-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KRNK 221753
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
153 PM EDT Mon May 22 2017

.SYNOPSIS...
A weak cold front will slowly cross the region from the
northwest today before stalling over southern sections this
evening. Low pressure riding up the boundary overnight into
Tuesday will likely bring another round of significant rainfall
late tonight into Tuesday. Yet another area of low pressure will
arrive midweek with added rainfall likely for Wednesday and
Thursday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1115 AM EDT Monday...

Meso-low has moved into southeast Virginia with the western
edge of the precipitation swirling around the low exiting out of
Charlotte and Halifax Counties. Warm front is retreating north
slower than cold front is advancing from the west. Winds had
shifted to the west and northwest at the surface. Low clouds
were filling in behind the cold front over the mountains and
will continue to move as far east as the Blue Ridge but there
will be some breaks in the clouds in the Virginia and North
Carolina foothills and piedmont. Clouds will limit temperature
rise in the mountains but any sin in the foothills and piedmont
will bring temperatures into the 70s. Maintained chance of
showers along and ahead of the cold front with isolated to
scattered thunderstorms, mainly in the piedmont.

Surface front likely only to make it into southern sections
this evening before strong southwest flow aloft starts to ripple
more energy northeast along the boundary with lift quickly
returning north after midnight. However models continue to have
timing issues with the GFS and new EC quite wet by daybreak
Tuesday while the Nam a few hours slower. Since late day showers
may linger and then become more widespread sooner given such
fast flow aloft and quickly developing southeast flow aloft,
will lean toward likely pops returning north, espcly along/east
of the Blue Ridge late with decent chances elsewhere by morning.
Will again leave out thunder per redevelopment of the low level
wedge/stability overnight with forecast QPF not enough to
warrant any added flood watches at this point. Low temps again
cool with cool lows in the lower to mid 50s mountains to 60-65
east.

&&

.SHORT TERM /TUESDAY THROUGH THURSDAY/...
As of 310 AM EDT Monday...

The 00z/22nd deterministic and ensemble guidance remain consistent
in a big-picture sense, with a continuation of unsettled weather
conditions into the middle of the week. While finer details still
remain an open question on some days (described below), continued
periods of at least light to steady rain can be anticipated.
Intensity and duration of rainfall will govern potential hydro
concerns; while not presently high, it`s not out of the question.
Temperatures will trend near to cooler than normal initially, and
then cooler than normal into Wednesday and Thursday.

In mid to upper levels, broadly diffluent mid/upper level pattern
will exist across the mid-Atlantic, Ohio Valley and Southeastern
CONUS...downstream of a large upper trough expected to close off
into an upper low by Wednesday morning. Mid-level vorticity maxima
pivoting around the base of the trough/closed low, interacting with
a remnant frontal zone will induce surface wave low development
affecting the region on Tuesday. While guidance agrees on a period
of at least moderate rainfall focused by zone of 925-700 mb
frontogenesis, differences in timing and spatial extent continue
between the GFS and ECMWF. Feel best chance of steady/perhaps heavy
rain would be greatest in the NC foothills/Piedmont northeastward
into the Southside of Virginia late morning into mid-afternoon where
highest PoPs are focused, and lower from the western Blue Ridge
north and westward. Potential may exist for sharper northern
gradient in QPF than presently reflected in the coarser-resolution
guidance as well. Convective potential appears limited at best, due
to the expectation of poor/isothermal lapse rates limiting
instability values to no higher than 500 J/kg CAPE based on most-
unstable parcels.

Tuesday night should see a reduction in PoPs/more limited rainfall
chances with the passage of the weak surface low. However there are
still some questions as conveyed in guidance on if we can build a
weak CAD/wedge for the evening (GFS/SREF camp) or with a return to
south-southwest flow ahead of the closed low (ECMWF). In either
outcome, it`s a dreary outlook with lots of cloudiness and either
periods of drizzle/light rain showers or intermittent to steady
rain. Offered light rain/drizzle for this period with lower total
QPF.

