Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 281513
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
1113 AM EDT Sun May 28 2017

.SYNOPSIS...
A frontal boundary stretching across Indiana, southeast to NC
will shift northward today. as a low tracks into New York. A
stronger cold front will move through the region on Wednesday
morning.

&&

.NEAR TERM /THROUGH MONDAY/...
As of 1030 AM EDT Saturday...

Getting very concentrated area of moderate to heavier showers
along the Blue Ridge north of Peaks of Otter, with isolated
lighter coverage elsewhere. The 13z HRRR and RAP13 showed this
fairly well, can hang onto a higher pop regime from Southeast WV
to the Blue Ridge north of Roanoke, east to Buckingham through
early afternoon.

At the surface there is an area of convergence in the wind field
over northern NC. Vis satellite indicated strong insolation
taking place in the south, with heavier cloud cover north of
Danville. This looks to be a place where differential heating
and some upper vorticity advection takes place through early
afternoon. However, 12z RNK sounding even modified is not too
aggressive with severe threat today, in terms of an organized
threat. Airmass is highly moist so will have to watch for
downpours as low level flow is weaker and storms may get
anchored. Severe mode today will be pulse to multicellular with
isolated large hail or damaging winds. The front should act to
focus these storms. SPC still has slight risk out.

No flood watches at this time, see hydro section below for
details.

Forecast update to lower pops based on radar in most of the
forecast area, and adjust sky cover to have more sun in the
south per latest satellite trends and 12z NAM.

Previous discussion from early morning...

Large upper low will deepen over southeast Canada today and
tonight and gradually back the mean flow over the Mid Atlantic
region from the west to the southwest. Models showing a series
of small short waves tracking through the southern stream around
the base of this trof. Any of these features may aide in
occurrence of precipitation.

At the surface low pressure will be just off the southeast Virginia
coast this morning along a warm front trailing into central Ohio.
This boundary will provide some low level surface convergence in the
Virginia piedmont this afternoon. By late today the low and front
move northeast, with the models showing the front east of Farmville
by 03Z/11PM. A weak cold front approaching from the northwest will
reach the mountains by Monday morning.

Amount of high clouds left over from the line of showers and
thunderstorms across Tennessee and North Carolina will impact amount
of heating this morning. Expecting enough sunshine to support
MUCAPES in the 500-1000J/kg range this afternoon.

No good consensus between models today on where most likely areas
and times where there will be thunderstorms.  Will have the highest
probability when the instability will be largest, between 18Z/2PM
and 03Z/11PM.

&&

.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 propagage 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 initally. 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 SATURDAY/...
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 /15Z SUNDAY THROUGH THURSDAY/...
As of 755 AM EDT Sunday...

Expecting isolated showers this morning for southwest Virginia,
northwest North Carolina and southeast West Virginia. Expect the
IFR clouds and fog to dissipate around 14Z.

High confidence that scattered thunderstorms will develop this
afternoon, but low confidence as to where. Some storms will be
along the front from central West Virginia to southeast
Virginia. Models also suggested that a ling of storms may cross
the region out of the Tennessee Valley from west to east,
possible between 20Z/4PM and 03Z/11PM. There is a slight risk
for severe storms...containing large hail and potential for
downburst winds.

Have added fog in again for tonight once a majority of the
thunderstorms dissipate. High confidence of MVFR visibilities.

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.

&&

.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...AMS/PM/WP
SHORT TERM...AL
LONG TERM...AL
AVIATION...AMS/PM
HYDROLOGY...WP



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