Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
750 PM EDT Tuesday may 26 2015
high pressure off the southeast coast will maintain a broad
southerly flow of deep moisture into the region through much of
this week. An upper level trough of low pressure to the northwest
will gradually slide east and interact with this moisture across
the area. This will result in periodic rounds of showers and
storms over the next few days.
Near term /through Wednesday/...
as of 600 PM EDT Tuesday...
Made some adjustments...mainly to pop/weather grids at this time in
concert with latest hrrr and NSSL WRF simulated radar
forecasts...which seem to be in line with latest synoptic scale
model trends as well regarding precipitation. Initial area...now
exiting the southern Shenandoah Valley into lwx County Warning Area...will
continue to move northeast away from the area. The next area to
watch is now down in Georgia packing quite a punch there. Feel
that the remnants of this will work its way northeast into our County Warning Area
toward midnight and several hours thereafter...albeit in a
diminishing state. Thus...have decreased probability of precipitation for the next few
hours...then increased from the southwest from 06z Onward toward
daybreak western half. Used a blend of synoptic and mesoscale
model probability of precipitation through 12z...leaning heavily toward the hrrr.
Temperatures were a little too warm in rain cooled areas and a little too
cool across the Piedmont. Made some minor adjustments to bring
better in line with current observations. Otherwise...remaining
grids look good at this time and did not need adjustment.
As of 215 PM EDT Tuesday...
Bands of convection continue to develop and push slowly NE mainly just
west of the Blue Ridge where 1000 j/kg ml cape exists and moisture is
deeper near a faint shortwave lifting NE. Most short term guidance
continues to follow suit with the latest hrrr and hires models sliding
clusters of deeper convection NE...passing across The Highlands and southern
Shenandoah Valley by early this evening. Either side of this
axis...expecting only isolated coverage mainly along the Blue Ridge
with little over the far east per ridging...and far west where in
between lift sources. Thus will have continued likely probability of precipitation for a while
longer over parts of the west into this evening and low to slight
elsewhere before trimming back toward sunset.
Main concern overnight shifts to energy now well to the SW along the
leading edge of the next upstream 500 mb trough that will help eject a
shortwave NE into the western portions after midnight. Models disagree
on the degree of residual deep convection with this feature with some
holding it together over the far west late and others fizzling
coverage into lighter showers by daybreak. Given speed of movement with
this feature and prognosticated strong 850 mb Theta-E along with good moisture
transport off most guidance will reintroduce likely probability of precipitation west
after this evening...and slide better coverage toward the Blue
Ridge early in the morning as convection weakens. Left temperatures close
to the latest mav MOS with lows mostly in the 60s per deep
Weak dampening wave slides east across the region Wednesday followed by
another weak shortwave over the SW late in the day. Despite deep
moisture and high precipitable waters espcly east...timing of this feature and degree
of heating/instability remains in question. Latest sref consensus would
suggest initial batch of rain showers will fade near the Blue Ridge Wednesday
morning followed by redevelopment across eastern sections in the
afternoon where heating and forecast convective available potential energy appear best. Potential
more of a west wind and drying aloft per latest soundings could
limit things in the mountains where clouds may linger longer.
Therefore trimmed back probability of precipitation a bit but upped along/just east of the
Blue Ridge where convergence looks better. However overall likely
to chance probability of precipitation with some heavy rain potential given even lighter
steering that today. Followed closer to the cooler mav temperatures given
more clouds and moisture/rain showers around early.
Short term /Wednesday night through Friday night/...
as of 225 PM EDT Tuesday...
The area resets in the sense that the extra influence of an upper
trough heading through the northern end of the upper ridge ends.
What is left of the trough is almost washed out and heads through
southeast Canada Wednesday night into Thursday. What remains is the
persistent southeast U.S. Upper ridge...and there will be a new
upper trough deepening in northern plains states. This will place
our area in a scenario comparable to the conditions of today. There
will be plenty of moisture in the low levels with good daytime
heating. Upper level support will be at a minimum...so shower and
thunderstorm activity during this portion of the forecast will be
primarily diurnal in nature. A rouge bit of energy could get ejected
eastward off the main upper trough to help enhance or prolong
coverage...but at this time...projecting accurately when or if that
happens would be challenging.
Long term /Saturday through Tuesday/...
as of 300 PM EDT Tuesday...
Saturday into Sunday the next upper trough makes headway into the
ridge...just like its predecessor a few days prior. Anticipate this
feature to generate a greater coverage of showers and
storms...especially across the western half of the area. This
associated surface cold front is expected to the area Sunday
night....and stall over or just to our south on Monday. The front
will buckle back north early next week as southerly flow around the
west side of the high re-establishes itself. This will keep the
chance of showers and storms in the forecast...again with the focus
with the greatest concentration in the west.
Aviation /00z Wednesday through Sunday/...
as of 740 PM EDT Tuesday...
Forecast area was well into a moist air mass this evening but
radar showed the only isolated showers were just west and north of
kblf. High confidence there will be a break in the precipitation
over the mountains through midnight. Then a weakening cluster of
showers and thunderstorms will move into the region from the
southwest. Broad SW flow will bring the precipitation into kblf
and krnk first and with the slow eastward movement of the
showers...it may not be until 11-12z/7-8am before the rain reaches
NAM BUFKIT and other short term models bring MVFR clouds into
klyh and kdan overnight. Low confidence as to how low ceilings
will be but any MVFR clouds will erode after sunrise on Wednesday.
Low level winds may briefly turn to the west over the mountains
Wednesday afternoon as the showers and thunderstorms progress into
the foothills. Low confidence that showers and thunderstorms will
make it as far east as klyh and kdan before the 00z end of the taf
Extended aviation discussion...
Showers an thunderstorms may be somewhat more focused along and east
of the Blue Ridge Thursday.
For the remainder of the period...a moist unstable air mass
remains across the region with weak triggering mechanisms and
dynamics. A frontal system will approach from the north by the
weekend...but will likely remain well north of the County Warning Area through
Sunday. Thus...no change in air mass or the overall synoptic
pattern is expected through the period.
Early morning low clouds/fog and associated IFR-LIFR ceilings/visibilities become
increasingly more likely through the period...especially following
any late day rainfall. Otherwise...expect mostly VFR-MVFR ceilings
outside heavier showers/thunderstorms.