Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
557 am EDT Tuesday Mar 31 2015
a deep low pressure system will pass east across the southern Great
Lakes today...bringing gusty winds and warm temperatures. The low
will drag a cold front across the area tonight...triggering spotty
shower activity across the mountains. Another cold front will move
into the region and stall Thursday through Friday...bringing an
extended period of rain and mild temperatures to the region.
Near term /through tonight/...
as of 430 am EDT Tuesday...
Starting the morning off with high pressure situated over the
southeast states...while a deep low pressure system is passing over
the Chicago area. Winds across the central Appalachians have yet to
respond to the low's influence...and remain out of the west...with
the downslope windflow helping to keep temperatures across the area
ranging from the middle 40s to the upper 30s.
Winds across our area will shift increasingly southwesterly this
morning as the low passes east across the southern Great Lakes. Warm
air riding into the area in the southwesterly flow will combine with
abundant sunshine under mostly clear skies to push afternoon
temperatures in the middle to upper 60s across the mountains...and into
the low 70s east of the Blue Ridge. The passage of the low across
Ohio/Pennsylvania during the afternoon will cause the pressure
gradient along the appalachian chain to tighten...resulting in gusty
winds from late morning into the evening. Most of our area can
expect wind speeds sustained between 15 and 20 miles per hour...with gusts as
high as 30 miles per hour. Higher ridges across the mountains may see gusts
reach as high as 40 miles per hour at times. That stated...do not believe we
will reach criteria over an large enough area to justify a Wind
Advisory today...although it will be close in some areas.
Of greater concern are the very dry conditions already in place to
start the day. Warm temperatures and dry air will cause afternoon
relative humidity values to fall to around 20 percent for most
locations. Combine this with gusty winds and dry fuels such as Leaf
litter and grasses...and any fires may quickly burn out of control.
See the fire weather section below for further details.
The low pressure system will continue to pass across the Delaware-Maryland-Virginia
during the evening...and will drag a cold front across our area. Not
much moisture for this front to work with...so expect only a few
showers through midnight...mainly across the western ridges. High
pressure will then build in from the west...settling across the
central Appalachians toward dawn on Thursday.
Short term /Wednesday through Friday night/...
as of 415 am EST Tuesday...
Upper flow will becoming increasingly progressive through the time
frame as the northeast U.S./Eastern Canada upper trough finally
lifts out. Wednesday should be a pleasant day and the best day of the
short term period as weak high pressure drifts across the region
under a short wave ridge. 850mb temperatures drop 4-5c from readings on
Tuesday but remaining mostly in the 5c range which with ample sunshine
should still support near to slightly above normal temperatures.
Winds will be much lighter than on Tuesday.
The remainder of the short term period becomes messy and
complicated as the northern and southern streams remain out of
phase. Despite the complications...the medium and long range
models remain in general agreement on the pattern supporting a
frontal system drifting into the northern part of the
region...mainly north of I-64...late Thursday associated with a
northern stream short wave. The front will become quasi-stationary
along or just north of I-64 awaiting the next upstream short wave
to kick the front further south. With the antecedent high pressure
drifting off the southeast U.S. Coast early in the
period...increasingly moist Gulf flow will overspread the region.
This will support waves of precipitation along the
boundary...mainly north of the region Thursday...drifting southward
through the region primarily Friday/Friday night...finally shifting
south-east of the area early Sat as upper flow amplifies slightly
and high pressure builds eastward from the central/Southern
Superblend probability of precipitation were way overdone for Thursday. This type of pattern
often exhibits such characteristics as moisture streams across the
region from the southwest...but forcing remains well north-west of
the region. Thus...reduced probability of precipitation considerably for Thursday from model
advertised probability of precipitation...with just slight chance-low end chance probability of precipitation far
western areas...with little or no precipitation expected across
the Piedmont. Likely to categorical probability of precipitation were introduced for Friday-
Friday night from west to east as the upper flow amplifies providing
better support for the frontal boundary to finally shift southward
through the County Warning Area.
Given the reduced probability of precipitation for Thursday and the overall lack of
forcing...expect thunder threat to remain west of the Alleghany
front at that time. For Friday...most models advertise enough
instability to support the continued mention of thunder during
the afternoon/early evening. The best instability is realized east
of the Blue Ridge Friday afternoon/early evening...but falls far
short of any severe criteria at this time.
Quantitative precipitation forecast is also modest...ranging from 1 to 1.5 inches west to less
than 1 inch east Thu-Sat. Given the recent dry weather...this
rainfall will not cause any issues.
As noted above...Wednesday should provide seasonable temperatures. Thursday-
Friday should see above normal temperatures...especially east of the
Blue Ridge with increasing southwest surface flow. Ample sunshine
Thursday east of the Blue Ridge could boost temperatures well into the
70s there...with 50s and 60s to the west. Friday morning should be
quite mild with lows mainly in the 50s...even near 60 across the
Piedmont. Model guidance temperatures Thursday-Friday appeared a little on the
cold side and were adjusted upward...also for better agreement
with neighboring offices.
Long term /Saturday through Monday/...
as of 430 am EDT Tuesday...
The aforementioned frontal system will finally shift south-east of
the region Saturday morning allowing for drier conditions for the
weekend. Given that the upper flow is zonal and the Post high
pressure area is of Pacific origin...only modest cooling is
expected Saturday...recovering to near or above normal
temperatures quickly sun-Mon. A few areas could briefly touch the
freezing mark early Sunday...but overall temperatures are expected
to remain well above freezing through most of the period. Gusty
winds will likely be an issue behind the front Saturday.
Mild...dry high pressure will drift across the region and to the
south sun-Monday arrives for Sunday into Monday...moving offshore
early Tuesday. Beyond this...it appears yet another series of short
waves will bring unsettled conditions back to the area.
Temperatures are expected to remain above normal.
Aviation /10z Tuesday through Saturday/...
as of 200 am EDT Tuesday...
Another clipper low pressure system will pass north of the area this
afternoon...bringing another round of strong west southwest winds
with gusts again topping 25-30 kts at times. However other than for
some upslope driven late day MVFR/VFR ceilings...and spotty showers
kblf/klwb vicinity...cloud cover/precipitation should be limited.
Weak high pressure builds in late tonight...resulting in VFR under
lighter winds into Wednesday.
High confidence in ceilings...visibilities and winds through the taf
Extended aviation forecast...
Cold front will dip southeast into the area by the end of the week
resulting in periods of sub-VFR in lower ceilings/rain showers from Thursday
afternoon into early Saturday as the boundary oscillates across the
region. Improving conditions appear likely later Saturday as high
pressure starts to bring drier air in from the west on gusty northwest
winds Saturday afternoon.
as of 500 am EDT Tuesday...
Westerly downslope windflow has maintained low dewpoint temperatures
and limited moisture recovery through the night. A deep low pressure
system will pass across the southern Great Lakes today...causing
winds across the central Appalachians to shift more southwesterly.
Warm air building in and abundant sunshine will cause temperatures
to rise quickly this morning...and relative humidity to fall to near
20 percent by middle afternoon. The passage of the low pressure system
will cause wind speeds to increase as well...with winds during the
afternoon becoming 15 miles per hour to 20 miles per hour sustained...with gusts as high
as 30 miles per hour for much of the area...possibly reaching 40 miles per hour across the
higher ridges. The combination of dry air and fuels will heighten
fire danger for Virginia and North Carolina...especially within a 2
to 5 hour window during the afternoon. Conditions will improve
during the evening as a cold front brings cooler air into the area.
Virginia...red flag warning from 11 am this morning to 6 PM EDT this
evening for vaz018>020-022>024-034-035.