Area Forecast Discussion
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FXUS61 KRNK 230854
Area Forecast Discussion
National Weather Service Blacksburg VA
454 AM EDT Sat Jul 23 2016
Strong high pressure aloft to the west will continue to bring
increasing heat and humidity over the weekend. A series of weak
surface cold fronts approaching from the northwest should
gradually bring an increasing threat for showers and storms early
.NEAR TERM /THROUGH TONIGHT/...
As of 235 AM EDT Saturday...
Strong upper ridge west of the mountains will continue to expand east
over the next 24 hours with heights slowly building as a piece of the
high noses east into the Mid-Atlantic. Resultant 850 mb temps near +24C
or better will accompany the ridging this afternoon as decent
subsidence remains in place despite lingering rather moist pwats.
This should bring one of the hottest days seen so far with
mid/upper 90s east and 80s to perhaps touching 90 in spots over
the mountains. However dewpoints again look to drop off enough
east of the ridges in the afternoon despite only weak flow to
preclude heat advisory headlines at this point. Still could see an
hour or so where indices touch 103-105F piedmont, espcly if the
flow ends up more southwest and dewpoints stay up, so continued
HWO mention for now.
Otherwise given little support aloft, expect more isolated convective
coverage at best, with storms driven by differential heating/weak
convergence mountains and perhaps the eastern lee trof. Latest suite of
solutions suggest best low chances across the northwest NC mountains
and possibly the Va Highlands, with shotgun 20ish pop elsewhere for
isolated afternoon pulse storms at this point.
Any residual convection should fade with loss of heating with another
warm overnight in place as higher dewpoints return and debris clouds
linger before clearing late. Will leave in some low pops mainly west
early on for more outflow driven shra/tsra, otrw cutting to patchy fog
overnight with lows from the mid 60s valleys to the low/mid 70s east.
.SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/...
As of 415 AM EDT Saturday...
This period will begin with a strong elongated upper ridge
extending across the entire U.S. through the mid-latitudes from
the desert southwest into the central Atlantic. The westerlies
remain to our north about 500 miles while a weak pseudo-tropical
upper low meanders westward through the Midsouth. For Sunday,
this leaves our region with very weak dynamics, mostly ridging
aloft with heights hovering near 549dm. One piece of the upper
ridge is actually progged to be centered across the VA/NC
Piedmont. As was the case Friday, differential heating will likely
result in isolated to scattered diurnally driven
showers/thunderstorms. Most favored areas will be across the NW NC
mountains and into southwest VA. Cannot rule out an isolated
shower/thunderstorm in other areas, but conditions are not
For Monday, heights begin to drop slightly as the upper ridge
shifts off the NC coast and a trough moves through the Great Lakes
toward New England with a weak trough extending into the Mid-
Atlantic region. An associated weak front will drift toward the
PA/MD/WV border by evening, then slowly southward overnight.
Shower/thunderstorm chances may be slightly better Monday during
the evening in the far west/north, but overall it looks like the
remnant upper ridge will dominate one more day and keep most of
the CWA dry, especially the Piedmont closest to the upper ridge.
