Area Forecast Discussion
Issued by NWS Blacksburg, VA

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS61 KRNK 180820

Area Forecast Discussion
National Weather Service Blacksburg VA
420 AM EDT Sun Mar 18 2018

High pressure over the region today will drift east as a complex
storm system approaches our region from southwest. This next system
will initially bring rain to the region late tonight and Monday, but
wintry precipitation is possible by Tuesday night and Wednesday.

As of 315 AM EDT Sunday...

A robust upper level shortwave seen on GOES-E water vapor is just
entering our forecast area early this morning. This wave and its
associated UVM has helped generate scattered showers and even some
isolated thunderstorms from Roanoke and points east. LAPS data and
SPC mesoscale analysis indicated weak positive CAPE and DCAPE values
near this region on north side of a narrow band of higher dew
points. Kept a small chance for thunder in the south through 10Z.

As this wave zips past our area this morning, subsidence and drier
air arriving from the northeast should erode clouds from north to
south during the day. This southward push of clearing near the Ohio
River seen on satellite at 07Z matches well with most of the model
guidance, which projects clearing into our area by midday into early

With limited mixing today and lower 1000-850mb thickness values
forecast, leaned toward the cooler side of guidance for highs today.

With a progressive upper level flow in place, low level moisture
quickly returns tonight as low level winds back ahead of the next
system. Good isentropic lift develops and the nose of an 850mb theta-
e ridge arrives late in the southwest which will bring a return of
stratus to the southern Blue Ridge and even the chance of light
rain. Temps may fall quickly after sunset, then become steady
overnight as clouds thicken.


As of 415 AM EDT Sunday...

Quite active weather pattern indicated in the 00z suite of
deterministic and ensemble guidance for the Monday through Wednesday
time period. Pattern aloft begins roughly zonal but becomes a highly-
amplified trough and cold pocket of air aloft toward the middle of
the week. The main highlights in this period include a rather wet
warm frontal passage Monday into Monday night, and then a prolonged
cold rain to accumulating wet snow event Tuesday night into
Wednesday. Specific to the latter, guidance differences still exist
on thermal structure, with a resulting great deal of uncertainty as
it pertains to snow accumulations. Elevation changes, marginal
temperatures and time-of-day are among other considerations in the
snow or rain setup for around midweek.

Initially potent, though deamplifying 500 mb shortwave and
associated surface warm frontal feature progresses across the
forecast area from the TN Valley Monday into Monday night. PoPs
steadily increase to Categorical levels by late Monday areawide in
association with the frontal system. Global models are steadfast in
showing rather significant QPF into the Monday night timeframe,
perhaps enhanced by frontogenetic forcing which skirts across
northern North Carolina into southern Virginia. QPF values range
from three quarters to an inch of rain, with the heaviest amounts
from the foothills of the Blue Ridge into the Piedmont. Though 850
mb temperatures would argue for milder daytime temperatures than the
official forecast, with lots of clouds and rain I sided temperatures
on the colder side for highs and somewhat milder on the lows for
Monday. Highs should top in the mid 40s to low 50s, with lows upper
30s to mid 40s.

Tuesday should generally feature a lull in precipitation, in the
wake of the first frontal system affecting the region from Monday.
ECMWF is slowest with moving this shield of precip out, while the
NAM and GFS show clearing early Tuesday. Did show a general decrease
in PoPs first half of Tuesday, but majority of guidance shows in
this lull develops a strong wedge with northerly/northeasterly flow
drawing colder 850 mb temps southward. 850 mb temperatures should
trend somewhere around -1 to +3C areawide, coolest further north in
the southern Shenandoah/Highlands areas. Exactly how cold low-level
temperatures become in this period then become critical to how the
Tuesday night-Wednesday period play out. Kept highs only in the
upper 30s to mid-40s on Tuesday under cloudy wedge conditions.

Tuesday night into Wednesday is about when model guidance indicates
a mean trough and cool pocket of air aloft develops over the region.
Before this process occurs, another strong mid-level shortwave
trough induces surface low development in the southern Carolinas.
Nearly all guidance show an inverted trough and/or mid-level
deformation band focusing a precipitation shield north/northwestward
across the Blacksburg forecast area, which begins Tuesday night and
lingers into Wednesday. PoPs though Tuesday night - Wednesday become
Categorical again. Dynamic columnar cooling should be taking place
throughout this period as well, which suggests a changeover from
rain to wet snow at least in the Blue Ridge/Greenbrier mountains
Tuesday night. As the column keeps cooling into Wednesday, rain may
mix over to wet snow even in lower elevations pending surface
temperatures. Have the higher confidence in elevation-based
significant wet snow accumulations, but given accumulations fall
over a long duration it`s not clear if warning criteria would be met
in a 12- or 24-hour period of time. Could at least envision Advisory
type snowfall across the higher elevations. Feel the best chance for
at least some limited wet snow accumulation in the lower valleys and
into the Piedmont would be Wednesday with 850 mb temperatures
between -4 to -8C and isothermal temperature profiles just off the
ground. Will therefore mention accumulating wet snow potential in
the HWO. For temperatures, kept lows Tuesday evening in the upper
20s to low 30s, with Wednesday highs ranging from 31-37 degrees.

As of 415 AM EDT Sunday...

As the closed upper low moves across the region synoptic forcing
lingers along with the decreasing possibility for wet snow through
early Thursday. Northwest flow will also be on the increase and will
support upslope snow showers at the higher elevations west of the
Blue Ridge into Thursday night. The upper trof axis finally pushes
to our east and allows for a break of fair weather on Friday before
a warm front starts to push precipitation back in our direction from
the west late Friday night.

Temperatures will start the period well below normal, then moderate
a bit but remain on the cool side through the end of the week.

As of 0135 AM EDT Sunday...

Widely scattered showers and a few stray storms ahead of an
upper level disturbance will continue to move southeast and
are expected to exit the WFO RNK forecast area by 10Z (6 AM
EDT). Kept a few hours of SHRA for all TAF sites early this

LIFR ceilings at KBLF, KLWB and KBCB are expected to improve
during the morning hours as light northeast/east sfc winds
ushers in drier air. By 16Z, all TAF sites should be VFR, but
confidence on the timing of this improvement is low. Higher
confidence for VFR conditions for this afternoon and this

Extended Aviation Discussion...

Moisture returning in advance of the next storm system for
early next week could bring a return to MVFR/IFR ceilings for
the mountains TAF sites late tonight/early Monday morning.

This system should push eastward from the Plains on Monday. The
warm front associated with this system could bring IFR/MVFR
conditions due to ceilings and showers for Monday night into
Tuesday. As the low pressure system heads offshore on Tuesday
night, precipitation wrapping along the western periphery of
this system could bring a period of snow showers, MVFR/IFR
conditions, and gusty north to northwest winds on Wednesday.
Drier air should finally bring conditions back to VFR by
Thursday as high pressure builds from the northwest.


As of 340 AM EDT Sunday...

The Mount Jefferson NWR remains off the air and will likely be
sometime next week before it is operational again.




EQUIPMENT...JH is the U.S. government's official web portal to all federal, state and local government web resources and services.