Area Forecast Discussion
Issued by NWS Blacksburg, VA-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KRNK 131203
Area Forecast Discussion
National Weather Service Blacksburg VA
703 AM EST Fri Jan 13 2017
A cold front will sag southward across the region today, then
stall across northern North Carolina Saturday before returning
north as a warm front late Saturday night. This front will then
oscillate north and south across the region into early next week
keeping an unsettled and wet weather pattern in place. By the
middle of next week, a stronger frontal system will finally
approach from the west and move the pesky front out of the area.
.NEAR TERM /THROUGH TONIGHT/...
As of 430 AM EST Friday...
Tricky forecast this period as a 1045mb surface high moves east
from the Great Lakes into New England pushing a backdoor front
southward into our region before stalling across northern NC as
a strong upper ridge holds across the southeast states and
prevents further southward movement. The front was drifting
southward through the region this morning and at 4 AM was near a
KBLF to KDCA line. Light rain was noted across western parts of
the CWA. This precipitation will tend to refocus further
southwest into eastern TN through the daylight hours.
Temperatures were very mild this morning ahead of the front with
many areas still in the lower to mid 60s. With the exception of
the Piedmont, high temperatures today will generally be this
morning. Potential for temperature forecast busts today are
fairly high as it was a struggle to forecast max temps only a
few degrees higher than current temperatures. However, with
persistent cold advection, feel temperatures will remain steady
to slowly fall as the day progresses. Greater amounts of
sunshine than currently expected could throw that off, but given
a persistent stream of mid/high clouds from the west, sunshine
should be limited. Again, the exception will be areas south of
the front, NC and the VA Piedmont, where cooling will hold off
Given the synoptic setup, the cold air will be very shallow,
confined mainly to the surface to 925mb layer, with temps at
850mb remaining above freezing. Meanwhile, a series of
disturbances emanating from an upper low in the southwest U.S.
will track along the baroclinic zone parked over our area. This
will generate periods of light precipitation that will spread
back into the region overnight. Temperatures across the I-64
corridor area and as far south as Craig and Monroe will dip just
below freezing after 4 AM and hold near there until Saturday
afternoon as a result of wet bulbing and evaporative cooling.
Given the very shallow nature of the cold air, precipitation
will primarily fall as freezing rain in these areas. There may
be a brief period of sleet early Saturday as the depth of the
cold air mass approaches 2000 ft. thick but the predominate
p-type will be freezing rain. Given the well above normal
temperatures the past couple of days, current thinking is that
accumulations will be confined mainly to elevated objects such
as trees and power lines. QPF with the overnight/Saturday event
should only be around 1/10 inch or less, yielding 1/10 inch or
less ice accumulation. In consideration of all of this, have
issued a Freezing Rain Advisory for areas generally along and
north of a Union-New Castle-Pleasant View line from 4 am to 4 pm
Saturday. For now, have left western Greenbrier out of the
advisory given the difficulty the cold air wedge should have
pushing west of the Alleghany front. Included Botetourt and
Amherst counties mainly for scattered higher elevations in the
northern and western parts of those counties. Later shifts may
need to evaluate the need to included Western Greenbrier county
as well as any other further southward areas.
Have leaned heavily toward the NAM guidance for temperatures
this period. The GFS is quite warm and would suggest no ice
keeping most temperatures above freezing. The ECMWF is a good
compromise between the two confining freezing temperatures early
Saturday mainly to Rockbridge, Bath, and eastern Greenbrier
.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
As OF 430 AM EST Friday...
