FXUS61 KRNK 180939

Area Forecast Discussion
National Weather Service Blacksburg VA
439 AM EST Mon Dec 18 2017

Weak high pressure to our south with a passing upper level
disturbance will contribute to variably cloudy skies today into
tonight. Turning warmer into Tuesday ahead of a frontal system
emanating from the Tennessee Valley, which will spread rain along
and south of Route 460 on Wednesday. Cooler wedge conditions then
set up for the rest of the workweek.


As of 400 AM EST Monday...

We often mention how weather conditions can vary depending on what
side of the Blue Ridge one resides. Well, that really couldn't be
more true than this morning. If you live along or west of the Blue
Ridge into the Appalachians, you're waking up to overcast
skies/stratus and patchy mist/fog. Get out into the Piedmont and
you're getting into thinner sky cover. Influx of shallow moisture
continues to be pumped northeastward from the Tennessee Valley,
underneath westerly confluent mid-level flow. Relatively non-
descript surface pressure pattern, with a weak high pressure
area centered over northern Georgia.

We'll be looking at continued moisture advection today given the
west-southwesterly component. For areas along and west of the Blue
Ridge, BUFKIT forecast soundings off the NAM, RAP and GFS each
reveal this low-level moisture will be trapped underneath dry
subsidence layer aloft for a good part of the day. Meanwhile in the
Piedmont/foothills of Virginia and North Carolina, skies should tend
to clear with intervals of high clouds spilling over in upper-level
southwest flow. Unfortunately am not all that optimistic on much
clearing and warming temperatures this afternoon west of the Blue
Ridge - if it happens, it would be late in the day. Would therefore
expect to see overcast skies with periods of drizzle at times in the
overcast. Mist or fog should continue into part of the morning until
we can tap into west flow that may help to disperse. All in all a
pretty dreary day seems likely for these locations. Though MOS
guidance wants to push highs into the upper 40s to lower 50s here,
I've cut these down by 2-4 degrees given the expected cloudiness to
values in the mid/upper 40s. From Roanoke into the Piedmont and
foothills, a more diurnal temperature trend is expected with highs
reaching well into the 50s. May even be a spot 60 degree reading in
Stokes, Rockingham or Caswell.

Continued variably cloudy skies should be the rule into tonight,
greatest furthest west. As an upper-level disturbance passes to our
north through the overnight, that should enhance westerly flow as
depicted by the 00z guidance. However warm advection occurs
overnight and it's unlikely lower-elevation locations mix GFS-
modeled westerly 50-kt jet in that regime, but terrain above 3000'
stands a fair shot at seeing breezy conditions (gusts to 35 mph).
Light west to calm winds appear more likely in the lower elevations.
Late in the overnight, will have to watch the northern NC Piedmont
into Southside for patchy fog. NAM-based BUFKIT soundings at GSO
reflect a potential fog layer developing that could advect
northward, and these areas may also stand to radiate better. Lows
should range from the mid/upper 30s in most locations, coolest in
the Piedmont given better prospects for radiational cooling. With
stronger winds/better mixing along the ridges, I opted for values in
the mid 40s.


As of 400 AM EST Monday...

A confluent flow will be in place across the Mid-Atlantic region
at the beginning of the period with distinct southern and
northern streams evident. The northern stream will continue to
be influenced by a strong upper low/Polar Vortex anchored over
Hudson Bay. However, this upper low has lifted somewhat north
and west of its position last week as a discontinuous
retrogression process appears to be in place with respect to the
large scale pattern, with the upper trough gradually shifting
westward through the extended periods. A notable strong
subtropical ridge (eventually 595 dm!) will be anchored off the
southeast coast generally attempting to expand northwestward
into the weekend.

