Area Forecast Discussion
Issued by NWS Blacksburg, VA

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099
FXUS61 KRNK 161511
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
1011 AM EST Tue Jan 16 2018

.SYNOPSIS...
Low pressure tracking through the Great Lakes will gradually push a
cold front through the Mid Atlantic region today and tonight. A
front will form along the front, which will then become incorporated
into a developing coastal low off of the Mid Atlantic and New
England coast. High pressure builds in behind this system for
Wednesday into Friday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1005 AM EST Tuesday...

Adjusted timing of snow entering extreme southwest Virginia and
southeast West Virginia this morning by an hour or so. Light
snow and/or flurries will start around noon. Accumulating snow
may not begin across the Mountain Empire, New River Valley and
the Alleghany Highland of Virginia until 3 or 4 pm. Amounts have
not changed.



As of 405 AM EST Tuesday...

Low pressure develops along the front in the Southern Appalachians
this afternoon before tracking northeast then being absorbed into
the deepening coastal low Wednesday morning.

Surface and low level winds do not back ahead of the front but there
will be some better convergences at 850 mb tonight. Large bulk of
the lift with this system will be post frontal with the vorticity
advection and minimal upper diffluence.

Enough of a pressure rise and cold air advection behind the front
for and increase in northwest winds from southeast West Virginia
into northwest North Carolina. This will promote upslope snow
showers through Wednesday morning.

Timing of the onset of snow looks similar to past few model runs.
Will be reaching a Lewisburg to Boone line after 12Z/7AM. HRRR, Hi-
Res guidance and RAP bring leading edge into southeast West Virginia
around 15Z/10AM. More of an eastward push does not occur until after
sunset. Based on this timing and expected snow amounts have added
another segment to the Winter Weather Advisory roughly east of
Lynchburg to Martinsville.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/...
As of 455 AM EST Tuesday...

The deep upper trough over the region at 12Z Wed will quickly
lift out of the region as strong Pacific energy moves into the
West Coast of the U.S. and breaks down the eastern U.S. trough.
The large-scale synoptic driven light snow event will quickly
exit the region by midday or early afternoon in the east.
Upslope snow showers may continue or redevelop across the
Alleghanys, but conditions will not remain favorable for long
because of rapidly trending less favorable wind flow and upper-
level ridging/zonal flow. Total snow amounts are expected to be
in the 1-3 inch range for most of the CWA.

The biggest story on Wednesday will be another surge of bitter
cold true Arctic air. Currently there is a pocket of such
centered over Kansas City where temperatures are in the 0F to -10F
range. The 850mb temperatures in this pocket of Arctic air are
around -24C, which rivals the coldest air we have seen in
previous cold outbreaks this winter. Fortunately, by the time it
arrives in our region, it has "warmed" to -18C, which should
hopefully spare us the below zero temperatures we saw with the
last cold outbreak. Nonetheless, we are still looking at low
temperatures in the single digits west to the teens east both
Wednesday and Thursday morning. Winds will be increasing during
the day Wednesday and Wednesday evening as a coastal low
develops off the Mid-Atlantic coast. This will drive wind chills
below zero for many areas west of the Blue Ridge and to near
zero for the Piedmont, more so Thursday morning than Wednesday
morning, when such wind chills will be more confined to areas
just west of the Blue Ridge. Given that a Winter Weather
Advisory has been issued for the expected snow accumulations
across the CWA, the wind chill issue has been included in the
WSW for snow.

Unlike previous Arctic surges the past several weeks, especially
since Christmas, as noted above, the upstream Pacific energy
will quickly break down the eastern U.S. trough and allow the
flow to transition to zonal rather quickly. It will do so
quickly enough that 850mb temperatures will moderate quickly to
above zero readings in about a 24-30 hour window after bottoming
out around -18C early Wednesday. This will allow a fairly rapid
warmup to begin Thursday and continue into the weekend as 850mb
temperatures rise to +10C or better by Saturday.

Thus, temperatures will start out well below normal Wed and Thu
with highs in the teens west to 20s east and lows from 0 to 10
west and 10-15 east, warming to highs in the 40s west and 50s
east by Friday with lows mainly in the 20s. Expected snow cover
could play havoc with low temperatures (e.g., the -11F that LYH
saw a couple of winters ago when in some areas Thursday
morning, but much will melt Thursday afternoon and much of the
remainder by Friday afternoon, especially in the east.

&&

.LONG TERM /SATURDAY THROUGH MONDAY/...
As of 510 AM EST Tuesday...

Quite a change in the overall upper atmospheric pattern during
the extended will lead to a period of much above normal
temperatures that will last into the first half of next week.
Broad upper ridging across the southeast will expand northward
and deep troughing shifts to the western U.S. 500 mb heights
over our area will rise from the current 540dm range to 570dm or
so by this weekend. Highs temperatures will warm well above
normal into the 50s west and 60s east by early next week. Most
will enjoy the milder temperatures after the bitter cold we have
endured most of January as well as late December.

A Pacific based system will arrive from the Midwest early next
week. With 850mb temperatures in the positive double digit
range, no concerns with any winter weather and on the other hand
there does not appear to be enough instability for thunder yet
either in our region. However, rain showers will be likely by
late Monday into Monday night. Some snow showers may develop on
the back side of this weather system into Tuesday as a brief
surge of moderately cold air follows (850mb temps only to the
-6C range).

&&

.AVIATION /15Z TUESDAY THROUGH SATURDAY/...
As of 720 AM EST Tuesday...

Ceilings have lowered to around 5000ft in western and central
West Virginia with light snow. Ceilings will lower to MVFR from
west to east today into tonight across southeast West Virginia,
northwest North Carolin and southwest Virginia in advance of a
cold front and band of light to moderate snow. Medium to high
confidence on the arrival time of the snow and associated MVFR
visibilities. The general trend this morning had been for the
HRRR and RAP guidance to slow down the arrival of snow until
after 10AM.

A low will develop along the front this afternoon and evening
which will expand the areal coverage of snow in the central and
eastern Virginia and North Carolina. Winds will turn to the
northwest behind this system bringing in much colder air.

Extended Discussion...

Dry air will restore conditions to VFR by Thursday under high
pressure. VFR conditions should persist into the weekend with
warming temperatures. Moisture will return to west Sunday night
into Monday.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...Winter Weather Advisory until 10 AM EST Wednesday for VAZ007-
     009-010.
     Winter Weather Advisory from 6 PM this evening to 3 PM EST
     Wednesday for VAZ034-035-043>047-058-059.
NC...Winter Weather Advisory from 6 PM this evening to 3 PM EST
     Wednesday for NCZ004>006.
WV...Winter Weather Advisory until 10 AM EST Wednesday for
     WVZ042>044-507-508.

&&

$$
SYNOPSIS...AMS
NEAR TERM...AMS/RCS
SHORT TERM...RAB
LONG TERM...RAB/RCS
AVIATION...AMS/PM



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