Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000
FXUS61 KRNK 230559
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
1259 AM EST Wed Jan 23 2019

.SYNOPSIS...
High pressure will continue heading east overnight while an area of
low pressure and its associated cold front approach from the west.
The cold front will cross our region Wednesday night and be followed
by a reinforcing cold front Thursday night. The area will continue
within a regime that offers quick moving, but weak, disturbances
cross the region over the weekend into early next week.

&&

.NEAR TERM /THROUGH TODAY/...
As of 925 PM EST Tuesday...

Low level moisture will continue to surge up from the south
overnight with precipitation chances increasing as the low level
jet starts to ramp up. Dew points at the surface remain quite
low with most locations in the single digits this evening.
Expect cooling as initial precipitation evaporates as virga
before reaching the surface will help keep the boundary layer
cold enough to support wintry precipitation, and even where air
temps manage to eventually get above freezing the threat for
some glazing of ice will persist for a bit since the ground has
been so cold of late. Believe the current winter weather
advisory looks to be in good shape covering the bulk of the
sleet/freezing rain, but will continue looking at the 00Z data
closely to see if there are any signals that wintry
precipitation may be more robust east of the Blue Ridge deeper
into Virginia and warrant further expansion of the advisory.

Previous discussion...

The axis of a ridge of high pressure was centered from Upstate New
York south into South Carolina. An upper level trough/low was
centered over central Kansas with its associated cold front trailing
southwest into New Mexico. Low level southerly flow in advance of
this front was robust over the Southern Plains States and the Lower
and Mid Mississippi River Valleys. As the surface high shifts east,
and the cold front approaches from the west, this moist jet of air
will gradually make headway our direction. While no
precipitation is expected through this evening, we will see a
progression of low level cloudiness making its way into the
area.

Light precipitation is expected to arrive across the southwest
portion of the region around midnight, then spread northeast across
central and eastern parts of the forecast area during the night and
into the morning on Wednesday. Initially, the precipitation will be
hard pressed to reach the surface given it will falling into very
dry air, and will serve mainly as a means to moisten the low level
profile. However, once precipitations starts reaching the ground, it
is expect to be falling in various forms. Initially the column
may be cold enough for some very light snow over the Northern
Mountains and Foothills of North Carolina. This is expected
quickly transition to sleet and eventually mainly light freezing
rain as the magnitude and depth of the warm nose aloft
increases.

We will continue our Winter Weather Advisory and include Floyd and
Henry County/City of Martinsville Virginia. While not a lot of
wintery weather is expected, the bulk of it will be falling
around the height of the morning commute on sub-freezing
surfaces. Other areas north and east of this area may also
experience some brief freezing rain Wednesday morning. However,
their duration is expected to be very short before a switch to
plain rain, and the amounts will be mainly on the order of a
trace. These areas will be highlighted in the Hazardous Weather
Outlook.

Light rain will increase across the area during the course of the
day, with the best coverage and amounts along and east of the crest
of the Blue Ridge. Western sections of the area will have the least
amount of rain thanks to the predominate flow having a downslope
component. While the cold front to our west will be getting closer,
the models have trended slower with its arrival, so have decreased
slightly the higher precipitation chances associated with its
arrival in the west in the late afternoon. Its arrival in the west
is now slated more for the evening hours.

Another element of interest for late tonight into Wednesday will be
the potential for gusty winds across the southwest portion of the
forecast area. The 850mb jet is forecast to increase to around
50kts. Initially its orientation is forecast to be parallel to most
of the ridgelines, but back slightly southerly adding a small ridge
crossing component by late Wednesday morning. Wind advisory criteria
may be met at some of highest peaks across the Northern Mountains of
North Carolina and the Mountain Empire region of southwest Virginia,
but not enough coverage to warrant an advisory. After collaboration
with neighboring offices, have selected to table any wind advisories
at this time. Will mention gust wind potential in the Hazardous
Weather Outlook.

Low temperatures tonight will be mainly in the middle 20s. Readings
around 30 are possible across the far southwest portion of the area,
and readings in the mid 20s are possible north of Interstate-64.
High temperatures on Wednesday will follow an atypical pattern.
Values will be lowest, mid 40s, in areas along and immediately
either side of the crest of the Blue Ridge. They will be warmest
across the far western and far southeast sections where temperatures
are expected to reach the lower 50s.

&&

.SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/...
As of 345 PM EST Tuesday...

The next in the series of deep upper-level troughs will be
moving through the region Wed night into Thursday morning. The
bulk of the QPF will fall in the 00Z-06Z time frame west of the
Blue Ridge and in the 06Z-12Z time frame east of the Blue Ridge.
The main concern, at this point, as it has been with the past
several systems, is the potential for flooding. While this event
is very similar to last weekend`s event, namely QPF on the order
of 0.8 to 1.5 inch and clearly wet antecedent conditions, the
ground is frozen from the past several deep freeze days with
temperatures in the single digits and teens. How quickly the
ground thaws out as the rain falls and temperatures rise above
freezing will largely be the key to the flood threat. Given very
limited flooding with last weekend`s system, the only difference
is the frozen ground for this event. In coordination with our
neighbors, decided to not issue a Flood Watch at this point.
Minor nuisance flooding and potentially minor river flooding are
definitely possible and will continue to be addressed in the
HWO/eHWO at this point. A Flood Watch remains a possibility with
later forecasts.