Meanwhile, the southern stream will be quite active during this
period with a classic wintertime strong, yet compact, highly
kinematic disturbance evident in central TX at the beginning of
the period tracking across TN/GA/NC through Thursday. The
confluent flow in place across the Mid-Atlantic will limit the
northern extent of the associated precipitation Tue-Wed with
likely a sharp contrast in pops from north to south (i.e., below
15% far north to categorical/definite southern areas,
especially the NC counties. In essence the disturbance will
track almost due east from south central TN along or near the
TN/GA and VA/NC border, exiting the coastal sections of eastern
NC by Thu morning. QPF will range from zero or trace amounts in
the far north, especially north of I-64 to 1/2 to 2/3rd inch in
the NC counties and adjacent tier of southern VA counties.

No concern with winter precipitation during this event as 850mb
temperatures remain above freezing for most if not all of the
CWA through the beginning of next week with the 850mb 0C
isotherm remaining just north of the CWA through the period. The
strong ridge just to our southeast certainly will contribute to
the lack of winter weather threat during this period.

Temperatures are expected to be well above normal most of this
period. In advance of the upper-level disturbance Tuesday,
temperatures will average some 10-15 degrees above normal with
much of the CWA seeing balmy 60+ degree readings, even closing
in on 70 in the eastern Piedmont. Cloud cover and precipitation
will knock about 10 degrees off maximum temperature readings for
Wednesday, with the passage of the disturbance, temperatures
should fall another 5 degrees or so compared to Wednesday.
Minimum temperatures will show the greatest departure from
normal averaging 10-20 degrees above normal much of this period.
Lows should be largely in the 20s at this time of year, but
instead will be mostly in the 30s and 40s, with many parts of
the CWA only falling as low as 32F during the period.


As of 430 AM EST Monday...

Quite a pattern change evident during this period as a strong
subtropical ridge amplifies across the southeast and adjacent
western Atlantic in response to the southeast Canada upper low
retrograding westward into south central Canada with a deep,
positively tilted upper trough developing into the southwest
U.S. With time, significant discrepancies appear between the
GFS and ECMWF as to how quickly an associated cold front will
move into the eastern U.S., with the ECMWF slower than the
GFS. With the GFS much more progressive, precipitation and cold
air will reach the CWA much sooner than with the ECMWF, which
keeps our region on the east side of the trough axis through
Christmas, but by the same token is much wetter in the later
periods, and also much slower to move the precipitation out of
the area as well. It should be noted that there will be a sharp
contrast in temperatures from the central U.S. air mass, which
will show some true Arctic signs to the eastern seaboard
dominated by the strong upper ridge and thus much above normal
temperatures. Along and near the associated frontal boundary
separating these two air masses, will be a corridor/conveyor
belt of heavy rainfall. Model consensus, especially favoring the
ECMWF, is that this will remain just west of our area for the
most part. Certainly the potential for heavy rain and event
winter precipitation will become a greater likelihood as we
move into the week of Christmas.

Otherwise, temperatures are likely to remain above normal as
long as we are on the east side of the upper trough axis/frontal
boundary/baroclinic zone. Based on the current extended models,
especially the ECMWF, am not advertising any winter
precipitation through the extended periods. If you were looking
for a White Christmas this year, the odds are definitely against
having such as it appears right now.


As of 1140 PM EST Sunday...

Sub-VFR in the mountains will persist through the period, with
BLF/LWB at times dropping below 1000ft through 10z. Westerly
flow is going to keep MVFR cigs at BLF at least until Monday
evening while LWB may jump above MVFR by the end of the period.

The other taf sites will also have MVFR at times early before
scattering out after dawn.

Extended Discussion...

Through Friday, the bulk of the region will maintain VFR
conditions as a split-flow pattern establishes itself across
CONUS, with our little part of the world remaining in between
systems. The exception to this rule may be on Wednesday when a
southern stream system may be far enough north to provide
sensible weather to at least the southern half of the region.

Our next cold front may arrive Friday night into Saturday. The
latest model guidance offers increasing, gusty winds both ahead
and in the wake of the front, along with sub-VFR conditions.




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