Scientific Forecaster Discussion
fxus61 krlx 201911
Area forecast discussion
National Weather Service Charleston WV
311 PM EDT Sat Jul 20 2019
potentially dangerous heat conditions expected today. Sunday
will only be slightly less hot. Cold front Monday/Monday night.
Cooler, drier next week.
Near term /through Sunday/...
as of 245 PM Saturday...
Today, strong ridging aloft and high pressure keeps the heat
and humidity around with mostly dry conditions. The only possible
caveat is the possibility of a few showers and storms in SW Virginia and
perhaps southern WV this afternoon associated with a mid-level ridge
weakness, dying down this evening. Slow movement and high available
moisture make for a very localized flood threat. Otherwise, hot and
humid conditions continue, with 22c 850mb temps yielding surface
temps in the mid-90's across much of The Lowlands. Corresponding dew
points well into the 70's result in heat indices maxing out 98-108
degrees below about 2000 ft elevation, hence the excessive heat
warnings and heat advisories currently in place. No changes were
made to heat headlines today.
Tonight, ridging across the eastern Continental U.S. Begins to break down.
This takes the prohibitive pressure off the aforementioned mid-
level weakness in the vicinity of the smokeys, so showery
conditions are more likely early Sunday morning heading into the
day Sunday across roughly the southern third of the County Warning Area. Href
members and other hires models suggest a good amount of cloud
cover associated with this wave during the morning, primarily
tied to the western upslopes perhaps to the 79 corridor, and
along/south of 64. This complicates temperatures during the
first half of the day, ultimately limiting afternoon maximum
Sunday, heat and humidity remain entrenched across the area,
particularly in the Ohio Valley where dew points remain in the mid-
70's and clearer skies earlier on allow temperatures to rise to
near or just above 90. Thus, will likely need another heat
advisory for zones mainly in southeast Ohio and western WV where heat
indices are expected to top 100f, once the current headlines
expire, for simplicity. Will have to keep a close eye on cloud
coverage and convective chances through the day as variable
conditions are expected with storms briefly dropping
temperatures and cranking dew points up. Should cloud cover be
less abundant than is forecast, the heat advisory may be needed
for a larger area.
Precipitation wise, heights aloft do gradually fall with the
approach of a positively tilted trough descending into the Great
Lakes region, whose embedded shortwaves push along a surface
cold front through the Northern Ohio River Valley. While the cold
front doesn't arrive until well into the short term period,
diurnal destabilization and increasing baroclinicity amid
increasing low- to mid-level flow will spur scattered storms
across the area during the afternoon. Mesoscale convective system development along the
front off to the west may spill into portions of southeast Ohio at the
very end of the period given adequate momentum, but more likely
into the next. Overall convective threat is twofold; scattered
convection during the day carries a relatively low severe storm
threat with brief, pulsating storms packing a few gusts. Later
on, severe threat grows marginally to correspond with mesoscale convective system
activity, but again is more of a concern Sunday night. The
flood threat mantra continues with high moisture availability
spelling a heavy rain and thus localized flood threat, however
increasing flow and storm motion will mitigate some flood threat
spare for repetitious storms over similar areas.
Short term /Sunday night through Tuesday night/...
as of 310 PM Saturday...
This period begins with the hottest air mass of the season so
far over the area, in southwest flow around surface high
pressure that will have retreated to the south of the area by
Sunday evening. However, temperatures modify this period, in
response to a long wave pattern change, bringing a break in the
heat. A long wave trough over central and eastern Canada will
have already begun to break the upper level high, centered over
the Ohio Valley this morning, in two, one over the western
Atlantic, the other over The Rockies and intermountain west.
This process continues, so that, by Tuesday, the long wave
trough occupies much of the eastern U.S.
A weak upper level short wave trough is likely to be associated with
a cluster of convection that develops near and south of a baroclinic
zone that will have pushed into the upper Ohio Valley and toward the
middle and lower Ohio Valley by Sunday night. Pooling of moisture
ahead of the associated front, and daytime surface heating, is
likely to result in cape values of 3 kj/kg amid precipitable water values nearing
two inches, and h85 Theta E values in the 350s k. Due to high
freezing levels and a lack of shear, the severe threat would be in
the form of wind derived from dape enhanced downburts. Training
or sufficient organization.Coberage could also lead to a flash
flood threat. The mesoscale convective system that moved across the upper Ohio Valley this
morning, in a not too dissimilar environment, managed to
produce wind damage despite the nocturnal surface based
inversion, along with flash flooding.
