Area Forecast Discussion
Issued by NWS Blacksburg, VA-- Remove Highlighting --
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FXUS61 KRNK 201419
Area Forecast Discussion
National Weather Service Blacksburg VA
1019 AM EDT Wed Jul 20 2016
A weak cold front will remain to the south of our forecast area
today with high pressure in control through the remainder of the
week. This will result in warm and drier weather, but an isolated
shower or storm still cannot be ruled out.
.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --As of 1015 AM EDT Wednesday...
Latest satellite images show that fog/stratus west of the Blue
Ridge has just about burned off, leaving mostly sunny conditions
across the region. Morning sounding is quite dry but some short
wave energy rotating through the northwest flow aloft will help
generate some Cu this afternoon. Expect CAPE values to be modest
at best but orographic effects along the Blue Ridge may be able
to squeeze out a few showers/rumble of thunder so POPs of 15 to 20
percent range look good. The NAM is a persistent outlier solution
in generating convection across the piedmont late today into early
this evening. Other meso models do not support this solution and
previous runs of the NAM too moist based on latest RNK sounding
and satellite imagery. Will restructure POPs a bit to reflect this
thinking and go with essentially a dry forecast with POPs of only
10 to 12 percent for the piedmont into this evening.
previous discussion as of 405 AM EDT Wednesday...
Difficult to find exact location of diffuse cold front any more,
but based on mean sea level pressure trough it may be from far
southeastern VA into northern NC and then southwest into far
northern GA. Surface dew point front may still lie just to the
north of our CWA. Still, it is clear much drier air has moved in
aloft behind last evenings upper short wave and with mixing during
the day should expect the dew point front to shift to the south of
the area as well. SBCAPE values may be moderate during much of the
morning, but with daytime mixing of drier air these should drop
during the afternoon. Therefore, would expect a drier day, but
with more sunshine still quite warm, or at least seasonable
daytime temperatures. Some very weak convergence in the NW NC
mountains and perhaps still close to the frontal boundary may be
just enough to squeeze out an isolated storm or two, but anything
trying to form will be battling entrainment of drier air aloft so
not especially confident in anything forming. Hardly any hi-res
models suggest this either. One small concern is that by evening
as dew points increase again with loss of mixing that surface-
based instability increases again. While there is hard to find any
kind of triggering mechanism, any lingering outflow boundary may
be enough to pop something but would tend to be weak and short-
lived. The NAM is suggesting scattered storms across central part
of forecast area by evening but think its low- level moisture is
overdone. However, one or two other hi-res models or ensemble
members suggest some signals similar but not as agressive as NAM.
Based on this idea, have added a slight chance PoP in basically
from ROA southeastward where these models suggest the best
potential is, and only from about 6pm through 10pm. There may end
up being nothing at all, but feel this pattern can produce
isolated overnight weak convection and since just a couple of
models hinted at it, felt it worthy of a very small PoP.
Otherwise clearing skies again overnight with light winds under
influence of surface high pressure. Patchy early morning fog a
decent possibility in favored locations and especially if there
are any pop-up evening showers which help to re-moisten the
-- End Changed Discussion --
.SHORT TERM /THURSDAY THROUGH SATURDAY NIGHT/...
As of 200 AM EDT Wednesday...
During this portion of the forecast a broad upper ridge will be
centered over the central portion of the country. The center of this
high will very gradually retrograde westward. The result will be a
trend towards better chances of disturbances across the Great Lakes
region heading southeast towards our region.
This looks to happen on Saturday when a cold front works its way
into the area and brings isolated to scattered showers to the region
Saturday into Saturday night.
Prior to this time, precipitation on Thursday and Friday will be
limited to primarily isolated orographical activity along the crest
of the Blue Ridge.
Temperatures will trend warmer through the period with readings on
Saturday averaging five to ten degrees above normal. Also on
Saturday, the heat index is expected to be in the 100 to 105 range
generally along and east of a Lawsonville, NC to Lynchburg, VA line.
.LONG TERM /SUNDAY THROUGH TUESDAY/...
As of 230 PM EDT Tuesday...
By Sunday the upper pattern will have transitioned to a quasi zonal
regime with the jet stream riding well to our north. This will keep us
in the heat and humidity through the first part of next week. The zonal
flow will result in little push to get fronts trough the region and as
a result, a washed out frontal boundary combined with a lee/thermal
trof will keep an elevated chance for thunderstorms across the region
on Sunday, followed by another weak front moving in from the northwest
and continuing our chances for thunderstorms as it washes out over the
area Tuesday. Air temperatures and heat index values will remain at
hazardous levels on Sunday especially across the piedmont, with only a
slight tempering of the heat through the first part of next week.
.AVIATION /14Z WEDNESDAY THROUGH SUNDAY/...
As of 740 AM EDT Wednesday...
Weak and slowly moving cool front near the VA/NC border this
morning and continues to drop southward. Patchy IFR fog mainly
impacting KLWB this morning but should lift by 14Z. Otherwise
clear skies under ridge and north of frontal boundary will give
way to some scattered cumulus clouds by midday, closer to blue
ridge and just east this deck may be broken at times and an
isolated early evening shower or storm is not out of the question
but confidence way too low to consider any mention at TAF sites,
but if it were to occur most likely from KROA to KLYH to KDAN
through 02Z or so. With upper ridge dominating today, expect
light winds of less than 7 kts from a generally northerly
direction. VFR the rule for the day and this evening unless an
isolated storm does happen to move over a TAF site.
Late tonight toward Thursday morning patchy IFR fog is likely
again at KLWB under the higher pressure and clearing skies, but
less confident in fog at other locations. Brief MVFR vsbys would
likely be the lowest at KBCB and KLYH.
Extended aviation discussion...
Strong high pressure aloft will move east across the region
Thursday into next weekend. Cannot totally rule out an isolated
storm each afternoon/evening through the period but appears
overall VFR outside of the typical late night valley fog in spots.
Hot and humid conditions east of the Blue Ridge this weekend
which may impact density altitude, and also slightly better
thunderstorm chances across the Appalachians.