Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 241120

Area Forecast Discussion
National Weather Service Blacksburg VA
620 AM EST Wed Jan 24 2018

A strong upper disturbance moves across the mountains this
evening bringing scattered snow showers to central and southern
Appalachians. High pressure builds in Thursday from the
Tennessee Valley. Temperatures will moderate to above normal
again later in the week before the next frontal system arrives
during the weekend.


As of 305 AM EST Wednesday...

Velocity still up there on the ridges with winds gusting to 40 to 50
mph from Roanoke to Boone, and another surge of pressure rises
arriving between 3 to 7am as another shortwave passes. Should see
winds die down after 7am, but will let advisory run til 10am.

Another issue will be the snow showers already pushing into
southeast WV with flurries further south. Since the track of the
vort is about on the same latitude as southern WV will mainly see
best accumulations over the mountains of western Greenbrier county
this morning where a half inch could fall. A dusting is possible in
the higher ridges as far south as the NC mountains.

There will be a lull between shortwaves with a stronger one digging
further south this evening. Dynamics with this system favor an
increase in snow amounts across the mountains of WV as models
showing some increase in Fn north of I-64 in WV. Timing of this
favors from around 6-7pm this evening through early overnight. At the
moment dendritic growth layer is somewhat dry but models showing
moisture at least getting to the -10C to -12C range but it is dry
above it, so will potentially 1-3 inches, in western Greenbrier
while everyone else over the higher ridges from Summers to Watauga
come up with a dusting to < 1 inch. Will highlight western
Greenbrier in the HWO, and potential winter wx advisory may be

Winds behind this vort overall will pickup again, but models showing
weaker 8h jet around 35 kts, so expect sub-advisory winds, but still

Temperatures today will be closer to normal for late January with
mid 30s to around 40 across the mountains, to mid 40s to around 50
from Roanoke to the Piedmont of VA/NC.

Expect gusty winds tonight and cloud cover in the mountains to keep
temperatures elevated some but still with cold advection lows will
be in the 20s, with teens in the higher ridges of WV/SW VA and NC.


As of 329 AM EST Wednesday...

Overall highlights in the period feature a warming trend to
temperatures, plentiful sun and dry conditions to close the
workweek. Frontal system offers next chance for rain later Saturday
and especially Saturday night. 00z NWP in good agreement overall
through the period, with some timing discrepancies evident with
frontal system late in the period.

We open Thursday under cold pool of air aloft and broad cyclonic mid-
level flow. Combination of increased subsidence behind an early-day
mid-level shortwave trough passage will dry up residual upslope
clouds/leftover -SHSN in southeast WV. Moderate warm advection then
begins with 850 mb temps warming to -3 to -6C by. Under full sun
thereafter, highs mainly in the 40s though some spot 50 degree
values in the foothills/piedmont in NC. Large dry surface ridge then
settles across a large part of the eastern states Thursday night.
Good radiational cooling expected with valley lows in the mid 20s
with lower 30s/near freezing along the ridges.

Friday is high and dry. Low level temperatures warm each day a
couple degrees under light southeast return. However dewpoints are
slower to recover and should mix lower in the afternoon to RH`s in
the upper 20s-low 30s. Should see highs well into the 50s Friday.
Will start Friday night mostly clear, but better tap of lower-level
moisture in southeast flow will start to increase cloud cover
particularly along the southern Blue Ridge in NC. Should see lows
ranging in the 30s, except upper 20s in the Greenbrier Valley.

Mid-level heights then slowly fall through Saturday per 00z model
consensus as we await the approach of a digging northern stream
disturbance aloft and surface cold front. This shortwave feature
looks to interact with at least modest Gulf moisture return. Today`s
00z guidance have slowed timing of meaningful QPF until later into
Saturday night with slowing front/upper flow becoming parallel to
baroclinic zone. There is some token light QPF values late Saturday
afternoon in southeast upslope and will carry slight Chance PoPs for
sprinkles or light rain from Tazewell southward into Watauga
Counties. Better frontal forcing then begins to encroach mainly
after midnight, so will then taper PoPs up to high Chance/low Likely
between 06-12z. Passage of the front really is not until Sunday but
do envision most locations receiving some modest rain amounts. Highs
Saturday have been warmed a couple degrees into the mid/upper 50s
generally, to close to 60 in Southside. Kept lows in the 40s but
these may need to be raised in later updates if the front is slower
to advance east.


As of 330 AM EST Wednesday...

Aforementioned frontal passage looks to be during the Sunday
timeframe. PoPs are relatively high for Sunday but then decrease
steadily with a slower dropoff in the customary northwest upslope

Given the differences in the intensity and timing between the ECMWF
and the GFS, the degree of cold air that follows in the wake of this
frontal system early next week and the amount of post frontal
upslope snow showers will largely depend on which solution verifies.
For now, will carry snow showers across just the Alleghanys
following frontal passage Sunday night into Monday. 850mb
temperatures fall back toward -8C per the ECMWF, but barely reach 0C
per GFS with a flatter, more zonal flow.


As of 617 AM EST Wednesday...

MVFR to occasional IFR cigs in the mountains this morning will
improve to VFR, though appears BLF may stay MVFR through the
period, maybe improving late tonight.

A few snow showers skirt across BLF/LWB mainly early this
morning, and again this evening which could reduce visibilities
to IFR.

Gusty northwest winds will persist with another burst tonight
behind the upper disturbance.

Medium to high confidence in ceilings and winds in the TAF
period. Medium to low confidence on visibilities in snow
showers during the taf period.

Extended Discussion...

High pressure then builds into the region Thursday through
Friday night with conditions trending VFR all terminals. Next
chance at sub- VFR is later in the weekend with a frontal system
moves across the region.


VA...Wind Advisory until 10 AM EST this morning for VAZ015>017-022.
NC...Wind Advisory until 10 AM EST this morning for NCZ001-002-018.


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