966
FXUS63 KLSX 120353
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1053 PM CDT Thu Jul 11 2024

.KEY MESSAGES...

- Isolated showers and thunderstorms are expected tonight through Saturday
  across portions of the area. Despite these chances (20- 30%),
  most will not see much, if any, showers or thunderstorms.

- Temperatures will continue to rise to above climatological
  normal through early next week. By Sunday, heat index values
  could reach the 100s.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 213 PM CDT Thu Jul 11 2024

Current water vapor imagery shows a trough over the western Great
Lakes region with a positively tilted axis extending across the CWA
from northeast to southwest. Along this axis are weak disturbances
that will provide support aloft for isolated to scattered afternoon
convection mainly south of I-70. Any thunderstorms that do develop
are expected to remain sub-severe as there is little forcing other
than weakly aloft and daytime heating. Expect the strongest storms
to have brief downpours.

Winds are expected to remain light and variable as surface high
pressure over the Upper-Mississippi River Vally expands southward.
Light winds, a saturated near surface layer, and little cloud cover
will create conditions favorable for fog. Patchy fog is likely
mainly across northeast and east-central Missouri (including the St.
Louis Metropolitan area) and western Illinois. It is possible that
low-lying areas in these aforementioned locations could see periods
of dense fog overnight. Apart from the potential for fog overnight
in the eastern portions of the CWA, there is a 20% chance for
showers and a brief thunderstorm across central Missouri along the
nose of a low-level jet.

The upper-ridge over the western CONUS will continue its eastward
expansion overnight and as it does so will help push a weak warm
front into the southern portions of the CWA by Friday morning.
Daytime heating will allow for the atmosphere to become modestly
unstable with SBCAPE values between 1000-2000 J/kg and in
combination with the weak warm front, isolated showers and
thunderstorms are possible through the day Friday primarily along
and south of I-70. Otherwise, temperatures on Friday will be near
normal.

MMG/Elmore

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 213 PM CDT Thu Jul 11 2024

A weak region of positive vorticity advection will round the
expanding upper-level ridge late Friday night into Saturday morning.
As it does so, it will coincide with an area of increased moisture
convergence and lingering instability from daytime heating across
eastern and northeast Missouri and west-central Illinois. This will
help focus very isolated (20% chance) showers and thunderstorms
across the aforementioned locations late Friday night through mid-day
Saturday. Most will not see much in the way of any showers or
thunderstorms. After convection wanes early Saturday afternoon,
allowing for daytime temperatures up warm-up into the upper- 80s
through mid- 90s. An additional chance (20%) for showers and
thunderstorms exists late Saturday through early Sunday across
western Illinois as a vorticity maximum rounds the ridge and the low-
level jet ramps up over northwest Missouri and southern Iowa.
Overall thunderstorm coverage across our far northeast CWA currently
looks very limited and unsupported given height rises aloft.

The main forecast concern comes as a potentially substantial heat up
gets underway Sunday through Tuesday. The upper-level ridging will
continue its eastern expansion across the CONUS with an 850-mb ridge
over the southeast CONUS. Current ensemble guidance is in consensus
that 850-mb temperatures areawide will be between 22-24 C Sunday
through Tuesday, supporting near surface temperatures in the high
90s and even push 100 degrees in some locations. There is also
agreement (at least 70%) among ensemble guidance that dewpoints will
reach at least the low 70s. This combination of hot temperatures and
high dewpoints will allow to heat index values to reach well into
the low 100s. Should these values be realized, heat headlines will
need to be considered over the next few days for early next week.
Confidence in these hot temperatures continues to increase as the
atmospheric set up corresponds to the region`s favorable
climatological for past heat events.

The hot and humid conditions are not expected to last more than a
few days as a shortwave associated with an upper-level low over the
Hudson Bay traverses the Mid-Mississippi River Valley by Wednesday.
This shortwave will help provide support aloft for the passage of a
cold front sometime late Tuesday into Wednesday. The cold front is
forecast to bring both precipitation and near normal temperatures to
the area for the second half of next week.

MMG/Elmore

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1053 PM CDT Thu Jul 11 2024

Fog is expected to develop in northeast Missouri through south-
central Illinois tonight, dropping flight conditions to MVFR through
sunrise before quickly burning off. Despite incoming cloud cover
from thunderstorms in western Missouri, left MVFR fog at KSUS and
KCPS due to their lower elevation, quickness to fog in calm, clear
conditions, and uncertainty in eastern extent of clouds. Decided to
maintain dry TAFs at KCOU and KJEF despite convection as well since
thunderstorms have been very slow moving and have dissipated before
they`ve reached these terminals all evening. Otherwise, calm to
light and variable winds will increase for most locations generally
from the south Friday morning. There is a slight chance (20%) of
convection through the day, but confidence in impacts to any
terminals are too low to mention in the TAFs at this issuance.

Jaja

&&

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

$$

WFO LSX