Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
735 am EST sun Dec 21 2014
high pressure over the Ohio Valley will move east today and wedge
down the east side of the Appalachians tonight. Low pressure will
remain along the Carolina coast today through Monday. A very
strong cold front will move through the region on Wednesday...bringing
rain and gusty winds to the area.
Near term /through tonight/...
as of 330 am EST Sunday...
Surface high pressure over the Ohio Valley moves east today then
wedges down the east side of the Appalachians tonight. By late
tonight low pressure will develop along the southeast coast and help
re-enforce The Wedge.
Deep westerly flow on the 00z soundings. Once weak short wave
crossing out of the Tennessee Valley crosses the forecast
area...winds diminish which will result in most low clouds eroding
and light precipitation will end. BUFKIT showed there will be some
breaks in the cloud cover after noon.
Clouds fill back in from the south tonight. Models similar with
timing of isentropic lift and upslope southeast winds...bringing
the probability of precipitation into the southern County Warning
Area after midnight...then spreading north through 12z/7am Monday.
Stayed close to mav guidance for temperatures through the period.
Short term /Monday through Wednesday night/...
as of 330 am EST Sunday...
For Monday...an upper level trough approaching from the west will
support low pressure deepening off the Carolina coast...while high
pressure will be wedged against the eastern face of the
Appalachians. Moisture radiating inland from the low will already be
spreading across our area by sunrise...and forecast temperature
profiles support a mixture of sleet and freezing rain across the
mountains throughout the day...as rain falling from the warm air
riding in aloft from the South Falls through the cold air being
drawn in by the high pressure from the northeast. East of the
mountains...surface temperatures are expected to be warm enough that
all precipitation will remain liquid. Best chances for precipitation
will be east of the Blue Ridge...closer to the low passing along the
coast...with precipitation chances diminishing further west as the
moisture fights the dry air already in place. Precipitation will be
most widespread during the morning and afternoon...and will decrease
by early evening as the coastal low begins to push away from the
area. Strong northeasterly flow and widespread precipitation will
strictly limit heating through the day...and have undercut model
guidance for temperatures...holding highs in the 30s and low 40s
For Tuesday...the high pressure wedge will only slightly weaken...
but will do so sufficiently to support warmer temperatures...making
for rain across the region...perhaps with a residual and isolated
pocket of light freezing rain in the mountains. Rain chances will
begin to increase during the evening as a deep upper level trough
approaches from the Southern Plains...supporting development of a
low pressure system along the Gulf Coast. The low will lift north
into the Tennessee Valley overnight...resulting in widespread rain
reaching the central Appalachians before sunrise Wednesday.
As the low lifts toward the Great Lakes on Christmas evening...it will
continue to deepen. With it...the low will drag a cold front
eastward...which will enter our western most counties by around
sunset. Strong low level winds associated with the low may reach to
the surface at times...and may result in sporadic tree damage and
power outages. Also...with precipitable water values reaching the
ballpark of 1.2 inches...may see locally heavy rain as the cold
front begins to pass across the area.
The cold front will pass rapidly east late Christmas evening evening...
and may see a brief shot of strong winds in the wake of the
front...especially along the higher ridges. Precipitation will
rapidly diminish however as drier air works in from the
west...resulting in only residual upslope showers that will change
over to snow by sunrise Christmas day.
Long term /Thursday through Saturday/...
as of 300 PM EST Saturday...
West to northwest upper flow will continue into Thursday with
mountain rain and snow showers. Highs Christmas day will vary from
around 30 degrees in the northwest mountains to around 50 degrees
in Piedmont. Winds will be quite strong on Thursday as a 50 to 55
knot 850mb jet pivots across the region. At this time...looks like
values should remain below Wind Advisory levels. As high pressure
slides off the coast...dry weather is expected Christmas
night/Thursday night with low temperatures generally in the upper
20s and lower 30s.
The upper flow shifts to the southwest Friday into Friday night as
trough curves out in the central US with surface low pressure developing
over the Midwest with a frontal boundary trailing south. Highs on
Friday will vary from the lower 40s in the mountains to the middle 50s
in the Piedmont. Moisture slowly return Friday night into Saturday
as frontal boundary approaches. Lows Friday night drop into the
lower and middle 30s in most locations. A cold front moves across our
region Saturday into Saturday night.
Aviation /12z Sunday through Thursday/...
as of 720 am EST Sunday...
Low clouds covered the region overnight. Ceilings were mainly MVFR
in the mountains and VFR along and east of the Blue Ridge. The
clouds were clearing slowly from west to east with satellite
pictures showing the western edge very close to kblf. Clearing
will continue during the day.
A wedge of high pressure will build south down the east slopes of
the Appalachians tonight and will be enhanced due to low pressure
off the Carolina coast. Rain will spread from south to north after
00z/7pm. High confidence that kroa/kdan/kbcb/klyh will have rain
after 06z/1am...but it may be at the end of the taf period that
the precipitation reaches kblf and klwb. Some freezing rain and
sleet are possible late tonight...especially at klwb. Ceilings
will lower to MVFR as the rain begins.
Monday afternoon through Tuesday...MVFR with showers.
Tuesday night and Wednesday...MVFR with showers and the passage of
a strong cold front. Low level wind shear possible as well as gusty surface winds
ahead of the front and following the front.
Wednesday night and Thursday...gusty northwest winds. MVFR clouds
and rain/snow showers in the mountains. VFR conditions east of the
near term...air mass