By Tuesday, the heights continue to slowly lower into the 588 dm
range as a series of weak upper troughs gradually return the
region to a pattern we have seen repeatedly this summer, namely
northwest flow aloft. One center of the upper ridge will remain
over the western U.S. with the one that was over our region Sun-
Mon shifting off the southeast U.S. coast. The aforementioned weak
front will continue to drift very slowly south and should be near
the NC/VA border by Wed morning. Shower and thunderstorm chances
should increase substantially Tuesday in response to the lower
heights, weak upstream northwest flow short waves, slight cooling
aloft, and the proximity of the front. Dynamics overall will
remain very weak, so instability/thermodynamics will be the main
driving force for the convection. SPC has advertised a marginal
risk near the I-64 corridor late Monday, but for our CWA Tuesday
should offer a better bet for isolated strong to severe
Temperatures will remain warm to hot through the period, but edge
back a few degrees by Tuesday. Sunday should be the hottest day
across the region as 850mb temps peak out around +24C under the
594 dm upper ridge. Even though guidance has consistently been a
few degrees too warm most days this summer, mid to upper 90s lower
elevations of the Piedmont to the lower to mid 80s mountains seems
reasonable in comparison to previous events where the 850mb temps
reached these high a levels. SPC climatology page shows that +26C
is the warmest 850mb temperature for BCB, which was reached during
the infamous heat wave/derecho day of June 29, 2012. So it is
quite possible that BCB could reach the 90-degree mark over the
next few days for the first time since July 2012. We were in the
mid 90s back then. By Tuesday, max temps should drop back into
the lower to mid 90s with increased cloud cover, showers, and
moisture content. Low temperatures will remain a good 5-10 degrees
above normal through the period.
.LONG TERM /WEDNESDAY THROUGH FRIDAY/...
As of 430 AM EDT SATURDAY...
The broad west-east oriented upper ridge gets nudged further
southward by broad troughing across central/eastern Canada. Upper
heights will continue to gradually lower. More importantly will be
a series of weak short waves embedded in the upper flow that will
traverse the region and impinge upon a weak front that will remain
stalled in/near the area. This should prove to be an increasingly
active convective period with scattered to numerous
showers/thunderstorms each day. Models present varying solutions
as to when the best chances for showers/thunderstorms will occur
within this period, but the overall consensus supports high
chance pops during the afternoon/evening and low chance pops/night
and mornings through the period. Too much noise and chaos in the
signal at this point to specify much further, so have broad
brushed pops with the previously mentioned methodology. Day-by-day
details are of the mesoscale level and yet to be resolved by
The greatly increased convective coverage and associated clouds
and moisture will result in max temperatures returning to more
normal levels mostly in the 80s to around 90 Piedmont. Min temps
will remain well above normal levels, mostly in the mid 60s west
to lower 70s east.
The large upper ridge will extend from the West coast to the East
coast through Wednesday. Towards the end of the week, there is a
gradual lowering of H5 heights as ridge splits and develops a
Bermuda ridge and desert SW ridge.
In the northern stream, a shortwave will drop southeast out of
eastern Canada and crosses through New England on Tuesday. A frontal
boundary will settle across the Mid-Atlantic and Southeast Wednesday
into Thursday. A wave of low pressure along the front will slow the
progression of the boundary. The placement of fronts are always a
challenge in the summer. Low level moisture will continue to
increase into the end of the period. Afternoon and evening showers
and thunderstorms are possible Tuesday into Midweek. Temperatures
will be above normal early next week and return to normal by end
of the week.
.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
As of 1250 AM EDT Saturday...
Continued mainly VFR this period with fog being the only real
issue late tonight into early Saturday. As such will stay with
the previous forecast of bringing KLWB to LIFR around 08z/4AM,
while including a brief tempo for MVFR at KBCB.
Should see good flying conditions prevail on Saturday under
scattered/broken cu including isolated convection, mainly along
the southern Blue Ridge espcly toward KGEV/KTNB. However
uncertainty too great to include any shra/tsra mention given
likely spotty nature to convection.
Extended aviation discussion...
Strong high pressure aloft centered across the Midwest will
attempt to build eastward into Sunday, then retreat back to the
west early next week. Our region will remain on the eastern
periphery of the upper high and thus subject to weak disturbances
in northwest flow aloft tracking around the Great Lakes and into
the Mid-Atlantic. Moisture and instability will be sufficient for
isolated pop up thunderstorms, but a slightly better coverage will
be possible early next week as weak front moves into the area,
stalls, and lingers over the region for several days. Hot and
humid conditions east of the Blue Ridge this weekend which may
impact density altitude. Late night and early morning fog will
still be possible at the usual sites almost every day, and those
chances increase if any rain occurs during the afternoon or
evening at any of these sites.