Debate de meteorólogos científicos
Area forecast discussion
National Weather Service Blacksburg Virginia
414 PM EST Thursday Dec 5 2013
a slow moving cold front moves east into the Appalachians
tonight. The boundary crawls across Virginia on Friday. Arctic
high pressure well behind the front will wedge south along the
mountains Saturday...before interacting with precipitation
returning north by Sunday. Southwesterly flow returns Monday
before a stronger cold front arrives late Monday night.
Near term /through Friday/...
as of 320 PM EST Thursday...
A strong cold front located to our west this afternoon from cle to
Lex to mem will move east tonight into Friday. Weak shortwaves move
northeast in the warm section will result in scattered showers.
Leaned probability of precipitation this evening into tonight towards a Bland of the
rnkwrfarw and NAM. The GFS also showed some light precipitation
along the southern Blue Ridge mountains. Increased probability of precipitation along the
western mountains after 06z Friday. The main body of rainfall
should remain will remain to our west tonight. A very warm and
muggy overnight expected with readings from around 50 degrees in
the northwest mountains to the lower 60s in the southeast.
Surface cold front will be entering the mountains Friday with a wave
of low pressure which develops along the boundary and ripple
northeast across the region. Main axis of rain should be focused
along and just west of the frontal boundary Friday. Felt there was
enough instability to include the mention of an isolated
thunderstorms mainly west of i77. Played high temperatures close
to the adjmavbc with values from the lower 50s in the mountains to
the middle 70s in the Piedmont.
Short term /Friday night through Sunday night/...
as of 315 PM EST Thursday...
Not a lot of change early on with the slow moving cold front and
associated wave crossing the region from the northwest Friday night and
spilling south of the area early Saturday. Models again weaken the
axis of lift along/behind the front Friday evening with heaviest quantitative precipitation forecast
and highest probability of precipitation along the western slopes through late
evening...then less along/east of the Blue Ridge except southeast where the
boundary may stall and the low level NE flow may enhance showers
late. Still appears enough moisture across the northwest late Friday night
to warrant keeping in some light mix mention at elevation but
overall think anything frozen at this point quite spotty across southeast
wva. Cold air will be sliding in from the north/west although temperatures
may actually get colder after daybreak Saturday when the better 850 mb
cold advection arrives.
Guidance has continued the trend from yesterday in pushing the
deeper moisture to the southeast Saturday as the large Arctic high builds
in from the northwest and initially gives the moisture a push. May see the
entire County Warning Area go dry by Saturday afternoon but for now leaving in some
pop mainly eastern half through midday and mainly far southeast late in the
day. Could even have some clearing take place northwest pending how strong
the dry advection is so cut back on cloud cover some north half
during the afternoon. Highs chilly with low level north/northwest flow with
temperatures 30s west to 40s/near 50 far south.
Main concern then with the potential for a significant winter event
from very late Sat night into Sunday and perhaps Sunday night across
the north. Latest guidance has again slowed things up a bit with
lots of cold/dry air sliding down east of the Blue Ridge as the
strong wedge develops with the high building farther south. This
will make for colder temperatures Sat night before thickening clouds and
light precipitation return from the south/SW after midnight per isentropic
lift ahead of another wave heading up west of the mountains.
Latest forecast soundings show dry air holding longer with precipitation
not reaching into the southern half of the area until after midnight
and points north Sunday around daybreak. Precipitation then looks to
accelerate into The Wedge Sunday morning from the SW but remain
inconsistent in quantitative precipitation forecast amounts which are critical given colder profiles
from 12z runs. However most showing the best lift with a warm
front aloft type feature that develops over the central/northern
County Warning Area Sunday morning and shifts north during the afternoon leaving
more spotty nature precipitation elsewhere into Sunday evening. Both the NAM
and GFS are colder than the earlier solutions with less of a warm nose
early on and deeper low level cold air supportive of more of a
snow/sleet mix at the onset espcly hiway 460 north. Thus including
some light accumulations northwest Sunday morning as things remain below
freezing at 850 mb and things become more isothermal until the
stronger warm advection arrives in the afternoon. Some sleet also
possible elsewhere as things wet bulb down at the onset but thinking
more of a prolonged freezing rain event espcly southern Blue Ridge and
across the Virginia counties south of 460. Wedge may be enough to force
temperatures down enough to produce some spotty freezing rain into northwest NC but
holding off going that cold at this time. Otherwise few changes to high probability of precipitation
Sunday with high probability of precipitation for low quantitative precipitation forecast Sunday night when the Greenbrier
valley and southern Shenandoah Valley may hold onto freezing rain
overnight. Since mostly a 6th period and beyond event...will hold
off on headlines for now and keep mention in the severe weather potential statement.
