Lou Del, Illinois Weather Forecast Discussion

340
FXUS63 KLSX 191658
AFDLSX
Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1158 AM CDT Fri Sep 19 2025
.KEY MESSAGES...
- There is a low chance (~30%) for isolated to scattered showers
  and thunderstorms this afternoon and evening. Thunderstorms
  will be capable of frequent lightning and gusty winds.
- Multiple chances of rain are forecast through the next week,
  with chances peaking Saturday morning (50%), Sunday (60%), and
  Tuesday (60%).
&&
.SHORT TERM... (Through Saturday Night)
Issued at 235 AM CDT Fri Sep 19 2025
A series of shortwaves can be seen in early morning water vapor
imagery digging southward across the Plains into the Midwest, with
the leading wave being responsible for the rain last evening that
has been dissipating up through this morning.
Through the day today, the shortwaves will phase eastward just north
of the CWA, though subtle lift due to weak vorticity advection
aloft, decreasing heights, and potentially remnant outflow
boundaries will serve as forcing mechanisms for widely isolated to
scattered convection mainly across southeastern Missouri and
southwestern Illinois this afternoon. Then, hi-res guidance shows
that 1,500-2,000 J/kg of SBCAPE will have built across the CWA,
leading to robust updraft potential. Weak deep-layer shear (10-20
kts) will keep storm mode single cellular, short-lived, and
disorganized. There is a low chance that a storm or two could
produce strong wind gusts as they collapse thanks to the weak shear,
strong lapse rates, and high LCLs/dry low levels. As the instability
dissipates with the setting sun and more conducive synoptic-scale
forcing departs the area, convection will dwindle this evening.
There is a low chance (30%) that convection could reform late
tonight across the same area as the axis of the shortwave swings
overhead, departing eastward around daybreak.
For Saturday, the upper-level pattern in the immediate vicinity of
the CWA will be similar to that of today, with the main exception
being a more potent shortwave moving through the region during the
day. This will be associated with a decaying convective complex
moving eastward out of the Plains through the daylight hours
Saturday. How quickly this complex decays as it moves through the
area in the morning and the amount of convective debris present in
its wake will dictate how much the atmosphere can recover and become
conducive for additional convection in the afternoon. Even if the
atmosphere is able to destabilize for another round of convection
Saturday afternoon, SBCAPE tops out around 1,000 J/kg in the most
aggressive guidance. However, guidance consensus is that the
vertical CAPE profile is skinny, lapse rates poor, and subsidence
will be present. This leads to low confidence in strong convection,
with isolated to scattered showers and weak thunderstorms being the
most-likely outcome.
Elmore
&&
.LONG TERM... (Sunday through Thursday)
Issued at 235 AM CDT Fri Sep 19 2025
At the start of the extended period, guidance consensus is that the
CWA will be beneath southwesterly to quazi-zonal flow aloft as a
trough slowly moves through the Great Lakes. Subtle disturbances
will continue to ripple through this flow over the Middle
Mississippi Valley, aiding in the forcing of multiple rounds of
convection along and in proximity to a front west-northwest of the
CWA through Tuesday. Despite repeated periods of rain forecast now
through then, rain will not be continuous over this stretch. The
windows during which rain chances will be greatest is early Sunday
and then Tuesday.
On Sunday, deterministic guidance depicts a strong low level jet
veering toward the CWA through the early morning hours, with
rainfall focused at its nose. Current indications are that this
precipitation will either pass over or very close to the CWA. If the
former, then portions of the CWA could pick up very beneficial
rainfall. As for Tuesday, the general consensus among deterministic
guidance and ensemble clusters is that the upper-level trough will
dig southward as it axis pivots eastward. This will be coincident
with a shortwave digging into the Plains and Mid South, both
combining to force a cold front through the CWA. The exact phasing
of both of these features, though particularly the shortwave, vary
drastically among guidance, leading to low confidence in FROPA and
rainfall timing and accumulation. Whether or not the front clears
the CWA Wednesday through the end of the week is also uncertain. If
the front is able to move well clear of the CWA, then rain chances
through the remainder of the period will be low to null. If the
front lingers near the CWA, which a majority of guidance seems to
favor right now, then chances for periods of rain will continue
through the end of the week.
As for how all of this impacts temperatures, the CWA remaining
beneath southwesterly to quazi-zonal flow through Monday will keep
temperatures generally at to above climatological normals. Warmer
values (highs in the mid to upper 80s) are favored generally along
and south of the I-70 corridor. North of that corridor and in closer
proximity to the front, cloud cover and rain chances will be more
prevalent, keep temperatures 5-10 degrees cooler than elsewhere in
the CWA. For Tuesday onward, the FROPA, cloud cover, and rain
chances will begin pushing temperatures downward areawide. Even with
the uncertainty of the frontal positioning and spread in
temperatures among guidance, the 75th percentile of ensemble
guidance is right around climatological normals for highs (upper
70s), leading to high confidence in cooler, more seasonable
temperatures toward the end of next week.
Elmore
&&
.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1151 AM CDT Fri Sep 19 2025
High cloud cover is lingering over the St. Louis metro terminals,
but is expected to continue to push east this afternoon.
Meanwhile, destabilization is occurring in the clear sky to the
west, and scattered to broken cumulus around 5,000 ft is expected
to develop and persist until sunset. This afternoon isolated to
scattered showers and thunderstorms are expected to develop across
the region, though exactly where remains uncertain. As such, I
have left the mention of -TSRA in the PROB30 group for the St.
Louis metro terminals. Conditions are most favorable at these
terminals compared to KCOU, KJEF, and KUIN where slightly more
stable air may be enough to inhibit thunderstorm development.
Where thunderstorms develop ceiling and visibility reduction to
MVFR or even briefly IFR flight conditions.
There is another chance (20%) for showers and thunderstorms
tomorrow afternoon, but confidence is even lower in where and when
these storms will occur. Therefore, I have left the mention of
showers and thunderstorms out of the St. Louis TAF for now.
Otherwise, winds will prevail largely from the south today,
picking up in speed during the afternoon. