Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
951 am EDT Tuesday Jul 29 2014
upper level low pressure will remain centered over eastern Canada
through the middle of the week...while cool high pressure settles
over the region through Wednesday. These cool airmasses will keep
temperatures below normal over the next few days. During the
latter half of the week and heading into the weekend...the upper
trough will retrograde westward...allowing for an increase in
moisture...moderating temperatures...and potential for showers and
Near term /through tonight/...
as of 950 am EDT Tuesday...
Current projection of arrival of isolated showers in the west
later this morning looks on track based upon the latest regional
radar trends. Have make minor tweaks to the temperature and dew
point grids based upon the latest observations...and expected
trends into the early afternoon. Made minor adjustments to the sky
cover to reflect a greater abundance currently over Southside
Virginia and neighboring sections of north central North
Carolina. Other than this minor adjustments...no other notable
changes for this update.
As of 400 am EDT Tuesday...
For the remainder of the morning...will see a few showers develop
along the western ridges from time to time...perhaps passing as
far east as the New River valley before dissipating in the
northwesterly downslope flow. A few hundredths of an inch rainfall
is likely across portions of our West Virginia/mountain Empire
counties...with amounts tapering down to a trace further east.
Winds across the area appear to be holding just strong enough most
places to keep the atmosphere mixed...which should limit fog
formation to isolated pockets in the deeper mountain valleys.
The main story for Tuesday will be another day of cool...early
autumn-like weather as a deep upper level low continues to drift
slowly near Hudson Bay Canada. Low clouds across the mountains
will lift and scatter by late morning...resulting in a mix of
clouds and sunshine through the afternoon. Will also see more
spotty shower activity develop across the mountains as subtle
disturbances pass overhead. With high pressure beginning to settle
into the area from the central Mississippi Valley...we can also
expect calmer wind speeds through the day which will cut down on
the chill that the stronger gusts produce. Still...if outdoors for
extended periods of time today across the mountains...you may
consider keeping a light jacket handy as afternoon highs range
from the middle 60s to the low 70s. Highs east of the Blue Ridge will
be warmer...climbing into the middle/upper 70s...perhaps touching 80
over the Southside.
Mostly clear skies and light winds will allow for strong
radiational cooling Tuesday night...allowing overnight
temperatures to fall into the 50s most locations...and in the low
to middle 40s in a few of the deeper mountain valleys. At this
time...think most locations will stay a couple degrees above
record lows...although it will be close for Danville.
Short term /Wednesday through Thursday/...
as of 230 am EDT Tuesday...
Models continue to be in good agreement that the full-latitude upper
trough across the eastern half of the United States relaxes some on
Wednesday as an upper level impulse moves south into the northern
plains. This...combined with the upper ridge in the Atlantic nosing
east...will shift the upper trough axis to the west on
Thursday...placing our area in an upper level southwest flow on the
east side of the trough.
Models are trending wetter in the mountains on Wednesday afternoon
as weak upslope flow and steep lapse rates ring out shallow
moisture. Without upper level support...coverage should be
very limited and inserted only a small chance for precipitation in
the western Greenbrier valley. Temperatures begin to modify on
Wednesday as 1000-850mb thicknesses increase. Guidance has shown
good run to run consistency with forecast highs from the 70s in the
mountains to 80-85 east of the Blue Ridge.
Center of a surface high is forecast to reside over southern WV/western
Virginia by 12z (8 am thursday)...leading to another cool night.
However...relative humidity cross sections indicate high level clouds begin to
stream over the region from the south...which could slow fall in
temperatures. Increased lows a degree...which keeps all climate sites at
least a few degrees above records for Thursday morning.
Moisture and warmer 850mb temperatures...along with an increase in cloud
cover surges north over our area Thursday. Increased highs a degree
or two...but abundant middle/high clouds will limit warming. Weak
southeast upslope flow and developing isentropic lift may generate
some showers along the Blue Ridge. Increased coverage for showers
Thursday and Thursday night along the Blue Ridge. Lows Friday
morning will also pushed up with anticipated widespread cloudiness.
Long term /Thursday night through Monday/...
as of 345 PM EDT Monday...
Upper 500 mb trough over the northeast United States amplifies on
Friday and Saturday with a significant short wave over the Ohio
Valley. Models similar in showing height falls on the southern end
of the trough and what may eventually be a cut off low over the lower
Mississippi Valley or southeast United States Monday.
Moisture begins to increase across the area on Friday but little in
the way of any organized lift to trigger any precipitation aside
from daily threat of airmass type showers and thunderstorms. With
little change in the air mass there will be minimal changes in high
and low temperatures each day. Temperatures will be near normal. Not
anticipating any extremes.
Aviation /14z Tuesday through Saturday/...
as of 715 am EDT Tuesday...
Patchy MVFR fog was noted at klwb and across the mountains early
this morning...but will lift shortly after 13z...leaving VFR
scattered-broken cloud cover. An upper level disturbance will arrive from
the northwest helping to trigger isolated to widely scattered
rain showers. Included vcsh at all mountain taf sites and
Otherwise...expect scattered to broken stratocu layer to develop by late
morning...greater coverage in the mountains...with cloud bases
ranging 3kft in the mountains to 5kft further east. Winds will be
lighter today compared to Monday with speeds generally 5-10kts.
Extended aviation discussion...
Persistent trough pattern in the east will gradually retrograde
west by the end of the week. Mainly VFR conditions are expected
through at least Thursday save for some late night/early morning
patchy MVFR fog in the mountain and river valleys. After
Thursday...with the trough axis farther west...this will allow for
a return of Atlantic moisture and a greater abundance of
showers...and potentially storms...in the forecast heading into
the weekend. This also will allow for increased chances of sub-VFR
conditions within the area of the showers...and then again
overnight in the form of light fog or stratus as boundary moisture
gets trapped under the nocturnal inversion.
July 30th record lows:
July 31st record lows:
the National oceanic and atmospheric administration weather radio at Wytheville Virginia...wz2500...broadcasting
on a frequency of 162.450 mhz...is off the air...due to phone line
issues. Phone company is working on it. No estimated time for
return to service.
long term...air mass