Area forecast discussion
National Weather Service Blacksburg Virginia
508 am EDT Thursday may 23 2013
Synopsis...
a strong cold front will move through the region from west to east
this afternoon and tonight. Meanwhile...a deep upper low will
swing through the Ohio Valley and middle-Atlantic region through
Saturday...then shift off the middle-Atlantic coast Sunday. High
pressure at the surface and aloft will build across the region
Sunday through the first part of next week. Showers and
thunderstorms today will give way to much cooler...drier weather
for the weekend.
&&
Near term /through tonight/...
as of 430 am EDT Thursday...
Main concern through the near term will be the passage of the cold
front and associated convection. Radar and surface observation suggest that
that the actual front is near a day-cvg line. Models in general
agreement that the front will move into the western part of the
County Warning Area during the afternoon...reaching the eastern part of the County Warning Area
before midnight. European model (ecmwf)/GFS both suggest convective activity will
increase through the morning with potential linear development
early in the day...perhaps as early as daybreak. Radar suggesting
at least weak potential for this at the current time. GFS/European model (ecmwf)
suggest that the best potential for rain showers/thunderstorms and rain activity will be
across the Piedmont as the day progresses NAM suggests more
scattered...unorganized activity through much of the day...with
more organized linear activity late in the day. This theory is not
totally discredited as the GFS...especially...attempts to move
frontal activity east too quickly. Considerable instability and
moisture remain in place...so have advertised categorical
probability of precipitation...focusing on the highest probability of precipitation in the west during the first
half of the day...and the east during the later half. Have also
trended the better threat for thunder from the west through 18z to
the east after 18z. There remains some threat for severe given
the degree of instability...convective available potential energy > 1000 across the Piedmont and
lifted indices during the afternoon to -4 eastern areas as well.
However...overall coverage of severe should be less than observed
Wednesday. As of now...Storm Prediction Center has the area in a see text...mainly for wind
threat...but with colder temperatures aloft spreading into the area with
an upper low approaching...there should be some hail threat as
well.
For tonight into Friday morning...our attention will turn to an
anomalously deep upper low...a recurring pattern since middle-
winter...prognosticated to settle across the middle-Atlantic into the
weekend. A secondary vorticity lobe will sweep through late
tonight/early Thursday...and bring winterlike upslope clouds/-shra to
the western slopes of the alleghanys...mainly in eastern WV.
Increasing downslope should end precipitation across the Piedmont...with
only small chances for the remainder of the County Warning Area outside the far
western slopes of SW Virginia/northwest NC...and southeast WV as mentioned above.
Temperatures will be noticeably cooler by early Friday as we enter a
period of well below normal temperatures that will last into the
weekend. 850mb temperatures will drop to near zero c across the northern
portions of the County Warning Area by the weekend.
&&
Short term /Friday through Sunday night/...
as of 400 am EDT Thursday...
For Friday morning...will be under a weather pattern found more
often during the winter months...albeit warmer...with an 8mb
pressure gradient situated across our area as Canadian high pressure
builds in from the upper Midwest. Will start the day with residual
shower activity across the mountains that will linger into early
afternoon...but the main concern will be strong northwest winds that
will remain across the region through the day. With cloud cover
clearing out early...the stronger winds aloft will mix down to the
surface during the morning...and will not be surprised to see 30+
miles per hour gusts common across the mountains and 25 miles per hour gusts across the
piedmonts. Gusts will be stronger above 3000 feet...and believe we
will see advisory level gusts across the North Carolina mountains
into southwest Virginia. The Canadian high pressure will bring
significantly cooler temperatures...with Friday afternoon highs
ranging from the upper 50s to the low 60s across the mountains...to
the low 70s across the piedmonts. Keep in mind that the gusty winds
may add an additional chill to the air...so keeping a light jacket
handy may be a good idea.
