to our southwest is progged to merge with a shortwave
now over the Mississippi Delta region. In the late afternoon
and evening, DPVA embedded in the combined trough will move
over our area, and that is when the best coverage is expected
overall. Guidance sources differ as to how long this will take to
push through the area, but a gradual reduction in precip activity
is likely as the remaining instability diminishes.
With the continued upslope enhancement, as usual the Blue
Ridge will be an area we are most likely to see flood issues
develop. Some gages in the most favored south-facing terrain are
already approaching 6-hour FFG. However, even away from the terrain,
meso models depict accumulations of 1 to 2 inches from individual
cells this afternoon and evening. A flash flood threat does exist
on the local level if convection develops as anticipated. At the
present time it appears the flood threat will be isolated enough
to preclude a flash flood watch.
Temps today will be generally near normal; mins tonight will stay
around 10 degrees above normal given the exceptional moisture.
&&
.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
As of 320 am Wednesday: A blocking pattern will maintain a trough
axis from the central Gulf of Mexico through the Mid-South and Ohio
Valley through the period, resulting in a deep fetch of Gulf
moisture into our forecast area into at least the early part of the
weekend. Vort maxes rotating around the low will generally be weak,
but with PWATs likely nearing record levels for this time of year,
it won't take much lift to focus bands of showers and thunderstorms.
Of course, timing of these individual waves and associated surface
reflections will be problematic, as will exact locations in which
heavy rainfall becomes focused. It continues to appear as if
Thursday will be the most likely period for organized heavy
rainfall, as guidance is in fairly good agreement in lifting a
surface wave north across the Upstate and the NC foothills through
the day, which would tend to focus heavy precip across the eastern
third or so of the forecast area. Having said that, the models tend
to focus the heaviest rain along a narrow (i.e., about 50 miles
wide) axis, and they do not agree exactly where the heaviest rain
will occur tomorrow. Nevertheless, a swath of 1-2 inch (with locally
higher) amounts appears likely across portions of the SC and western
NC Piedmont. While lesser amounts are expected west of the surface
wave track, numerous showers and storms will likely develop there
throughout the day, especially across the high terrain, where
antecedent conditions will be in a deteriorating state due to
today's rainfall. Thus there will be a threat for localized flash
flooding pretty much everywhere. While watch issuance doesn't seem
plausible at this time, it may become necessary later today or
tonight, depending upon how the hydro situation unfolds over the
next 12 to 18 hours.
Thursday night/early Friday may see some degree of a lull in
numerous/widespread coverage, as a bit of a subsident regime sets up
in the wake of the vort max. However, the pattern will support
development of at least numerous showers and storms with Friday
afternoon destabilization. The flash flood threat will continue to
increase, if for no other reason due to the deteriorating antecedent
conditions. But again, weak waves lifting north (that models may not
be handling well at this juncture) will be capable of focusing
mesoscale areas of very heavy rainfall. Coverage and intensity of
convection should diminish during the overnight hours, but some
degree of coverage will likely persist well into Friday night. As
usual in situation in heavy rainfall situations in which there is
such a heavy convective component, confidence in daily rainfall
amounts is lower than normal, but it appears likely that multi-day
rainfall totals of 2-4", with locally (possibly much) higher amounts
can be expected through Friday night.
&&
.LONG TERM /SATURDAY THROUGH TUESDAY/...
As of 355 am Wednesday: Blocking pattern will persist into early
next week, with mean trough axis likely persisting just west of the
area, albeit evolving into more of a weakness in the subtropical
ridge, as strong anticyclone/Bermuda high retrogrades toward the
Southeast Coast. This will allow surface ridge associated with the
Bermuda high to build into the Southeast. It's not completely out of
the question that this could somewhat limit diurnal convective
chances somewhere across the region (not nec in our area) early next
week, but the stronger signal in the guidance suggests that diurnal
convective chances will remain enhanced through the end of the
period, perhaps getting a boost by day 7 in association with an
easterly wave (ugh) hinted at in the global models.
&&
.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
At KCLT and Elsewhere: Ongoing showers will continue at/near all
sites except KCLT this morning, generally being associated with IFR
cigs and periodic MVFR vsby. A lull in precip activity is expected
in the late morning, and cigs are expected to lift out of IFR by
midday, and VFR at some sites in the mid afternoon. Destabilization
this afternoon should result in an uptick in convective activity;
where possible a TEMPO has been included for TSRA, except at KCLT
where it is prevailed. Convection, at least in the form of heavier
showers, will linger through the evening, with lesser coverage
expected after midnight. The moist pattern remains tonight, so
deteriorating conditions are generally fcst again. The presence
of showers and still relatively weak low level forcing suggests
a similarly tricky fcst for restrictions Thu mrng, so confidence
is rather low.
Outlook: Deep moisture will persist. Showers, thunderstorms,
and restrictions will continue into late week. With the overall
pattern changing little, unsettled weather is expected to persist
into early next week.
Confidence Table...
11-17Z 17-23Z 23-05Z 05-06Z
KCLT High 81% Med 78% High 94% High 85%
KGSP Med 75% Low 55% High 100% High 100%
KAVL High 100% High 94% Low 47% High 100%
KHKY High 87% High 89% High 94% Med 60%
KGMU Med 69% Med 61% High 100% Med 75%
KAND Med 69% High 94% High 89% Med 60%
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:
www.weather.gov/gsp/aviation
&&
.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.
&&
SYNOPSIS...Wimberley
NEAR TERM...Wimberley
SHORT TERM...JDL
LONG TERM...JDL
AVIATION...Wimberley