Area Forecast Discussion
Issued by NWS Blacksburg, VA-- Remove Highlighting --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KRNK 141457
Area Forecast Discussion
National Weather Service Blacksburg VA
957 AM EST Tue Feb 14 2017
Weak high pressure over the area this morning will move east to
along the coast this afternoon. A cold front moving across the
upper midwest will approach the region tonight. In addition, an
area of low pressure over Texas will be moving east across the
southern states. A period of light rain or snow is likely late
tonight and early Wednesday, rain for the lower elevations, and
snow for the higher elevations. Blustery conditions will return
Thursday will colder temperatures.
.NEAR TERM /THROUGH WEDNESDAY/...
-- Changed Discussion --As of 950 AM EST Tuesday...
Limited adjustments were made to the ongoing forecast. Changes
include minor tweaks to the cloud cover to reflect the leading
edge of the main area of clouds that is progressing southeast
through the area. Likewise, the breaks in the overcast behind th
leading edge are now also better represented in the sky cover
grids. The result though for the day will remain the same. Still
anticipate a trend towards more cloud cover, especially this
afternoon, and in the west.
Also, temperatures, dew points and winds were refreshed to
reflect the latest observations and expected trends as we
progress into the early afternoon.
As of 500 AM EST Tuesday...
Split flow in the upper levels of the atmosphere will make for a
little bit of a weather challenge over the next 24 hours. A
short wave trough will dig southeast through the Great Lakes
Region today, its associated cold front moving southeast across
the upper midwest and into the Ohio Valley. Meanwhile, a short
wave trough over Texas will move east, spreading clouds and
moisture northeast across the Mid-MS valley and toward our
Model consensus is for increasing cloudiness today with
potential for some light rain late in the day across our far
western CWA. Radar may indicate potential for precip to arrive
earlier, but antecedent airmass is dry with dewpoints in the
teens and 20s, and radar returns will likely be from virga, or
precip which evaporates before reaching the ground. Temperatures
today will be relatively cool, the increasing cloud cover
preventing much in the way of a warm up. Undercut MOS temps by 4
to 8 degrees to account for the increasing cloud cover, with
most areas climbing into the 40s for highs, maybe 50 across
southside VA and the Piedmont.
By tonight, the column is forecast to eventually saturate with
likely to categorical pops for most of the CWA after midnight
and into early Wednesday. Greatest QPF is expected across the
southern half of the CWA (south of highway 460), with a quarter
to a half inch liquid equivalent. The northern CWA is expected to
receive a tenth of an inch or less. The key word here is liquid
equivalent because some areas of the CWA will receive snow as
opposed to rain.
Temperatures early tonight will initially will be in the 40s,
but as evaporational cooling of the column takes place it will
gradually bring temperatures down with the Freezing Level
dropping to within about 4kft above MSL. This lowering of the
freezing level will result in a p-type change from rain to snow
across the higher elevations, with some minor to modest
accumulations anticipated for elevations above 3500 feet. Places
like Mount Rogers (elv. 5700 ft) will have the best chance for
the precip to be all snow, so a quarter to half inch QPF would
translate into several inches of snow for these higher ridge
tops. Model snow ratios are suggesting something close to 7:1 so
1 to 4 inches of snow would be possible for elevations above
3500 feet late tonight and early Wednesday with focus on the
far western CWA from Bluefield south, including the Mountain
Empire, Grayson Highlands and the higher terrain surrounding
Mount Rogers, and then into the high country of NC for the
higher elevations just west of West Jefferson and into western
Watauga including Beech Mtn.
At some point we may need to consider an elevation defendant
winter weather advisory, but want to take a look at the next
model run or two to assess amount of cold air available and
whether the cold front approaching from the northwest will aid
or inhibit precip production and/or lowering of the freezing
level below what is already forecast. Confidence attm is for at
least an inch or two of snow for the highest elevations and just
plain ol rain below 2000 feet MSL. It`s the layer between
2000-4000 feet MSL that could have more or less of a mix of snow
pending available cold air, and a lot will depend on how warm
the boundary layer gets today and whether this warmth can be
maintained during the balance of precipitation event. Based on
what I am seeing in the current set of models (00Z run), the NAM
is the coldest with temperatures getting into the 28-33 degree
range for elevations above 3500 feet. From 3500-2000 MSL
temperatures cool into the mid-upper 30s. And below 2000 feet
MSL in elevation readings will be closer to 40 deg F. The GFS is
a little warmer, by a degree or two F, and a little wetter, a
tenth of an inch more QPF compared to the NAM. This would have
little impact on the lower elevations, yeilding a little more
rain, but would mean elevations above 4000 feet, such as Mount
Rogers could have closer to 4-5 inches of snow. The most likely
scenario is something in between, so 1-4 should cover it.
-- End Changed Discussion --
.SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY NIGHT/...
As of 415 AM EST Tuesday...
