FXUS61 KRNK 201720

Area Forecast Discussion
National Weather Service Blacksburg VA
1220 PM EST Tue Nov 20 2018

A cold front will move east across the region this morning, giving
way to variable cloudiness with scattered mountain rain and snow
showers. High pressure then settles across the region tonight
lasting through the Thanksgiving holiday, with cooler than
normal temperatures and dry conditions.


As of 915 AM EST Tuesday...

Made a quick update to the forecast to push up temperatures east
of the Blue Ridge and increase wind gusts. Otherwise, cold air
will continue to advect into our region behind a cold front and
expect temperatures to fall this afternoon which was reflected
in the hourly forecast. Post-frontal northwest flow will lead to
a longer period of upslope clouds and intervals of mountain
rain and snow showers from NW Greenbrier Mtns southward into the
mountains of northwest NC. Great Lakes moisture advection in
northwest flow doesn't really shut off until late afternoon as
the mid-level trough seen in water vapor imagery over central MO
moves into the mid-Atlantic region. So by late afternoon into
early evening, we should see precipitation begin to dry up from
south to north along the Appalachian Divide. Forecast soundings
today show rather steep low-level lapse rates but an unsaturated
dendrite snow growth layer should lead to intervals of
generally low- SLR wet SHSN in the mountains in NW NC and NW WV
above 3000'. Shown snow accumulations of a dusting to a little
more than an inch at these elevations.

For tonight, high pressure is expected to ridge eastward from the
mid-Mississippi Valley into the VA/NC border for the overnight. Dry
advection should help partially disperse some of the western mtn
stratus/stratocu. While some clouds will likely linger into the
overnight in southeast WV, most elsewhere will turn clear with winds
slackening. Good radiational cooling should help plummet
temperatures quickly after sunset to lows in the 20s from the Blue
Ridge and west. Not out of the question the Piedmont and foothills
fall below freezing, but I've kept low temperatures in the low 30s
given some question on how quickly northwest winds can trend calm.

Forecast confidence is overall moderate to high, with some question
on how quickly clouds clear in the west and how fast winds die off
to calm tonight in the Piedmont.


As of 415 AM EST Tuesday...

Dry...seasonably cool conditions...leading to good travel
conditions through the period.

A large surface high pressure area will build into the region
from the Great Lakes Wed into Thu, then shift toward the
northern Mid-Atlantic region by Friday. Aloft, a vigorous upper
trough will lift out of the northeastern U.S. Thanksgiving day and
be replaced by a progressive upper ridge. Dry, yet unseasonably
cool conditions, will dominate the Wed-Fri Thanksgiving holiday
period as northwest flow aloft prevails. Our region will only
receiving a glancing blow of the bitterly cold air that will
sweep across the northeastern U.S./New England in particular
where 850mb temperatures will plummet into the -24C range. For
our region, 850mb temperatures are destined to drop into the -8C
to -10C range, which is not atypically cold at all for late
November. Lows will mostly be in the 20s west to the 30s east
Wed dropping into the 20s with a few teens at locations such as
Burkes Garden and Lewisburg Fri morning, which will be the
coldest. Afternoon highs will average about 10 degrees below
normal in the upper 30s to mid 40s.

A weak shortwave along the New Jersey coast Tuesday evening
will slide east into the Atlantic ocean. A few snow flurries may
linger in the western mountains early. High pressure in the
central U.S. will build eastward Tuesday night. Low temperatures
Tuesday night will range from the lower 20s in the northwest
mountains to the lower 30s in the piedmont. High pressure will
build east across the Tennessee Valley Wednesday. High
temperatures on Wednesday will warm into the mid 30s in the
northwest to the lower 50s in the piedmont. A very weak trough
passing to our north might bring snow flurries to the far
northwestern parts of the forecast area Wednesday night or early
Thursday. Low temperatures will range from the mid 20s in
northwest Greenbrier county in West Virginia to the lower 30s in
the piedmont. A large Canadian high will build southeast into
the area on Thanksgiving day into Friday. High temperatures
Thursday will be 5 to 10 degrees below normal with readings from
the mid 30s in the west to around 50 degrees along the eastern
slopes of the Blue Ridge mountains. Thursday night will be cold
with readings generally in the lower and mid 20s. Travel
conditions should be good these parts for the Wednesday through
Friday period.

Clouds will be to increase late in the day Friday, but
precipitation with the next upstream system is expected to hold
off until Friday night or more likely Saturday morning.

