Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 221942

Area Forecast Discussion
National Weather Service Blacksburg VA
342 PM EDT Tue May 22 2018

A weak cold front will approach from the northwest tonight
before sliding through the region on Wednesday. High pressure
noses down from the Ohio Valley and Mid Atlantic Wednesday night
into Thursday, then off the coast by Friday.

As of 250 PM EDT Tuesday...

Clusters of showers/storms continue to develop within an axis of
instability/convergence across the western mountains with some of this
now jumping to the eastern slopes of the Blue Ridge where the flow
remains more southwest. Latest short term guidance indicates a gradual
organization to more short/broken lines as the storms slowly propagate
east into better CAPE along/east of the mountains as indicated by the
latest HRRR, but less by the HREF which seems best based on current coverage.
Since have more instability out east, will go with more deeper convection
spilling into the foothills where did bump up pops a little espcly
northeast sections as the tail of the passing speed max to the north
clips across per severe watch 116. A lot more uncertainty exists
east/southeast into the Piedmont given better capping aloft and
less focus until around sunset so kept mostly chance to low
likelys there for now. Expect most of this convection to exit
the southeast later this evening with the actual cold front not
arriving across the far northwest until just before dawn. Thus
will leave in some low pops far northwest overnight with slight
pops or less after midnight elsewhere otherwise becoming partly
cloudy. Lows mainly muggy 60s with some dense fog again in spots
espcly where earlier heavier rainfall occurred.

Surface cold front sags through the region from the north/northwest
Wednesday preceded by deepening northwest flow in the wake of the pre-
frontal boundary from this evening. Guidance showing this trajectory to
allow some drying aloft with PWATs dropping off in all except the far
southeast where the boundary will likely end up by late in the day.
This looks to be the region for added deeper convection Wednesday
afternoon as a tight forecast instability/theta-e gradient exists.
However the majority of the stronger cells may end up just south of the
area as suggested by the Nam/GFS with more isolated coverage farther
north. Highs mostly low/mid 80s east and 75-80 west pending clouds far
west and strength of downslope out east.

As of 300 PM EDT Tuesday...

Our forecast pattern will be transitioning into a relatively brief
drier period during this portion of the forecast. An upper level
ridge is expected to become parked over the mid-Atlantic region.
This will help shunt moisture southwest of our region along with
most of the cloud cover. Temperatures will also tend slightly
cooler, mainly the result of lower dew points allowing for slightly
cooler low temperatures.

The exception to the above rule will be Wednesday night in the
southeast where there will be some lingering precipitation
associated with a departing cold front. Also, the Northern Mountains
of North Carolina may be far enough southwest of the center of the
ridge axis to have a bit of a moisture return that will allow for
some afternoon diurnal showers Thursday. On Friday a slightly larger
part of the southwest portion of the area may see some of these
showers, and a few thunderstorms, as the center of the upper ridge
shifts a bit east.

As of 330 PM EDT Tuesday...

This portion of the forecast period will be one characterized by a
generous return of moisture to the area. The upper ridge over the
mid-Atlantic will head southeast and become centered in the western
Atlantic, east of GA/FL. This will again open the door for Atlantic
and Gulf of Mexico moisture to stream into the region. Northern
stream energy will again progress into or just north of the region
along a baroclinic zone, allowing for coverage of showers and storms
similar those those of recent days. We even may be influenced by
moisture heading north from what may be a tropical system in the
east Gulf of Mexico by early next week.

Temperatures during this portion of the forecast will average five
to ten degrees above normal.

As of 1245 PM EDT Tuesday...

Clusters of showers and storms developing across the western
mountains will shift slowly east reaching the Blue Ridge by
19-20Z/3-4PM, and eastern locations between 20-23Z/4-7PM.

Will likely see more of a scattered coverage west to start with
most Taf sites having at least thunder in the vicinity but will
likely include a Tempo period of thunder from northwest to
southeast into this evening. Potential to see at least a broken
line along and east of the Blue Ridge later this afternoon so
boosting thunder mention east later on.

Looks like activity wanes in the evening with some clearing
taking place to allow for fog, dense at times to form. Should
see at least MVFR fog to IFR stratus at most sites, with dense
fog at KLWB/KBCB late.

Appears any low clouds/fog will fade by mid morning Wednesday
leading to more in the way of VFR across the region through
early afternoon. Added sub-VFR conditions will be possible
mainly southeast of a line from KLYH- KDAN with most activity
likely south of the region later in the afternoon.

.Extended Aviation Discussion...

Conditions are will be drier and are expected to be VFR
Thursday and Friday. Tropical like moisture streams back north
as a warm front moves through Saturday with a return to
showers/storms and potential sub-VFR conditions and heavy rain
by Saturday afternoon/evening.




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