Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
341 am EDT Tuesday Jun 2 2015
a cold front near the Appalachians will stall along the Blue
Ridge today into tonight before drifting south into the Carolinas
on Wednesday. This in combination with waves of low pressure
riding up along the boundary...and a developing wedge of cool air
will keep periodic rounds of showers...and storms going into
middle or late week.
Near term /through tonight/...
as of 240 am EDT Tuesday...
Overall forecast remains a rather wet one over the next 24 hours but
complicated in regards to placement of deeper convection today and
whether or not will see widespread enough precipitation for flooding
concerns espcly Blue Ridge east overnight. Main culprits remain
with a cutoff 500 mb low just west of the mountains today and a weak
surface front drifting east toward the Blue Ridge that will
interact with deep precipitable waters to get bands of convection going this
Latest guidance suggests best chances for slow moving rain showers/thunderstorms and rain to take
shape over the east ahead of the front by this afternoon...and then
across the SW later as the boundary swings over a bit more in backdoor
fashion under cooler air aloft. This supported by better diffluence
along the eastern quadrant of the upper dynamics out west...and
with convergence ahead of the boundary east where will slowly get
trajectories switching around to a more easterly component under
the 850 mb Theta-E gradient later on. Best instability although weak
also prognosticated over the south and east with broad upper diffluence
that could eventually cause rain showers/thunderstorms and rain to merge along/east of the
Blue Ridge by this evening per latest ensembles. Thus appears best
chances for isolated pulse severe mainly south/east with the
corridor of heavier rainfall southeast-SW but gradually filling in along
the Blue Ridge as the flow turns more upslope late. This supports
upping probability of precipitation from chance after a lull this morning to likely/categorical
in all except the far northwest this afternoon where will stay just behind
the front. High temperatures remain tricky given a mild start but with
more clouds and iffy timing of onset of the developing weak wedge
later today. MOS values have cooled quite a bit but likely are too
cool parts of the west/south where think any cool advection will
hold off until this evening. However likely enough convection to
limit rises so trended with the warmer mav which still keeps most
in the 70s this afternoon.
Convergence looks to get pinned in along a corridor from the Virginia Blue
Ridge east overnight with a conveyor belt of deep moisture running in
from the southeast along the stalled 850 mb front and under the 500 mb low. This
within a broad area of upper diffluence that should be enhanced by
upslope flow as The Wedge deepens. Soundings suggest much of this
should be more stratiform in nature given deep saturation and an
inversion aloft. However cant totally rule out elevated convection per
coolness aloft and prognosticated lift on the east side of the 500 mb low. This
would make a difference in rainfall rates overnight as could be enough
for flooding issues pending the coverage of heavier rain showers/thunderstorms and rain today.
Since coverage could again be more scattered nature until late this
afternoon and overall ffg rather high...will hold off on issuing a
Flood Watch for now. Otherwise keeping persistent likely to categorical probability of precipitation
going in a swath roughly from the Blue Ridge east in Virginia tonight and
chance elsewhere. Low temperatures should be a little cooler within The Wedge
with lows mostly upper 50s to lower 60s.
Short term /Wednesday through Thursday night/...
as of 310 am EDT Tuesday...
A well defined upper low centered on the central Kentucky/Tennessee
border this morning slowly shifts eastward across the southern
Appalachians into the Carolinas. On Wednesday morning...the upper
low is located in eastern Tennessee and travels eastward to The
Spine of the Appalachians and opened up into more of an upper trough
by Thursday night.
Along the low/trough axis and on its eastern flanks...expect daily
showers and thunderstorms across the region. The best chance of
precipitation will occur across the eastern half of the region. At
the surface...high pressure will push south along the Lee of the
Appalachians on Wednesday...resulting in cool wedge. Low level
stable air will limit the potential for thunderstorms on Wednesday.
High temperatures on Wednesday will be cooler than Tuesday with
readings from the lower 60s in the north to the lower 70s in the
south. Cloudy skies with scattered showers continuing Wednesday
night. Low temperatures Wednesday night will vary from the lower 50s
in the northwest mountains to around 60 degrees in the Piedmont. The
coverage of storms will increase again on Thursday the axis of the
upper trough passes overhead...and convection becomes enhanced by
the steep lapse rate of the corresponding cold pool aloft...despite
relative stability closer to the surface. Kept high temperatures
cool on Thursday with values from the upper 60s in the mountains to
the middle 70s in the Piedmont. Shaped probability of precipitation Thursday night towards
superblend with slight chance in the west to scattered in the east.
Low temperatures Thursday night will range from the middle 50s in the
west to the lower 60s in the east.
Long term /Friday through Monday/...
as of 230 PM EDT Monday...
The axis of the upper low will continue its slow progression
eastward. This will keep the area in a regime prone to additional
scattered showers and storms. Coverage will become more diurnal in
nature heading into the weekend as the upper level support shifts
east of the coast. Sunday into Monday...we expect another increase
in coverage across the area as a shortwave trough enters and moves
through the area from the Great Lakes region. Temperatures during
this portion of the forecast are expected to be slightly milder than
normal and slightly cooler than normal.
Aviation /08z Tuesday through Saturday/...
as of 1255 am EDT Tuesday...
Boundary layer is very moist from showers/storms which took place
Monday afternoon/evening. This will lead to areas of fog and
stratus late tonight and early Tuesday with MVFR ceilings/visibilities likely
with potential for a period of IFR ceilings/visibilities around daybreak.
However low confidence in exactly how low ceilings/visibilities will go
espcly in areas that will see more middle deck espcly along the Blue
Ridge. May also see added rain showers over southern areas around kdan
overnight with less coverage elsewhere where things have been
Considerable cloudiness is expected Tuesday per upper trough of
low pressure which is forecast to be centered over the central
Appalachians. This will lead to another day of numerous
showers/storms...especially for the afternoon and evening hours. Front
bowing south into the region will make for a lingering convergence
axis with more convective nature precipitation to the south and the
development of lower IFR type stratus over the north where will
see NE flow kick in. This may also tend to limit deep convection
to southern/eastern areas early Tuesday afternoon before shifting
west of the Blue Ridge as the front pivots SW under the developing
wedge. Thus will include a period of MVFR in rain showers/thunderstorms and rain at all
locations Tuesday afternoon.
Wedge deepens Monday night while the flow turns more southeast over the
cool pool making for a good overrunning setup to the east of the
upper low along the mountains. In addition...upslope combo likely
to cause widespread IFR ceilings with MVFR/IFR visibilities in fog and added
rain showers/rain into Tuesday evening.
Model trends suggest the upper trough will stall over the region
for most of the week (through at least thursday) resulting in a
prolonged period of low ceilings (mvfr and possible ifr) and
opportunity for showers and thunderstorms.
The trough of low pressure is expected to weaken by the weekend.
This will allow for higher cloud bases...however...there will be
a continued threat for diurnally driven showers/storms in addition
to late night-early morning fog.