Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 261947

Area Forecast Discussion
National Weather Service Blacksburg VA
347 PM EDT THU MAY 26 2016

A southwest flow of moisture around high pressure will push warm
unstable air into our region. High pressure staying off the coast
through Friday with typical summer like weather impacting us
increasing humidity. This weekend, a potential tropical low may
arrive toward the South Carolina coast and linger in the general
area into early next week.


As of 347 pm EDT Thursday...

An upper air disturbance rotating across our area will trigger
scattered showers and thunderstorms this afternoon into this
evening. SPC mesoscale analysis at 18z showed best moisture
convergences along the southern Blue ridge mountains. Sbcape values
have climb into the 1000 to 1500 j/kg range in the east. SB LIs were
minus 2 to minus 4 across our region. Looks like the best chance for
convection will be along the Blue Ridge. SPC has our area in general
thunderstorms in the day one convective outlook. Limited instability
and weak shear will hinder severe weather potential. For pops this
afternoon leaned towards the HRRR, then blended NAM and ECMWF for
tonight. With the loss of solar heating expect storms to diminish
this evening and end by Midnight. Areas of fog will develop
overnight into Friday morning especially in the river valleys. Low
temperatures tonight will drop into the upper 50s in the mountains
to the mid 60s in the Piedmont.

Surface high over the western Atlantic strengthens Friday as a cold
front approaches from the west. A deep conveyor belt of Gulf
moisture will be tracking northward into our area around the high.
An upper disturbance will moved into New England, leaving little
triggering mechanism to work with Friday. A very moist summerlike
atmosphere will be present across the region with PWATS in around
1.5 inch. Scattered convection should develop...especially across
the Alleghanys and western mountains in association with
differential heating. Steering currents will be weak and limited
dynamics. High temperatures Friday will climb into the mid 70s in
the mountains to the mid 80s in the Piedmont.


As of 330 pm EDT Thursday...

Scattered showers and thunderstorms Friday afternoon especially in
the northwest will diminish with the loss of solar heating Friday
evening. The combination of low level moisture and light winds will
result in areas of fog Friday night into Saturday morning.

The remainder of the forecast period continues to hinge heavily on
the track...intensity...and duration of a tropical system developing
across the Bahamas. The models move the coastal upper low to the SC
coast by Saturday evening, while the upper ridge weakens over the
Southeast. The upper low is shown to move very little over coast SC
through Sunday.

The 27/12Z ECMWF/GFS were in very good agreement until 30/12Z
along the SC Coast. Then, there are differences on how the models
handle the low and its associated moisture. WPC was leaning
towards the ecmwf solution over the aggressive and inland GFS
solution. The GFS solution would bring heavy rain into the CWA and
arouse flooding concerns...while the other models suggest much
less potential...barring a predecessor rain event (PRE). Will need
to watch closely...but the consensus among most models is for
scattered to numerous showers/thunderstorms by Sunday across the
region...especially east of the Blue Ridge...requiring at least
high chance pops. It will not be widespread continuous rain
event...but largely diurnally driven. A very moist summerlike
atmosphere will expected across the region with PWATS in around
1.0 to 2.0 inches.

Moisture/humidity will increase through the period and overall it
will feel very muggy and summerlike with mild to warm nights and
warm/humid days. Looks for low temperatures to be mainly in the 60s
with high temperatures in the the 70s for the mountains.


As of 345 pm EDT Thursday...

There remains a lot of uncertainty in the medium range forecast.
The forecast challenge through the period will depend on the track
of an area of tropical low pressure across the Southeast states. The
Question is whether it linger across the region through the period
thanks to a stagnant upper atmospheric pattern with an upper high to
our north and a broad upper low continuing in the western U.S.

If the tropical system does linger for several days with non-
existent steering currents and blocked upper flow, rainfall may
become a concern as a result. The other key player in the extended
period is a cold front to the west, which travels eastward during
the period. The strength of the ridge will determine how far the
front makes it. In any case, we can expect scattered diurnal
convection with warm/humid conditions through the period. used the
superblend for temperatures. In general, High temperatures will
remain mostly in the 70s mountains to the 80s elsewhere. Low
temperatures will be mainly in the 60s. The details of the extended
forecast will become clear during the next couple of days.


As of 139 PM EDT Thursday...

An upper air disturbance rotating around a broad diffluent upper low
will triggering scattered showers and thunderstorms, especially west
of the Blue Ridge. Isolated convection is forming along the
southern Blue Ridge with moisture convergence. Because of the
nature of the convection, have included VCTS for BCB/ROA/BLF/LWB.
Outside of this activity...expect mostly VFR cigs/vsbys through
the day.

Areas that see rain this afternoon into this evening will have a
better chance of seeing late night/early morning fog Friday morning
and the potential exists for ifr-lifr cigs/vsbys in such...mainly
thinking LWB/BCB for now...with less rain coverage east of the Blue

Winds mostly SSW-SW through the period 5-7kts...with some low end
gusts possible KBLF late morning through the afternoon. Gusty
winds may also be possible near a thunderstorm.

Medium confidence in ceilings, visibilities and winds during the taf

Extended discussion...

Friday and Saturday the region will be in between systems with the
upper low remaining west of the region and a tropical system
developing off the SC coast. Little in the way of forcing and
increased subsidence should confine any convection to the
mountains...isolated to sct diurnal activity at best. For the
remainder of the period...the forecast will hinge heavily on the
track of the tropical low pressure area...which at a minimum will
bring increased moisture and shower chances to the area...which
could linger well into next week. This will result in periods of
mvfr-ifr cigs and greater potential for late night/early morning
fog as well.




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