985
FXUS63 KLSX 111111
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
611 AM CDT Thu Jul 11 2024

.KEY MESSAGES...

- Scattered thunderstorms will continue to threaten parts of the
  region each day through Saturday morning, with plenty of dry
  time in between rain chances.

- Temperatures rise above normal through the weekend, eventually
  dragging heat index values into the triple-digits by Sunday,
  before increased rain chances and a cold front offer some
  relief.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 327 AM CDT Thu Jul 11 2024

Weak, isolated thunderstorms continue to percolate at this hour
across parts of Missouri along a lobe of mid-level positive
vorticity advection. Aloft, a sluggish shortwave is oozing east
across eastern Iowa and northwest Illinois. Several other areas of
PVA rotate around the cyclonic flow, stimulating additional
convection. It is this feature that will be the focus for activity
today across the region. As daytime heating sends temperatures into
the mid/upper 80s again and surface-based instability values into
the 1500-2000J/kg range, the threat for isolated to widely-scattered
thunderstorms will increase. Unlike yesterday, the mid-level forcing
will gradually weaken with time as most guidance begins to shunt the
shortwave east of the bi-state area. Despite this, the copious
instability that will develop and a weak surface boundary traversing
the region will be sufficient for some degree of convection that
threatens gusty winds and localized heavy downpours. The best
chances (30-45%) are east of the Mississippi River, where the
stronger forcing will exist during peak diurnal heating.

With the shortwave departing to our north and east overnight and
upper-level ridging building in from the west, weak high pressure
across the Upper Mississippi Valley will help to weaken winds out
of the northeast. Convective activity should wane overnight for
most of the region save for areas near the Ozarks, allowing for
clearing skies. All of this lends increased confidence in areas of
fog impacting parts of the St. Louis metropolitan area and areas
along and east of the Mississippi River. While dense fog is not
forecast at this time, localized low-lying areas could very well
see visibilities approach one-quarter mile overnight.

The strong upper-level ridge over the western CONUS begins to expand
east on Friday, which will slowly draw the surface boundary north
again as a warm front later in the day. Before this, weak and
isolated convection may fire in the vicinity of the front. That
said, the synoptic-scale subsidence aloft will overcome most
updrafts outside of the front itself. As the front begins to lift
Friday evening, persistent warm air aloft will cap any convection
and keep the region largely dry.

MRB

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 327 AM CDT Thu Jul 11 2024

As the western ridge broadens across the central CONUS late Friday
into Saturday, pushing a warm front into the region, there is
potential for convection Saturday morning along the front on the
nose of a veering low-level jet in northern Missouri. At this point,
most guidance has most surface-based/mixed layer instability well-
capped amidst rising heights aloft. However, if the subsidence is
weaker than currently expected, a cluster of stronger thunderstorms
would persist along the Missouri/Iowa border into central Illinois.
Given the uncertainty in thunderstorm intensity for the reasons
described above, we are not currently messaging the SPC Day 3
Marginal Risk for severe weather.

Behind the front, temperatures jump into the 90s areawide sending
heat index values into the low 100s. Ensemble guidance continues to
paint 850mb temperatures in the 90th climatological percentile
across the Missouri/Iowa border. This anomalous 850mb warmth expands
south by Sunday amidst persistent warm-air advection and southwest
boundary layer winds. The strong subsidence will suppress convection
through late Monday at the earliest, but the strong 850mb thermal
ridge and southwest surface winds will result in temperatures rising
further still into the next work week. All available guidance agrees
that Monday and Tuesday will be the warmest days of the forecast
period, with near-certainty (90%+ chance) of low to mid-90s each
day. The St. Louis and Columbia metropolitan areas will make a run
at 100 degrees, but it may be a bit too humid to get that hot. This
combination of heat and humidity will threaten vulnerable
populations and those outdoors, and if the current 105+ degree heat
index values persist, heat products will be needed in the next few
days. The largest source of uncertainty surrounds how humid we`ll
be. A southwest wind can sometimes help advect drier air from the
Ozark Plateau, which isn`t represented in this forecast. If that
occurs, it would act to keep heat index values in check.

By late Monday, a pronounced shortwave trough tracks along the
periphery of the ridge (now as far north as the U.S./Canada border)
and erodes its western extent. While this has little impact on us in
and of itself, additional shortwaves will work to deepen a longwave
trough across the Great Lakes region. This will draw a front slowly
but surely into the Mid-Mississippi Valley, and both increase our
rain chances and bring relief from the abnormal heat. While a
relative cooldown is all but certain according to available ensemble
guidance, exactly when the relief arrives and how far temperatures
drop is still not clear. Most reasonable scenarios support a slower
frontal passage, which would promote multiple rain chances going
into the end of next week but keep temperatures in check.
Regardless, by Thursday temperatures fall back to near-normal at the
warmest.

MRB

&&

.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 601 AM CDT Thu Jul 11 2024

Patchy fog is impacting the region this morning, however only KSUS
has fallen below VFR conditions among the TAF sites. Weak
convection continues to percolate over central Missouri, which
will briefly impact KCOU and KJEF but will most likely not have
lightning with it.

Most of the TAF period will be dry and VFR, but there is a general
threat for thunderstorms. Considerable differences exist in
exactly which terminals (if any) would be affected today, and
timing uncertainty also exists. As such, no thunderstorms are
mentioned in the TAFs but remain an outside possibility. Tonight,
winds weaken out of the northeast and fog will become a more
distinct possibility at a few terminals. The highest chance for
fog is at KUIN, though the river valley TAF sites (KSUS and KJEF)
may also be impacted.

MRB

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX