Area forecast discussion
National Weather Service Blacksburg Virginia
1001 PM EDT Sat may 18 2013
Synopsis...
a front extended from central Ohio into northeast North Carolina.
This front will linger across the middle Atlantic region through
Sunday night...acting as low level support to shower and
thunderstorm activity as an upper level trough passes across the
area through Monday.
&&
Near term /until 6 PM Sunday evening/...
as of 945 PM EDT Saturday...
Widespread light rain/drizzle covers much of the forecast area with
embedded heavier showers. So far not much thunderstorm activity as
it has been confined to the NC Piedmont. Heaviest rain activity is
moving up through the central and eastern Piedmont of NC from
Danville southeast to Raleigh NC. Some of this heavy rain will
likely skirt our County Warning Area overnight with potential for a half inch to
an inch of rain vicinity of yancyville and South Boston. Enhanced
rainfall...moderate rain showers...also likely to occur right
along the Blue Ridge where upslope enhancement will exist due to
the shallow easterly low level flow. The upper low is still
centered west of the Appalachians...so threat of showers can be
expected even west of the mountains overnight. Convergent flow from the
westerlies intersecting the low level easterlies still points
toward potential for persistent moderate to occasionally heavy
rain for the NC mountains will leave Flood Watch for this area intact
for now although rain amounts the last 6 hours has been under a
quarter of an inch. Our local WRF suggests there will be an uptick
in shower intensity over the NC mountains after midnight.
Despite the overall light rain so far this afternoon...the
ingredients remain for the possibility of heavy rain to pass
across the area through late Sunday evening. Still have a weak
wedge of high pressure situated against the Appalachians as
indicated by light easterly wind across our area...while a large
upper level trough approaches from the west. Precipitable water
values are approaching 1.5 inches east of the mountains...and are
expected to reach around 1.75 inches tonight/Sunday
morning...which is well above normal. Also...upper level winds are
also expected to be light...which will make for slow moving cells.
All of these factors may lead to very efficient rain producing
storms that will be more pulse variety compared to what was
observed today. Regardless...these rain makers may result in flash
flooding... especially if the cells begin to train over the same
locations.
As for temperatures...high dewpoints and abundant cloud cover will
make for a mild night...with overnight lows holding in the upper
50s west to the low/middle 60s east. Because of the very moist
conditions...will also be in for some patchy fog areawide.
Scattered rainfall and widespread cloud cover will limit heating
for Sunday...although middle and high clouds will decrease somewhat
as drier air works its way in aloft. As such...will be looking for
highs in the low to middle 70s.
&&
Short term /6 PM Sunday evening through Tuesday/...
as of 325 PM EDT Saturday...
The area will still be involved with showers and storms associated
with the current slowly moving upper level trough. However...most of
the guidance offer solutions that places the axis of the trough
over...or just east of the area by daybreak Monday. This will begin
a trend where we will experience less precipitation across the
western portion of the forecast area...and a greater amount over the
east. Model timing of the trough is fairly consistent that it will
be east of the area by daybreak Tuesday. Tuesday will begin a day of
transition between the departing trough on the East Coast...and our
focus turning to one that will be developing across the upper
Mississippi Valley. In between the two...very brief upper level
ridging will take place across the area. However...enough low level
moisture will remain so that we cannot rule out some isolated
showers or thunderstorms across the region on Tuesday. The influence
of the upper ridge should help keep the convection from becoming too
deep.
Temperatures through the period will trend milder. By
Tuesday...highs in the middle to upper 80s are expected across the
Piedmont with a mix of 80s in the mountains. The highest elevations
will be in the upper 70s.
&&
Long term /Tuesday night through Saturday/...
as of 1215 PM EDT Saturday...
The area only looks to get a brief break in significant generous
coverage of shower activity during the Tuesday night into Wednesday
time frame. It will be during this period that a brief upper level
shortwave ridge builds over the area between the parting closed low
that is currently giving the region precipitation...and the next
system that will impact our region late Wednesday night through the
start of the next weekend. Although...we cannot rule out isolated to
scattered showers or even a few storms during the Tuesday night into
Wednesday time period. Low level flow will already brining
additional moisture into the region from southwest in advance of the
next system.
Our next system is currently prognosticated to be associated with an upper
level closed low that will pinwheel around the upper Mississippi
Valley Wednesday through at least Friday. Plenty of Gulf of Mexico
moisture will be advected northward into the region in advance of
this system. Also...periodic bits of energy will spiral eastward
from the main system and combine with daytime heating to promote a
generous coverage of showers and storms over the region through at
least the start of Thursday night. Model guidance varies as to the
speed of the associated surface cold front. HPC guidance is favoring
a slower solution that is more in line with the operational 12z/8am
GFS that doesn/T have the front exiting the area until Friday
evening. Have follow closely with this scenario.
Temperatures through this period of the forecast will trend
cooler...with highs on Saturday expected to be in the low to middle 60s
across the mountains with middle 60s to near 70 across the Piedmont.
&&
Aviation /02z Sunday through Thursday/...
as of 800 PM EDT Saturday...
Easterly low level flow will result in MVFR and IFR ceilings
along and east of the mountains overnight and early Sunday.
Areas of light rain...drizzle...and fog will also restrict visibilities
to 1-3sm at times. Can't rul out a thunderstorm overnight...but
mainly over the Piedmont vicinity of kdan. Models indicate chance
that rain may become moderate to heavy vicinity of the Blue Ridge
overnight. Radar has not supported these trends...so will leave
out of the forecast for now. Think the more likely scenario is
formation of low stratus associated with a persistent drizzle or
light rain. Either way...flight conditions still poor...and are
expected to remain that way through Sunday night.
Conditions look to improve Monday as the upper trough moves
to the middle Atlantic coast. Expect Tuesday to be the best bet for
decent flying conditions although an isolated afternoon airmass
shower or thunderstorm is still possible. Next front approaches
on Wednesday with more showers/storms developing. This front does
not appear to be in a big hurry and looks to bring MVFR
conditions...possibly IFR at night...to taf sites for the latter
half of the week.
&&
Rnk watches/warnings/advisories...
Virginia...Flash Flood Watch until 8 am EDT Sunday for vaz009-015.
NC...Flash Flood Watch until 8 am EDT Sunday for ncz001-002-018.
WV...none.
&&
$$
Synopsis...nf
near term...PM
short term...km
long term...ds
aviation...PM
