Area Forecast Discussion
Issued by NWS Blacksburg, VA

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303
FXUS61 KRNK 281840
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
240 PM EDT Sun May 28 2017

.SYNOPSIS...
Warm front lifts north to the mid-Atlantic this evening, with a
cold front to follow across our area Monday. This front slows
and stalls from the Tidewater of Virginia to the Smokys
Tuesday, while a stronger cold front moves in helping to push
the deeper moisture out midweek.

&&

.NEAR TERM /THROUGH MONDAY/...
As of 230 PM EDT Sunday...

Cu field starting to grow over the Blue Ridge south of Mabry Mill to
the NC/TN border. Weak convergence boundary situated in this area,
and anticipate showers and thunderstorms to fire up along/east of
the divide in the next hour. Weaker showers further north where
clouds have kept it more stable. Still expecting good insolation in
the south with the boundary slowly moving north to produce a decent
enough baroclinic zone/differential heating zone to fire
thunderstorms after 2-3pm. At the moment high-res models coming in
tend to favor more coverage over the mountains from NC to WV with
less coverage east, Getting a little more wind aloft into WV should
send any storms just north of east or northeast. Upper flow becomes
a little more difluent ahead of next shortwave over VA so expect a
better coverage north of Highway 460, but with better severe threat
south of Highway 58 in VA, with more widely scattered coverage.

Should see subsiding convection this evening, with clearing thanks
to subsidence behind the wave expected east of the mountains. Still
have the cold front to shift to the mountain by dawn Memorial Day.
Will keep scattered threat of showers/isolated storms around through
Monday morning.

Monday seems like transitional day for us, with models mainly
emphasizing greater threat of storms south of the NC/VA border as
shortwave moves across by midday. Scattered strong storms appear
likely in our southern CWA thanks to the stronger dynamics per NAM,
but best thermodynamics stay south if I-40. Will be mainly dry north
of US HWY 460 Monday, so not looking like a bad day for outdoor
activities.

With some clearing tonight and high ground moisture should see
patchy fog mainly east of the mountains, and in mountain valleys.

Still muggy tonight with lows in the 60 to 65 degree range. Highs
Monday will range from the mid 70s mountains to mid 80s east of the
Blue Ridge.

&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/...
As of 1110 AM EDT Sunday...

With the potential for active weather again this afternoon,
the short and long-term forecast was completed early, mainly
using the 12z NAM, 09z SREF, 06z GFS and the 00z ECMWF.

While still some question as to its exact position leading into the
period, a poorly-defined northeast to southwest oriented surface
cold front should extend from southern Delaware southwestward
through the northern Tennessee Valley Monday night. Boundary will
lie roughly along the 500 mb gradient between subtropical ridge over
the Bahamas and broad cyclonic flow regime associated with the
southern end of an upper low over the northern Great Lakes region.
This is a setup does not typically encourage much southward progress
of the baroclinic zone; as such, chances of mainly showers will
remain focused across the Southside of VA southwestward across the
northern NC Piedmont and into the NC foothills/High Country, as weak
mid-level vorticity maxima propagate along the boundary.
Prospect of thunder is low and appears to be more focused from
the NC Triangle/Triad areas southward into Upstate SC. Will keep
slight chance but it probably is slight in every sense of the
phrase. Expecting a dry forecast outside of these areas; may
need to watch for potential radiation fog in the dry areas
conditional on winds staying light but confidence is not all
that high.

Cold front should have slipped just south of the forecast area by
early Tuesday morning, due to cold advection and effect of rain-
cooled air pushing the effective position south. This should lead to
a mainly dry forecast at least initially. Some question between
the GFS and the ECMWF on areal coverage of showers, with the
ECMWF shows some additional light QPF on the northern side of
the frontal zone associated with a weak vort max. I`ve therefore
kept a low PoP/slight Chance for the NC foothills. The area
where both NAM, GFS and ECMWF agree upon as far as shower
coverage goes is in eastern WV and the Mountain Empire along a
passing surface trough in westerly flow. Depicted PoPs here into
the lower Chance range; wouldn`t discount idea of a couple
thunderstorms but not thinking areal coverage of those would be
all that great. Surface trough advances eastward with little
fanfare into the evening hours with forecast trending dry for
Tuesday evening.

Broad longwave trough over the northern tier of states then digs
southward towards the central Appalachians on Wednesday. A more
potent mid-level shortwave trough in the west-northwest flow and
associated surface front moves eastward Wednesday. Wind fields are
rather strong ahead of the front, but poor low-level moisture,
limited degree of instability and questions about trough timing cast
uncertainty on potential shower/storm coverage and intensity. Due to
this uncertainty, kept a generic slight chance PoP for showers and
thunderstorms along and west of the foothills of the Blue Ridge.
This could be a day with more active weather if greater
heating/instability and better moisture can be realized but is
definitely conditional.

Temperatures through the period trend near to slightly above normal
Monday night into Tuesday (highs in the 70s-mid 80s, lows in the 50s
to near 60), then start to trend closer to normal toward Tuesday
night and Wednesday.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
As of 1111 AM EDT Sunday...

Heavily used a blend of guidance for this forecast period. Clearing
of the front Wednesday night with increasing subsidence follows for
overnight into Thursday. Thursday appears to be the next day with
fully dry and mostly sunny conditions for the forecast area along
with seasonable temperatures. Unfortunately, it appears that is
short-lived, as a trough digs into the southwestern CONUS and sends
weak vorticity maxima pivoting eastward across the central Plains,
Ohio/TN valleys and into the Appalachians. Difficult to time any
specific disturbances at this range but it does appear we return to
a more unsettled weather pattern for late week into the weekend.
Temperatures start out near normal, then begin to trend near to
slightly above along with rising dewpoints heading into the
weekend.

&&

.AVIATION /19Z SUNDAY THROUGH FRIDAY/...
As of 135 PM EDT Sunday...

Looks like VFR will be the rule over the area today into this
evening outside of any heavier convection. Latest sat/model
trends favor a LWB-LYH line for sub VFR with shower and possibly
thunderstorm threat, with less coverage to the south. Am
confident enough to have tempo TSRA at LWB/ROA/LYH along this
corridor for an hour or so between 18z-23z. Still some MVFR cigs
at LYH but should be VFR soon.

Appears showers/thunderstorms subside this evening, though a few
showers or storms may still be present ahead of the cold front
tonight in WV. With wet ground/moist low levels and some
clearing have added fog in mainly LYH/DAN/LWB/BCB for 3sm,
though lower is a possibility.

Extended Aviation Discussion...

Deep convection becomes less organized Monday into Tuesday with
only hit and miss sub-VFR conditions. Perhaps late night and
early morning sub-VFR river and mountain valley fog.

Better organization of convection Wednesday with the passage of
a cold front. Thursday is expected to be VFR and dry, with
widely scattered showers or storms starting up again Friday.

&&

.HYDROLOGY...
As of 1040 AM EDT Sunday...

No flood/flash flood watches at the moment for us. Will have to
watch the Blue Ridge north of Roanoke, as well as the
Alleghanys, as high-res models favor this area to get
showers/storms locked in over the terrain per weaker low level
winds this morning. WPC already highlighting this area for
possible hydro issues where the low level flow along the front
works to keep showers slower moving and meandering along the
boundary.

Given isolated threat will not put a flash flood watch out at
this point.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.

&&

$$
SYNOPSIS...AMS/WP
NEAR TERM...WP
SHORT TERM...AL
LONG TERM...AL
AVIATION...AMS/PM/WP
HYDROLOGY...WP



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