Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
651 PM EDT Monday may 25 2015
high pressure will continue its position off the coast of the
southeast United States. An upper level trough and associated
surface cold front will make gradual progression toward our region
from the central part of the country. This scenario will provide for
warm and humid conditions...with increasing chances of showers and
storms through at least Thursday.
Near term /through Tuesday/...
as of 645 PM EDT Monday...
Have backed off on the already low probability of precipitation
through 11pm in the far southwest part of the County Warning Area
and through 2am for the rest of the western County Warning Area.
Also raised dew points overnight based on the latest trends in the
observations and short term guidance.
Tonight into Tuesday the trend across the area will be for
increasing moisture levels in advance of an approaching upper
trough...and anti-cyclonic onshore flow around a surface high parked
to our east. The result will be increasing dew point values and
higher humidity. That moisture tonight is expected to generate low
level cloud cover under the base of the nocturnal inversion...so
that just about all locations will be mostly cloudy to cloudy by
sunrise Tuesday. Western parts of the area will become cloudy first.
This will also be the area to watch for the development of isolated
to scattered showers during the overnight hours. Precipitation
will start later and continue into Tuesday morning. Overnight low
will be milder than those of this morning thanks to the higher dew
point levels. Look for the upper 50s to around 60 across the
mountains...with low to middle 60s across the Piedmont.
Tuesday will be day with more cloud cover and better chances of
precipitation than that of today. First...the early morning low
level cloud cover will mix out as the day progresses. However...it
will only then redevelop as additional cloud cover at the lifting
condensation level by the afternoon. Showers will start to develop
by the late morning...with isolated thunderstorms a better potential
during the afternoon. Coverage will be greatest across the
mountains. While warm air advection will continue across the
area...the additional cloud cover and greater chances of
precipitation will keep high temperatures comparable to levels of
today. Areas in the west may be a few degrees cooler. Highs will
range from the middle 70s to around 80 across the mountains with low to
middle 80s across the Piedmont.
Short term /Tuesday night through Thursday night/...
as of 300 PM EDT Monday...
Axis of a highly amplified 591 dm upper 500 mb ridge will be centered
just off the East Coast at the start of this forecast period. The
ridge will resist displacement eastward as short-waves continue to
emerge from the trough over the central U.S. But eventually the
ridge becomes suppressed far enough to allow for more moisture and
energy to ride northward along its western periphery. Thus will be
increasing probability of precipitation with a distinct diurnal emphasis through much of the
period as the deep SW flow continues. Precipitable waters will slowly increase
through the period increasing the efficiency of rainfall in what
look to be mostly pulse-type thunderstorms. Storm Prediction Center outlook for
Wednesday shows only general thunder chances across the County Warning Area as the
shear and strongest dynamics remain to the west. Thursday-Friday
appear to be similar as the surface and upper ridge remain
influential in directing moisture around their western periphery
toward the southern appalachian region but best support for more
organized storms persists closer the mean trough over the central
and Western Plains. Again pulse-type afternoon convection will be
likely or at least at high chance with diminishing nocturnal
trend. Rainfall amounts likely to be highly variable as would be
expected in this convective environment but nothing at this points
suggests widespread enough for any significant flooding. Mainly
advisory level downpours are possible in slow-moving or stationary
storms. Temperatures will remain well above normal as the southwest flow
and considerable cloudiness do not allow for any incursions of
Continental polar air or overnight radiational cooling...but days
should not be extremely hot. Nighttime highs Wills run about +10
degrees above late may climatology while daily highs closer to +5 above
Long term /Friday through Monday/...
as of 300 PM EDT Monday...
In extended period the synoptic setup does not really change too
much. The ridge out over the Atlantic continues to direct a steady
supply of warmth and moisture into the eastern Seaboard. At the same
there will be a chance for somewhat more organized activity
associated with a slow-moving boundary trailing from a surface low
tracking into the Great Lakes on Saturday. The European model (ecmwf) shows this
feature becoming quasi-stationary to our north Ohio-PA-NJ axis on
late Saturday into Monday and then possibly sliding farther south
into our area by later Monday. GFS perhaps a bit further north
with this feature and more progressive and eventually builds high
pressure over the northeast which pushes a backdoor type front
toward the area late Sunday. GFS ensembles solution through this
period mostly maintain the ridge over the southeast in a position
that would hold the boundary and associated precipitation further
north. HPC guidance closer to Euro solution and was basically
followed for the extended. This all means continued well above
normal temperatures...especially at night...along with abundant
moisture and high chances for at least afternoon/evening
Aviation /00z Tuesday through Saturday/...
as of 125 PM EDT Monday...
VFR conditions will prevail through the remainder of the
afternoon into the the first half of the overnight hours. Isolated
showers or thunderstorms are possible late this afternoon into the
early evening along and west of a kblf-kukf line.
Late tonight into Tuesday morning...increasing low level moisture
will become trapped under the nocturnal inversion and yield
IFR/MVFR ceilings and patchy MVFR light fog. Once the inversion breaks
by late morning...look for an improvement to low end VFR ceilings
through mid-day. Isolated showers will continue to be a potential
across the western quarter of the forecast region late tonight
into Tuesday morning.
Extended aviation discussion...
An upper level trough across the central portion of the country
will make steady progress eastward during the week...and try to
squelch the influence of the upper ridge parked over the East
Coast of the U.S. This will allow for an increasing opportunity
for showers and storms across the area...with the focus for the
best coverage remaining across the western portions of the
Chances will increase through Thursday. Friday into
Saturday...troughiness refocuses across the central part of the
country and the upper ridge in the east re-intensifies over the
East Coast. This will return our pattern to one of more heat and
humidity with diurnally fired showers and storms in the west...as
compared to the ones with more upper level dynamics involved middle-