As cold advection spreads into the area early Thursday,
precipitation in the west will transition to snow showers with
minor 0-1 inch accumulations possible in the western mountains
Thursday morning. However, this is the typical case of will the
precipitation end before the temperatures drop below freezing
with mainly upslope snow showers being the issue in the west.

While another surge of Arctic/Canadian air is on the way, not
expecting the extremity of the most recent cold outbreak. 850mb
temperatures are expected to bottom out around -12C as opposed
to closer to the -20C range. Maximum temperatures Thursday will
range from the mid 30s to lower 40s across the mountains and the
upper 40s to lower 50s east of the Blue Ridge with
temperatures. Cold advection will begin to increase during the
afternoon and evening with overnight lows falling into the lower
20s west of the Blue Ridge to the upper 20s east. Highs Friday
will remain mostly below freezing once again west of the Blue
ridge and only in the 30s across the eastern areas.

Finally, wind could be an issue behind the cold front Thursday,
but given all of the upfront concerns and issues in the near
term, will address post-frontal wind issues in later forecasts.

/Confidence in Forecast Parameters/
Temperatures - High,
Precipitation Chances - Moderate to High,
Winds - Moderate to High.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
As of 355 PM EST Tuesday...

During this portion of the forecast, a deep upper low will
persist across eastern Canada. Climatologically this is a cold
pattern for the eastern U.S. Weaker troughs of low pressure and
their associated cold fronts will spiral into and across our
region in a clipper-like fashion. Timing and impact of these
features has not had model run- to-run consistency. Also, their
interaction, if any, with southern stream lows/troughs has not
been consistent, thus adding to greater uncertainty and lower
confidence farther in the latter portion of our forecast. One is
slated to cross the area as early as Friday and another over
the weekend, with yet another one early next week. Moisture is
questionable, especially with the first couple of these. Will
keep pops in the low end range of 20-30 and generally confine to
the western slopes which the persistent westerly flow favors.

If we include the potential for a southern stream low tracking
along the Gulf Coast to head northeast near or just east of the
east coast of the U.S. Sunday into Monday as both the Canadian
and European solutions offer, then precipitation coverage across
the Piedmont region will be greater, with a challenging
p-type forecast given vastly different forecasts of the low
level temperature profile across this region during this time
frame.

Temperatures will remain mostly below normal through the period
with the persistent anticyclonic northwest flow. Several
opportunities for lows to drop into the teens.

/Confidence in Forecast Parameters/
Temperatures - Moderate to High,
Precipitation Chances - Moderate,
Winds - Moderate to High.

&&

.AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
As of 1255 AM EST Wednesday...

Expect VFR conditions early this period, but satellite imagery
already shows clouds surging northward east of the Blue Ridge.
These clouds will eventually trend down to MVFR, then IFR for
the latter portion of the period. After 00Z/7PM LIFR ceilings
will be widespread across southwest Virginia and northwest
North Carolina.

Precipitation will accompany the lower cigs, with a period of
freezing rain and/or ice pellets for a couple of hours at the
onset, mainly at KDAN and possibly extending to KLYH and KROA,
before transitioning to plain rain through the end of the
period.

Surface winds will generally be light, but the low level flow
just off the surface will amplify considerably and create some
LLWS mainly west of the Blue Ridge.

Confidence in timing of the precipitation is above average.
Confidence in timing of the IFR and lower ceilings is below
average.
Confidence is average for all other elements.

Extended Aviation Discussion...

Winds shift northwest late tonight with the passage of a cold
front and remain gusty. Sub-VFR conditions will accompany the
front`s associated moderate to heavy precipitation.

Winds remain gusty from the northwest on Thursday into early
Friday morning, especially across the mountains. There also
will be isolated to scattered snow showers across parts of
southeast West Virginia. The gusts and snow showers will come to
an end by mid-day Friday. Conditions will be mainly VFR during
this time period.

VFR conditions will continue into Saturday.

A weak clipper system may bring isolated snow showers to
southeast West Virginia on Sunday along with patchy sub-VFR
conditions. However, the vast majority of the forecast region
will remain VFR.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...Winter Weather Advisory until noon EST today for VAZ015>017-
     032-043.
NC...Winter Weather Advisory until noon EST today for NCZ001>005-
     018>020.
WV...None.

&&

$$
SYNOPSIS...DS
NEAR TERM...DS/MBS
SHORT TERM...RAB
LONG TERM...RAB
AVIATION...AMS/MBS


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