/Confidence Levels in Forecast Values/

Moderate to High confidence in forecast temperatures (below
High confidence in low probabilities of precipitation (mostly
dry through the period)
Moderate confidence in projected wind speeds and directions.


As of 430 AM EST Tuesday...

Weather conditions are expected to deteriorate significantly over
the weekend, as low pressure organizes in the northeastern Gulf of
Mexico and lifts northeastward along the coast and a vigorous
southern stream upper trough moves northeast from the Southern
Plains. However, the synoptic details among the various models
remain highly discrepant yet today with a variety of solutions
involving potential for winter weather, heavy rainfall, and even
strong to severe thunderstorms early next week.

High pressure will slide east into the western Atlantic Friday
into Friday night. Then, a strong negatively tilted trough is
progged to rotate through the region Saturday into Sunday. The
ECMWF remains the weakest with the upper trough and the Canadian
remains the strongest. The main problems to consider with this
weather system will be the potential for winter precipitation,
mostly likely in the form of freezing rain or sleet early
Saturday as the precipitation moves over the top of the residual
cold/dry air mass yet in place from Thu-Fri. The second concern
will be the potential for heavy rainfall later in the day
Saturday into early Sunday, with already saturated ground and
low FFG values from our extraordinarily wet fall/summer.

With strong Canadian high pressure to the northeast Saturday, a
significant cold air wedge should be in place as the upper
trough rotates toward the area. Today the models overall and
especially the MOS guidance have trended a bit colder and dry
for antecedent conditions prior to the arrival of precipitation
early Saturday. This could easily set the stage for a notable
period of freezing rain and/or sleet, less likely snow.
Depending on how long the shallow cold air mass lingers and the
precipitation falling into the air mass enhance the wedge, will
determine how along the winter precipitation would last. Still,
there is not 100% confidence by any means that the air
mass/ground will be cold enough to support winter precipitation
with gridded model data suggesting temperatures may only be in
the mid 30 to near 40 early Saturday. I am leaning toward the
colder numbers provided by ECMWF and GFS MOS given the strength
of the cold air mass to our northeast and that being the source
of the high pressure area supporting the wedge. A situation
similar to last Thursday is still not out of the question and
this event will need to be carefully analyzed in the remaining
days forecasts. The good news is that the worst part of this
weather system and the most likely time frame for winter
precipitation/problems will be Saturday morning, typically well
before the big Sunday after Thanksgiving travel rush. Stay

After a cold Saturday and the potential for temperatures to
hover only near or a few degrees above freezing all day,
temperatures will trend toward above normal levels by Sunday
after the wedge breaks and southwest flow evolves behind the
retreating wedge.

Looking ahead to early next week, another very strong negatively
tilted trough embedded within the southern stream is forecast to
eject across our area Mon-Tue. Unlike the weekend system,
surface temperatures and dewpoints will be more springlike with
this system and the strong shear and marginal instability
advertised by several models suggests that this could bring a
line of showers and thunderstorms with gusty winds, perhaps a
QLCS with its passage. The potential for such will be analyzed
as well during the coming days. Needless to say, an active
weather pattern is expected much of next week.

/Confidence Levels in Forecast Values/

-Moderate confidence in forecast temperatures (below normal
beginning of the period to above normal by the end),
-High confidence in moderate to high probabilities of
precipitation (pops 50-100% Saturday),
-Low confidence in precipitation type Friday night/Saturday
morning (i.e., rain vs. sleet, freezing rain, or snow)
-Moderate confidence in wind speeds and directions.


As of 1210 PM EST Tuesday...

In the wake of a cold front that has passed through our forecast
area blustery northwest winds are gusting to 15-30 KTS, and will
only gradually diminish tonight. Otherwise, VFR conditions
observed at most TAF sites, with the exception of KBLF, where
MVFR cigs were noted at 17Z (Noon EST). Only a gradual
improvement in clouds are expected this evening, with MVFR cigs
at BLF and occasionally KLWB slowly clearing early Wednesday
morning. A few widely scattered rain showers will be possible in
the mountains and foothills through this afternoon, but precip
will be light and no restrictions to visibilities are expected,
but winds will increase under these showers.

Conditions will gradually improve for flying late tonight and
Wednesday as winds subside and clouds erode, with high
confidence for VFR conditions.

Extended Discussion...

Weak upslope flow increases Thursday which may be an increase
in clouds and some MVFR ceilings at Bluefield.

Strong wedge of high pressure sets up at the end of the week and
southeast to east flow may allow for some lower cigs to set up
along and east of the Appalachians, increasing threat of precip
Friday night into Saturday and sub-VFR conditions.




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