Quasi-stationary front will be over the area this weekend promoting
abundant cloud cover and threat for some light precipitation. Focus
early Saturday will be the low level cold wedge which will drive
boundary layer temperature into the 30s on the east side of the
Appalachians, with near freezing temperatures as far south as the
higher ridges surrounding the Roanoke Valley. For a 6 to 8 hour
period Saturday morning, expecting the Blue Ridge Parkway northeast
of Roanoke and the Highlands along the VA/WV border (Craig and
Monroe counties and points NE) to drop below freezing. This will
promote a period of freezing rain or freezing drizzle with impacts
mainly to elevated surfaces. Elsewhere expecting just a cold rain
and/or drizzle, although amounts are forecast to be light (less than
a tenth of an inch). Greatest isentropic lift is forecast to occur
north of Interstate 64, so rain amounts of a tenth to a quarter inch
are possible across the far northern CWA and areas to the north, but
lift for precipitation production wanes quickly as you approach the
NC/VA border with little or no rain expected across the Carolinas
and southern Appalachians.
Temperatures will be the warmest across the far western CWA (west of
the Appalachian Divide) Saturday where readings may test 50 degrees.
Within the wedge, temperatures will struggle to get much above 40.
Wavy surface front will be over the area Saturday night through
Sunday night, with cooler temperatures prevailing on the east side
of the Appalachian Divide vs. warmer air over the Tennessee Valley
again Sunday. Sunday morning is forecast to be a few degrees warmer
overall compared to Saturday, so no freezing rain issues expected
Sunday. Some of the models are hinting that slightly cooler air will
drift back into the area Sunday night as the front gets a little
nudge to the south. If this turns out to be the case, then some of
the higher elevations along and north of I-64 could see some
freezing rain/drizzle Sunday night. Again it would be low QPF and
mainly on elevated surfaces.
.LONG TERM /MONDAY THROUGH THURSDAY/...
As OF 220 PM EST Thursday...
Wavy front across the area Monday morning lifts north into New
York Tuesday as strong upper low moving across the central U.S.
amplifies ridge downstream. Will see less threat of showers
Monday but still looks cloudier, though some sunshine should
peek out. Warm air will stick around into midweek with models
slowing the front moving into the CWA until late Thursday into
Friday. Decided to keep pops low Tuesday with mainly dry in the
foothills/piedmont then chance pops start Wednesday with better
threat Thursday. Highs running about 10- 20 degrees above normal
.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --As of 645 AM EST Friday...
Generally deterioriating to poor aviation conditions expected
through the TAF valid period as a backdoor front sags southward
through the region, then stalls just south of the NC/VA border,
returning northward slowly Sat-Sun. The first wave of light rain
and associated MVFR-IFR cigs traversing southeast WV at this
hour. This precipitation will sag southwest into eastern TN/far
southwest VA by afternoon. The next upper disturbance and
stalled front will return rain to the region later this evening
and overnight. Cigs will remain/trend toward IFR-LIFR in the
west and become generally MVFR in the east as this area of rain
spreads east overnight. As cold air spreads southward from the
northeast overnight, a brief period of freezing rain will be
possible at KLWB late tonight/Saturday morning. Visibilities
will be mostly MVFR in areas of rain this morning and again
overnight, but VFR otherwise. Winds generally WNW-NW 5-7kts west
of the Blue Ridge, becoming NE-ENE east of the Blue Ridge with
speeds increasing to 9-10kts by afternoon. Later this
afternoon/evening, winds will also become ENE-ESE at 5-7 kts
west of the Blue Ridge as the wedge air mass builds westward.
Medium to high confidence in ceilings through the TAF valid
Medium to high confidence in visibilities through the TAF valid
Medium to high confidence in wind speed/direction through the
TAF valid period.
Extended Aviation Discussion...
A baroclinic zone will remain anchored across the region
sandwiched between a strong upper ridge over the southeast
states and an active jet stream from the southwest U.S. into the
northeast U.S. Thus, an unsettled, wet weather pattern is
expected to remain in place through the extended periods. The
frontal boundary may lift north enough by Monday for a brief
period of mainly VFR, but a stronger front will arrive By
Tuesday with more showers. Generally looking at MVFR much of the
period, although periods of VFR as well as IFR-LIFR can be
expected at times through the period. Overall, not the best
weather for aviation.
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VA...Freezing Rain Advisory from 4 AM to 4 PM EST Saturday for
WV...Freezing Rain Advisory from 4 AM to 4 PM EST Saturday for