035
FXUS63 KLSX 271952
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
252 PM CDT Thu Jun 27 2024

.KEY MESSAGES...

- Multiple rounds of showers and thunderstorms are expected Friday
  morning through Saturday evening. There is a low chance that
  some of the storms could become strong to severe Friday night
  and again Saturday afternoon and evening, with the main threats
  being damaging wind gusts and large hail.

- Near normal to below normal temperatures are expected through
  the weekend, with heat returning toward the middle of next week.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 250 PM CDT Thu Jun 27 2024

Current water vapor imagery shows ridging across the Plains, with
northwesterly flow over the CWA. An expansive surface high remains
centered over the Great Lakes and extends southwestward into our
CWA, keeping our weather relatively cooler and calmer compared to as
of late. With mostly clear skies and ample sunshine, temperatures
have warmed further than this point yesterday, but still at or below
climatological normals.

Tonight, the previously mentioned ridge will flatten and broaden
across the region as a warm front lifts through the Upper Midwest.
Warm advection associated with this front will spawn scattered
showers and thunderstorms mainly north of I-70 and particularly over
northeastern Missouri and west-central Illinois. This activity is
expected mostly Friday morning, but isolated convection could
continue through the afternoon in this part of the CWA. The warm air
advection will bump-up temperatures relative to today, but cloud
cover associated with the convection will keep temperatures from
increasing no more than a couple of degrees over today.

The shortwave and surface system responsible for Friday night`s
convection has slowed further, with the main focus for robust
convection remaining over the Central Plains. As it moves eastward
into the CWA very late Friday night/early Saturday morning, it is
expected to be in a decaying state given the lack of instability
across the area. Additional convection is expected along the nose of
the low-level jet mainly north of I-70 late through the overnight
period. Again, with relatively low instability (roughly at or below
1,000 J/kg of MUCAPE), wide-spread severe weather is not
anticipated, but a storm or two could become strong to severe and be
mainly capable of damaging wind gusts and large hail.

Elmore

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 250 PM CDT Thu Jun 27 2024

Early Saturday, there is a chance (50%) that convection from Friday
night will still be ongoing as the low-level jet continues to veer
across the CWA. Though, this convection is currently expected to be
weak and scattered, as the jet will be weakening and instability
will be at a minimum. Aloft, a shortwave will be departing the
region, leaving subtle height rises in its wake per deterministic
and ensemble guidance consensus as a cold front slowly sags
southward through the CWA through the day. However, given the
morning convection, outflow may force the effective boundary further
south than the synoptic front. The front or effective boundary will
be among an environment characterized by SBCAPE of around 2,000
J/kg, an ideal amount for robust updrafts to form along the
boundary. However, guidance consensus is that shear will be weaker
(20, possibly 30 kts at best), and a majority of model soundings
show some degree of capping or poor low-level lapse rates. These
factors, along with rising heights, keep confidence low in the
occurrence and coverage of convection during afternoon and evening on
Saturday. If convection is able to form and become strong to severe,
damaging wind gusts is the primary threat, but large hail could
occur within the strongest updrafts.

On Sunday, a trough deepening over the eastern U.S., will cause flow
over the Middle Mississippi Valley to become increasingly
northwesterly, pushing high pressure into the Midwest and the front
well south of the CWA. This will usher in an unseasonably cool
airmass for the area on Sunday, with ensembles continuing to cluster
around 80 degrees across the CWA. Despite guidance consensus
continuing to have an upper-level ridge building quickly into the
Midwest on Monday, the surface high is slow to depart the Midwest,
keeping temperatures relatively cool on Monday. However, this cool-
down will be short lived, as guidance consensus has the ridge
expanding across much of the southern CONUS and southerly low-level
flow returning to the region. In turn, warm air is expected to
advect into the region for the mid-week period, with ensemble means
supporting above-normal temperatures. How long this heat will last
is uncertain, as guidance indicates that an upper-level trough will
impede on this ridge and attempt to send a cold front through the
Midwest. However, guidance varies on the strength of the ridge vs.
trough, and therefore, how much southward progression the front
makes. The current forecast for Wednesday leans more toward a slower
FROPA, which is supported by a majority of guidance.

Elmore

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1228 PM CDT Thu Jun 27 2024

Confidence is high in dry and VFR flight conditions through today
and much of tonight for most of the local terminals. The exception
to this is KUIN where scattered showers and thunderstorms may
impact late in the period. These showers and thunderstorms are
expected to remain north of the other local terminals, though
very brief impacts are possible at any terminals. For terminals
that are impacted by this convection, drops in visibility are
expected.

Elmore

&&

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

$$

WFO LSX