Long term /Monday through Thursday/...
as of 350 PM EST Thursday...
Complicated forecast continues through the extended periods as
southwest flow aloft continues across the region at the same time
Arctic air continues to stream southeastward from western/central
Canada into the central and eastern U.S. Our region remains on or
near the boundary between the Arctic air and the warmer air to the
southeast...however...through the longer term periods it appears
that the Arctic air will win out across our region and the frontal
will finally be pushed into the southeast states as the upper flow
finally becomes northwest.
Monday morning we are still dealing with the end of the potential
significant icing event addressed in the short term periods as The
Wedge boundary finally lifts into northern Virginia. Models have
continued to trend colder...which requires holding onto the -fzra
a few more hours into middle-morning Monday than earlier indicated.
Will change all precipitation to -ra by 15z Monday as the forecast
area briefly moves into a warm sector sandwiched between the
retreating cold air wedge and the next surge of Arctic air to the
west. Through the daylight hours...we should see only liquid precipitation
across the region.
By 06z Tuesday...the next surge of Arctic air should move into our WV
counties and our far SW Virginia continues. Given the strength of this
second surge of cold air...would expect -ra to quickly change to
-sn from west to east during the evening. At the same time...the
deeper moisture is shifting east into the Piedmont where at this
point it should still be warm enough for mostly -ra. Thus...snow
accumulations outside mountain upslope -shsn eastern WV...should
be minimal...but could not rule out an inch or so in some spots
west of I-81/I-77. There is considerable discrepancy between the
GFS/European model (ecmwf) in this time frame...with the European model (ecmwf) pushing the moisture
well east of the County Warning Area before any appreciable cold air moves into
the region from the west...while the GFS tries to move yet a third
wave along the frontal boundary across the region from the SW.
Given the overall confluent flow in this time frame...not
expecting any significant precipitation or more importantly winter weather
issues. Again...however...there will be mountain upslope
-shsn...with minor snow accumulations eastern WV counties into far
SW Virginia and the northwest NC mountains.
For the remainder of the extended...at this point it appears dry.
The center of the Arctic air mass drifts over the region by Thursday
with a very dry/cold air mass. Again...there are considerable
differences between the European model (ecmwf) and GFS with respect to temperatures
in this time frame. Given the strength of this Arctic air
mass...have leaned heavily toward the colder GFS. The coldest day
should be Thursday when the Arctic high is centered right over the County Warning Area.
Would not be the least surprised to see min temperatures in the
single digits in locations such as lwb. However...the prevailing
west as opposed to a strong northwest-north flow aloft should result in the
colder air remaining north and west of the County Warning Area..so we should stay
at or above 20 degrees for mins across the Piedmont. Any precipitation
Wednesday-Friday should be confined to upslope mountain -shsn with drier
downslope west flow east of the alleghanys. Even across eastern
WV...snow accumulations should be fairly limited with overall
snow accumulations there even an inch or less.
Aviation /21z Thursday through Tuesday/...
as of 1230 PM EST Thursday...
Strong cold front will be approaching from the Ohio Valley this
afternoon...stalling along The Spine of the Appalachian Mountains
Gradual improvement is expected this afternoon...trending from
MVFR to VFR by late this afternoon. Light surface winds will
become south to southwest on this afternoon with moderate to high
confidence that tafs sites will improve to VFR by late this
afternoon. After sunset...cloud bases will lower back into the
MVFR category tonight.
Extended aviation concerns...a cold front will make slow progress
across the area Friday...resulting in MVFR and IFR conditions in
addition to widespread rain showers...before the front passes east
Friday night. May see a brief improvement to VFR on Saturday as
high pressure builds in from the north...however IFR will return
Saturday night as another system approaches from the
southwest...bringing freezing rain to the area Sunday. A cold
front will move through on Monday...bringing a slow improvement to
VFR after its passage for Tuesday.
High pressure is expected to provide fair weather with VFR
conditions from Tuesday through Thursday.
warm temperatures the next couple days will approach records at the
climate sites. Below are the current records.
December 5 current record high temperatures (year)
December 5 current record high minimum temperatures (year)
December 6 current record high minimum temperatures (year)
lwb 52 (1998)