Winds will remain somewhat gusty Saturday...although not as strong
as those on Friday...as a low pressure system deepens off the New
England coast. Will start the morning off with rather cool
temperatures...ranging from the middle/upper 30s across the mountains
to the middle 40s further east. Believe winds will remain strong enough
to prevent frost from developing in the cooler spots in the
mountains...however those with tender vegetation should keep an eye
on the forecast just the same...and be prepared to protect these
plants if necessary. At any rate...temperatures will rebound into
the low/middle 60s west...to the low 70s east by afternoon.
High pressure remains in control of the weather pattern on Sunday...
although temperatures will begin to moderate as low pressure over
New England begins to push northeast into Canada. Weather is
expected to remain dry once again...but will be keeping an eye on a
warm front stretching from the lower Great Lakes to the central
Appalachians that will lift northward during the day. This warm
front will play a role in our weather as we get into the next
workweek.
&&
Long term /Monday through Wednesday/...
as of 1245 PM EDT Wednesday
Expect cool trough to linger into Sunday before lifting east.
Meanwhile a strong trough digs into the Pacific northwest by Monday
with increasing heights arriving over our area next week. The
weather pattern will be mainly dry Sat night-Monday...but as we can
see with a westerly flow aloft the chance that convection that fires
along a warm front over the Ohio/middle Mississippi Valley could shift
east and affect our mountains.
Will lean toward an European model (ecmwf)/wpc blend solution. This will keep the
chance probability of precipitation in the area by Monday afternoon through Tuesday. The
warm front lifts north toward the Great Lakes by midweek with
heights continuing to build. At the surface...high pressure will be
situated across the middle Atlantic coast then shift off the coast
midweek. Dewpoints will be increasing again by next week with temperatures
warming back to normal or just above.
&&
Aviation /08z Thursday through Monday/...
as of 200 am EDT Thursday...
Frontal system responsible for convection last two days will
finally move through the forecast area late in the taf valid
period...generally from 20z west to 04z east. Pre-frontal
convection this morning should limit convective with actual
frontal passage. Main concern in the short term is residual
rain showers/isolated thunderstorms and rain traversing the region from SW-NE...mainly across
the Piedmont. See no reason for this activity to not continue
through the night. New and stronger convection...developing now in
western WV...west of crw...should reach lwb-blf late
tonight/early Thursday morning...reaching roa/bcb around
daybreak...then lyh/Dan during the middle to late morning into the
early afternoon. Expect mostly MVFR ceilings overnight...although VFR
at times with just middle-high clouds until stronger activity arrives
around daybreak. Scattered -shra should persist behind the stronger
convection until the front actually arrives during the afternoon.
At this point...pattern shifts to more of a winter pattern with
typical upslope clouds/-shra lwb-blf with MVFR ceilings. East...toward
roa/lyh/Dan...ceilings should improve to VFR with precipitation ending by late
afternoon/evening. Visibilities mostly VFR...except MVFR in areas of
br/-shra and/or thunderstorms and rain. Winds south-southwest-SW 5-8kts until frontal
passage this afternoon...then west-southwest-west 7-10kts...with low end gusts
blf and roa especially.
Medium confidence in ceilings and visibilities through taf valid period.
Medium confidence in thunderstorms and rain timing and coverage.
High confidence in wind speed/direction through taf valid period.
Extended aviation conditions...strong high pressure will bring
much drier air by Friday with VFR likely to prevail through the
weekend into early next week. Some upslope clouds linger into Friday
across southeast WV. Increasing potential for northwest flow driven thunderstorm
activity early next week...most conditions overall VFR for the
most part.
&&
Equipment...
the National oceanic and atmospheric administration weather radio at Roanoke Virginia is off the air this
evening due to a problem at the transmitter site. Restoration time
is unknown at this point.
&&
Rnk watches/warnings/advisories...
Virginia...none.
NC...none.
WV...none.
&&
$$
Synopsis...rab
near term...rab
short term...nf
long term...wp
aviation...rab
equipment...rab
