Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Blacksburg Virginia
619 am EST Tuesday Dec 10 2013
low pressure will track northeast across the Carolinas by this
afternoon and then well offshore this evening. Cold high
pressure will finally build in for the middle and end of the
week. Another low pressure storm system will affect the region
by this weekend.
Near term /through tonight/...
as of 545 am EST Tuesday...
Fast moving precipitation has overspread the region a bit sooner which
has gotten ahead of the colder air aloft that remains stuck from
just west of blf to lwb and hsp at this time. Latest 06z NAM suggests that
the thicker cold air may not move out of southeast wva until after 12z
and concern about much of the steadier precipitation moving out to the NE
shortly thereafter per regional radar loops. In fact dual pol
signatures show much of the precipitation even in southeast wva a mixed bag at this time
outside of western Greenbrier and suggested by observation up that way.
Some spotty icing ongoing in the normally colder spots over the
west but most temperatures remain enough above freezing to limit
widespread issues for now. Thus trimmed back snowfall amounts some
in the far west and New River valley where may just see a period
of snow before things end. Otherwise no changes to the going headlines
since primarily based on impact for the morning commute outside of
the north and northwest.
Previous discussion as of 255 am EST Tuesday...
Radar shows precipitation quickly zooming into the west at this time in advance
of the upstream 500 mb trough nearing the miss river and north of the
surface wave developing on the old front across the southeast states.
Expect most of this initially to take the form of rain over the
west per warm nose aloft before quickly transitioning to a brief
mix and then snow espcly southeast wva counties and the Virginia
Highlands. Models remain rather slow to kick the 850 mb oc line southeast
until after daybreak but in turn fold colder air in quicker across
the far north where the higher elevations of Rockbridge and
Amherst are likely to see more snow. Guidance also faster with
most of the sig precipitation exiting after midday with varied quantitative precipitation forecast
amounts from around a quarter inch west to better than a half NE
and east. Timing of this critical as faster movement may mean more
sleet and rain early before cold enough air works in which could
greatly limit accumulations. Latest short term guidance though
does show more banding potential northwest and across the north and even
NE counties this morning where will have to watch for locally
higher totals. For now will hold off upgrading to a warning but
will include Buckingham and Appomattox into the advisory with most
snow in the northern reaches of these areas. Elsewhere kept the winter
weather headlines the same but did up amounts western Greenbrier/Bath
and included more icing New River and along the Blue Ridge per
current temperatures and a bigger warm layer aloft. Still expecting a
sharp gradient to the rain/snow line this morning with locations
from northwest NC to south of the Staunton river seeing mostly rain. Otherwise
running with faster/higher probability of precipitation this morning and a quick end this
afternoon when expect quick clearing from west to east by middle
afternoon. Kept highs mostly in the 30s mountains with a bump into
the 40s south/se.
Low level jet also to ramp up in the wake of the system espcly as
some clearing develops and cold advection really takes shape this
afternoon/evening. Forecast soundings show a surge in speeds early
this afternoon then stronger gusts right after sunset when the
inversion lowers and the 850 mb jet touches 50 kts over northwest NC. Thus
hoisting an advisory for the northwest North Carolina counties through
Cold high pressure builds in overnight with winds gradually
diminishing under clear skies. This should allow for better
radiational cooling late and despite some slow warming aloft
appears teens to low 20s mountains to mostly 20s east on tap.
This will push wind chills into single digits by morning but
should stay above advisory levels outside the highest elevations
as winds fade late.
Short term /Wednesday through Friday night/...
as of 400 am EST Tuesday...
Looking at dry conditions across our region until the very end
of the workweek. Broad upper level troughing across the eastern
U.S. Will result in temperatures several degrees below average
through the period.
Will start Wednesday morning off with high pressure centered over
the central Appalachians...on its way toward the coastal Carolinas
by evening. This high pressure will make for mostly clear skies...
perhaps with passing high clouds...and winds becoming increasingly
westerly as the high shifts east. Will see afternoon temperatures
range from the middle 30s across southeast West Virginia...to the middle
and upper 40s across the North Carolina Piedmont.
A clipper cold front will pass across the area Wednesday night...
resulting in mainly increased cloud cover...but perhaps also light
upslope snow showers across southeast West Virginia. Can also
expect gusty northwest winds behind the front...but expect these
gusts to remain below advisory level.
Snow showers will end Thursday afternoon as another dome of high
pressure approaches the area...causing wind speeds to diminish. This
high pressure will pass across our area Thursday night...and shift
toward the coast Friday afternoon...resulting in mostly clear skies.
Will see our next storm system approach Friday night from the
southwest as an upper level trough moves across the plains...drawing
Gulf moisture northward toward our area. Long range models agree
that we will start the night off with a very cool and dry
atmosphere. Believe they are underestimating the extent of wet
bulbing that will take place...and believe we will see much of the
precipitation start out as snow and sleet...rather than freezing
rain as the models suggest...although freezing rain will be possible
toward sunrise as the warmer air aloft increases.
Long term /Saturday through Monday/...
as of 300 PM EST Monday...
Saturday into Sunday...the timing differences exist in the long
range models...but the general trend is for the parent upper
low/trough to move into and then through the Ohio Valley. This will
allow colder air to enter the region on gusty northwest winds. The
upslope snow machine will turn on across southeast West
Virginia...south into the northern mountains of North Carolina. The
northwest flow will also provide for clearing skies and little or no
precipitation east of the crest of the Blue Ridge. Given the model
timing differences...our forecast will not be that binary. Some
allowance will be given for enough cold air to reach eastern parts
of the area before the strong northwest flow begins to allow for a
chance of a rain/snow mix east of the Blue Ridge late Saturday night.
By Monday...high pressure will build into the area and low level
flow will veer more north or northeast. This will turn off turn off
the upslope machine and cloud cover across the mountains will erode
as drier air continue to move into the region.
Aviation /12z Tuesday through Saturday/...
as of 555 am EST Tuesday...
Flying conditions continue to deteriorate early this morning as
ceilings are slowly falling under widespread precipitation with mostly MVFR
visibilities in rain or mixed precipitation.
Still expecting precipitation to transition to -rasn or -rapl over the southeast
wva sites...then all -sn blf/lwb/bcb after daybreak. Further
east...stratiform rain/snow will end and transition to -shsn by
late morning/early afternoon as westerly downslope flow develops.
There could be a period of moderate snow for blf during the morning
hours with LIFR vsbys/cigs. Otherwise...expect IFR-low end MVFR
ceilings and mostly IFR visibilities in -rasn blf/lwb/bcb...with MVFR
ceilings/visibilities further east. By afternoon as precipitation briefly transitions
primarily to upslope -shsn...expect MVFR-VFR visibilities and MVFR ceilings
lwb/blf/bcb...with VFR ceilings lyh/roa/Dan. Winds will be a big
concern by afternoon as they become west-northwest-northwest 15-25kts with gusts to
30kts possible at roa especially. Winds will begin to diminish
after 00z Wednesday.
Medium confidence in ceilings and visibilities through taf valid period.
Medium to high confidence in wind speed/direction through taf valid
High pressure will build in for Tuesday night into Thursday with
a return to VFR. The next organized storm system will move into
the region on Saturday.
Virginia...Winter Weather Advisory until 4 PM EST this afternoon for
NC...Wind Advisory from noon today to midnight EST tonight for
WV...Winter Weather Advisory until 4 PM EST this afternoon for