A stronger short wave acts to carve out a deeper long wave trough as
it rotates through it. This drives the surface front through the
middle Ohio Valley Monday afternoon and evening and then on through
the central Appalachians Monday night. This is likely to bring
showers and thunderstorms Monday into Monday night, but
instability is likely to be quite limited by lack of diurnal
heating because of clouds including morning stratus left behind
by convection Sunday night, and early timing of new shower and
thunderstorm development with the arrival of the front and short
wave trough. This may keep the severe threat limited, but
continued high precipitable water values, narrow to medium cape, and high
coverage of rainfall may still present a flash flood threat
Monday into Monday night, prior to the front crossing.
Stratus and some fog are likely to set in by Tuesday morning, as
significantly cooler air behind the front arrives. Any fog will
burn off quickly Tuesday morning, and the stratus will mix into
morning stratocu, and then an afternoon cu field, as the mixing
layer deepens. Valley fog is likely to form give wet
conditions, if the pressure gradient ahead of high pressure
still out over the Midwest, becomes weak enough to allow
decoupling. In the much cooler air, steam fog is likely to form
along the rivers.
No major adjustments to central guidance temperatures and dew points
which, after another stiflingly humid night Sunday night, shows
cooling Monday on clouds and convection, and Tuesday on the passage
of the cold front.
Long term /Wednesday through Saturday/...
as of 310 PM Saturday...
A large trough over the eastern U.S. To start the period, moves
east during the balance of the work week, and a large high pressure
system slowly crosses beneath the height rises and the gradually
slackening upper level northwest flow. This brings dry weather,
with a couple of cool nights, and pleasant afternoons.
By Saturday, southwest flow around the exiting high will start to
bring about increasing warmth and humidity, with the chance for
afternoon convection at least in the mountains. In the larger scheme
of things, the long wave trough position remains over the eastern
U.S., As upper level high pressure remains anchored over the
Central guidance reflects the relief from the heat and humidity,
with lows in the 50s Wednesday and Thursday mornings and, for at
least parts of the area, Friday morning as well. Tuesday features
highs in the 70s.
Aviation /19z Saturday through Thursday/...
as of 145 PM...
VFR conditions prevail at all terminals through the period with
possible overnight restrictions, possibly lingering through the
morning hours, serving as the only caveat. Winds are generally
from the SW at 5-10 kts with a possibility of a couple gusts to
15 kts across northern terminals. Winds go light and variable
tonight, with chances for patchy fog development across the
north. Further south, primarily the bkw terminal but also
crw/heights to an extent, could be marred more by MVFR cloud cover
overnight. Sunday, winds increase by late morning out of the
southwest to 5-10 kts with vcsh bkw earlier on, expanding to
thunderstorms in the vicinity near crw/heights by the end of the period.
Forecast confidence and alternate scenarios through 18z Sunday...
Forecast confidence: high.
Alternate scenarios: uncertainty exists in the development of
fog and/or stratus late Saturday night/early Sunday. Convective
initiation will be tough to locate Sunday, so northern terminals
may get a rogue shower or storm earlier than currently forecast.
Experimental table of flight category objectively shows consistency
of weather forecast office forecast to available model information:
h = high: taf consistent with all models or all but one model.
M = medium: taf has varying level of consistency with models.
L = low: taf inconsistent with all models or all but one model.
UTC 1hrly 18 19 20 21 22 23 00 01 02 03 04 05
EDT 1hrly 14 15 16 17 18 19 20 21 22 23 00 01
crw consistency h h h h h h h h h h h h
heights consistency h h h h h h h h h h h h
bkw consistency h h h h h h h h h h h M
ekn consistency h h h h h h h h h h h M
pkb consistency h h h h h h h h h h h h
ckb consistency h h h h h h h h h h h h
After 18z Sunday...
restrictions possible in storms Sunday afternoon through the
overnight period. Late night/early morning IFR possible near
river valleys on Monday, and more rain/storm related
restrictions during the day. Stratus and fog is possible again
Tuesday morning, possibly valley fog on Wednesday.
WV...excessive heat warning until 8 PM EDT this evening for
Heat advisory until 8 PM EDT this evening for wvz024>026-
Ohio...excessive heat warning until 8 PM EDT this evening for ohz066-
Kentucky...excessive heat warning until 8 PM EDT this evening for
Virginia...heat advisory until 8 PM EDT this evening for vaz003-004.