Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 241801

Area Forecast Discussion
National Weather Service Blacksburg VA
201 PM EDT TUE MAY 24 2016

Our stretch of cool weather will be coming to an end as high pressure
builds over the southeast United States and ushers much warmer air into
the region. Temperatures will be well above normal for most of the week
with highs by Thursday ranging from the upper 80s across the piedmont
to the lower and middle 80s west of the Blue Ridge. However along with
the warm temperatures will come increasing humidity and the chance for
afternoon showers and thunderstorms.


As of 1155 AM EDT Tuesday...

Morning RNK sounding still pretty cool aloft. This will allow for
at least scattered clouds this afternoon per steep low/mid level
lapse rates. Forecast soundings indicate modest cu depth with
cloud bases around 4kft and max cloud depth to about 10kft. This
may generate a few sprinkles, but overall trend is for warming to
take place aloft, 700 mb temperatures increasing from m1 deg C to
+2 deg C by the end of the day. This should cap cloud depth and
prevent deep convection. The Meso models and HRRR generate some
isolated showers, but POPs AOB 10%.

The upper low which has been with us for several days will still
have enough influence over our weather today to keep a just a
slight chance of showers across the region...especially far north
and east...thanks to the combination of diurnal instability and
steep lapse rates in the cold pool of air aloft. The prospects for
thunder look to be very limited indeed so will not include in the
grids. As the day wears on and the low continues pulling off to
the northeast much warmer air will begin to surge in from the
southwest. This will give our temperatures a much needed boost to
slightly above normal...which should feel like a tropical heat
wave after our recent stretch of cool weather. Expect highs today
to reach the lower 80s east of the Blue Ridge with generally
middle and upper 70s to the west. Tonight looks to be quiet with
mostly clear skies and lows ranging from the middle and upper 50s
east to upper 40s and lower 50s west. There may also be valley fog
forming once again late tonight into early Wednesday morning.


As of 400 am EDT Tuesday...

Upper heights and 850mb temps continue to rise through the period
as the northeast U.S. upper low continues to lift northeast into
the north Atlantic and troughing deepens and shifts inland across
the southwest U.S. Upper heights across our region will be
nearing 588dm with 850mb temps averaging near +16C through the
period. This will result in much warmer temps than we have seen
over the past 14 days...with temps actually above normal through
the entire period...generally lows in the 50s to lower 60s and
highs in the 70s mountains and 80s elsewhere...nearing 90 across
the Piedmont.

Little if any synoptic-scale forcing is evident on Wednesday with
weak upper ridging in place...but all models depict an area of
moisture pooling across the Appalachians..typical of differential
heating across the mountains...which could yield isolated to
widely scattered afternoon showers/thunderstorms. Instability is
minimal at this with no dynamics...activity would be
diurnally driven and non-severe.

On Thursday...the pattern becomes slightly more interesting and
more active convectively speaking. While the region is still under
broad upper ridging...the southwest U.S. tough continues to shift
inland...but more importantly a notable short wave is ejected to
the northeast of the parent upper low into the TN/OH valley by
afternoon. The NAM hits the feature the hardest...but it is
evident via all of the longer range models. This increase in
forcing along with increased thermodynamics...should yield
scattered showers and thunderstorms...again largely diurnally

By Friday...the aforementioned disturbance has moved to our
northwest...but again a notable area of moisture pooling is
evident across the mountains...but the best area of such has
shifted slightly northwest from Thursday`s location. Of more
importance at this point is a potential tropical system apparent in
most models developing off the southeast U.S. coast. While all
models indicate this system in varying degrees...there are vast
differences in the track of such system...with the GFS most
aggressive in taking in inland into SC/NC...where it circulates
about into the weekend and early next week. This feature may serve
initially to induce increased subsidence across the south and east
portion of our CWA Friday...but beyond that...cannot hinge on any
one solution at this point.


As of 415 am EDT Tuesday...

As noted in the section conditions across the
region through the weekend and into early next week will depend
largely on the track...intensity...and evolution of a potential
tropical system moving from the Bahamas northwest into the
southeast states per GFS...or hugging the SC/NC coast line per
most other models. Clearly...the GFS solution would have a
significant impact on our region in terms of cloud cover and
precipitation...while other model solutions keeping the low track
closer to the coast...would result in more subsidence across our
region and minimal impacts. For sure temperatures will remain on
the warm side with no significant change in air mass underneath a
continuing large-scale ridge...with humidity levels high through
the period. Meanwhile...the western system remains mostly intact
across the western states while overall weakening and lifting more
north than east. Thus...outside a potential tropical system...the
pattern is becoming more-and-more summerlike than we have seen so
far this year. Look for a continuation of highs in the 70s
mountains and 80s elsewhere...with lows mostly in the 50s and 60s.


As of 200 PM EDT Tuesday...

Closed upper low continuing to spin off the New England coast,
this feature gradually moving northeast and away from the
forecast area. Influence less than in previous days, but still
close enough to generate high based cu/sc and some mid
clouds, mainly east of the Blue Ridge. Cannot completely rule out
an afternoon shower east of the Blue Ridge, coverage certainly
not enough to include in the tafs. if there is a risk it would be
northeast of KLYH through sunset. after sunset...expecting clear
skies all terminals.

Wet antecedent conditions from rainfall previous days in
combination with light winds and clear skies will promote valley
fog formation in the mountain valleys tonight. Main impact will be
KLWB/KBCB and vicinity. Expect fog to lift fairly quickly after 9
AM Wednesday with return of widespread VFR.

Winds generally light WNW-WSW through the period...speeds 10 mph
or less...near calm after 00Z/8PM...increasing again after 13Z/9AM
Wednesday but generally less than 10 mph.

Extended discussion...

Wind flow through Friday will be out of the southwest with the
weather pattern more reflective of summer conditions, primarily
VFR except for widely scattered afternoon and evening showers
during the peak heating part of the day. Nightime fog also
possible in the mountain valleys, but mainly after midnight up
until an hour or two after daybreak.

Potential wind flow change is advertised by the models this
weekend per development of a storm system off the southeast
Atlantic Coast. Models differ quite a bit on timing and movement
of this feature so low confidence forecast beyond Saturday attm.




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