The next round of at least steady rainfall appears to be on
Wednesday into Wednesday evening. Energy associated with closed
upper low and associated surface cold/occluded front approach our
southwestern counties Wednesday morning and move across the region
through the day on Wednesday. Here too, there are differences in
overall evolution between the GFS and ECMWF. The GFS solution
verbatim would introduce a potential strong/limited severe
thunderstorm threat Wednesday afternoon, limited by modest
instability values generally below 1000 J/kg MUCAPE albeit with high
vertical wind shear. The ECMWF on the other hand keeps deeper
moisture/QPF confined to the Gulf Coast, with comparatively
lighter/more gradual rainfall associated with weaker northward
moisture return. Couldn`t rule out the GFS solution yet but I feel
the ECMWF probably is closer to the right track. PoPs are again on
the high side for Wednesday, though begin to taper into Wednesday
evening with intermittent showers associated with upper low
overhead.

Into Thursday, upper low establishes itself along the western
Appalachians into central Georgia. Cold thermal profiles aloft
(below-normal 850 mb temperatures between +6 to +10C) owing to the
upper low. With some daytime insolation, expect cumulus to develop,
a few of which may develop into scattered slow-moving thunderstorms.
While instability values are low, atypically low freezing levels/wet-
bulb zero heights for late May would support potential for small
hail. It`s overstating things to call Thursday a washout, but be
aware that there should be a few showers/storms around. Will keep
lower-end Chance PoPs for now in the Piedmont and Blue Ridge
foothills, with higher-end Chance/lower Likely along and west of the
Blue Ridge with greater coverage closer to cold pool. Timing and
coverage would more closely follow the diurnal trend (i.e.
weakening/diminishing in coverage with loss of sun).

&&

.LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/...
As of 320 AM EDT Monday...

The upper pattern will then be progressive into the weekend as the
upper low moves off to the northeast Friday allowing for a brief
period of ridging, followed by a transition to a zonal regime by
Sunday. This will give us a mostly dry Friday after any lingering
upslope showers west of the Blue Ridge dissipate, but another
frontal system will push in from the north and stall over the region
on Saturday and bring an increasing chance of showers/storms to the
forecast for the weekend.

&&

.AVIATION /18Z MONDAY THROUGH SATURDAY/...
-- Changed Discussion --
As of 145 PM EDT Monday...

Last of the stable air and IFR stratus was moving out of
southwest Virginia as seen on satellite pictures and as
depicted on the Lifted Index analysis. Winds had come around to
the west for the rest of the area. A cold front extended from
southwest Pennsylvania to northern Georgia with scattered
thunderstorms beginning to develop along and ahead of the front
in northern North Carolina. Have a TEMPO group in the KDAN taf
for thunderstorms late this afternoon.

There will be a break in the precipitation this evening then once
this front advances farther into North Carolina deeper moisture
will return into southern Virginia with a wedge of high pressure
developing along with widespread light to moderate rain. Models
showed the best lift from 12Z/8AM through 18Z/2PM Tuesday.
Expect MVFR to IFR clouds to develop after midnight as surface
and low level winds back to the southeast Tuesday morning. High
confidence on the occurrence of MVFR ceilings and visibilities
with medium confidence on the timing.

Extended Aviation Discussion...

The chance of precipitation continues Wednesday and Thursday
with sub-VFR conditions associated with any of the
precipitation. Friday will be drier with a better chance of VFR
ceilings and visibilities. Another front reaches the area for
Saturday with more precipitation and sub-VFR conditions.
-- End Changed Discussion --

&&

.HYDROLOGY...
As of 333 AM EDT Monday...

Ongoing light to moderate rain should slowly taper from west to
east early this morning with locally heavy rainfall possible
over the next few hours. However not expecting rainfall rates to
be high enough to cause much in the way of flooding unless the
axis of heavier rainfall slows down and persists.

Otherwise will still have to watch late tonight into midweek as
more rainfall is expected, which could lead to small stream and
river flooding. Models continue to favor areas along/east of
the Blue Ridge with multiple rainfall events into Wednesday or
Thursday, possibly resulting in another 1 to 3 inches of rain
at this point.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.

&&

$$
SYNOPSIS...JH
NEAR TERM...AMS/JH
SHORT TERM...AL
LONG TERM...AL/MBS
AVIATION...AMS/JH
HYDROLOGY...JH/WP