Classic split flow pattern will remain in place into Wednesday,
then evolve into a dominant northern stream flow across the
Mid-Atlantic region. Southern stream closed low coming out of
Texas will begin to fill and lift northeast as a deepening
northern stream upper low across the Great Lakes digs toward the
Mid-Atlantic. The RNK CWA will find itself on the northern
extent of the baroclinic zone associated with the merge of
these two systems, which will take place overnight Tuesday into
Wednesday. The best lift will reside near the I-40 corridor,
just to the south of the CWA. However, substantial lift will
exist to the north with the strengthening northern stream upper
trough such that an additional 1/4 inch of QPF can be expected
along and east of the Blue Ridge Wednesday morning. In the 15Z-
18Z time frame, models show a sharp northwest shift to the low-
level flow, which will result in a rapid recession of the
precipitation to the southeast, generally exiting the Piedmont
region around or shortly after 18Z. Pops at 12Z are generally
categorical along and east of the Blue Ridge to just slight
chance by 18Z southeastern Piedmont. By late morning/early
afternoon, any remaining precipitation will transition to
upslope across the usual western upslope areas.
As has been noted the past two days, there will be a race
between the advection of cooler air/evaporational cooling from
precipitation western mountains and the exit of the
precipitation to the east. Much of the time, temperatures will
be above freezing while precipitation is occurring. However,
toward daybreak and into the first half of this period, through
roughly 15Z Thursday, precipitation and temperatures at the
higher elevations of the western mountains should combine such
that minor accumulations of snow can be expected. This would be
mainly for elevations above 3500 ft. Looking at BUFKIT model
soundings, it is a fairly classic case where the sounding cools
from top down, leaving the surface layers, even at locations
such as Boone (TNB) just above freezing until right at daybreak.
Using a general snow/liquid ratio of 7-8:1 gives an additional
one inch or less of additional snowfall at these higher
elevations in the 12Z-18Z time frame Wednesday morning. Other
p-types do not appear to be an issue with precipitation
transitioning simply from snow to rain, or must remaining all
rain throughout the event at lower elevations and across the
Through Wednesday afternoon and into Thursday morning,
precipitation will end east of the Alleghany front as activity
transitions entirely to upslope snow showers on the heels of
another significant surge of Arctic air. 850mb temps will plunge
toward -12C by 12Z Thursday. Gusty northwest winds will become
a concern once again and appear to approach Wind Advisory
criteria in some areas along and west of the Blue Ridge once
again. However, the 850mb jet is generally in the 30-40kt range
as opposed to the 50-60kt range with the most recent event. Some
MOS guidance suggests, however, that some locations as noted
will be near Wind Advisory criteria. For now, just plan to
highlight this in the HWO and evaluate the need for an advisory
with later shifts.
Temperatures Wednesday with cloud cover and modest cold
advection will be near seasonal values for lows, while below for
highs. Thursday will be a cold day and generally about 10-15
degrees below normal, especially maximum temperatures. Friday,
the upper trough shifts east of the area and a rapid warmup
ensues once again. Several models depict an area of warm
advection clouds and perhaps a few sprinkles traversing the area
in the departing northwest flow, but this is likely overdone.
However, an area of dense cirrus could develop in such a pattern
and retard the otherwise expected warmup.
.LONG TERM /SATURDAY THROUGH MONDAY/...
As of 445 AM EST Tuesday...
This period will be characterized by an extended time frame of
above normal temperatures as the flow aloft amplifies across the
U.S. in response to an eastern Pacific trough/energy shifting
into the western and eventually central U.S. early next week.
Long wave ridging sets up just to the west of our region over
the weekend and gradually translates eastward into early next
week. Expect a mostly precipitation free period with well above
normal temperatures. A weak southern stream weather system may
mar an otherwise precipitation free weekend as it lifts
northeast from the southeast states early Sunday. Given that
this system is moving into an area of large scale ridging, only
minor amounts of precipitation are expected and it is expected
to lift out of the area by Sunday afternoon. 850mb temperatures
will approach +12C by late weekend into early next week,
supporting maximum temperatures in the 60s west to lower 70s
Piedmont by Sun-Mon. We will have to wait until the mid part of
next week for the aforementioned weather system to reach our
region and bring the next significant chance of precipitation.
.AVIATION /15Z TUESDAY THROUGH SATURDAY/...
As of 800 AM EST Tuesday...
VFR conditions will prevail today...or the first half of the TAF
cycle. Mid/High level cloudiness will be increasing in advance
of a storm system over Texas, but the low levels of the
atmosphere is dry, thus little or no low cloud element attm.
By tonight, moisture will increase enough to begin saturating
the lower levels of the atmosphere. MVFR cigs are expected to
develop from west to east across the region along with the
development of light rain. Freezing levels will lower to near
4kft tonight, yielding some snow accumulation in the mountains
of NC and far western VA.
Extended Aviation Discussion...
Sub-VFR conditions likely Wednesday. Passage of a cold front and
development of a coastal low will bring another round of
stronger winds and colder temperatures for Wednesday night and
Thursday. The northwest flow will continue to promote some MVFR
cigs in the mountains through Thursday. Friday and Saturday will
be milder and dry with VFR conditions.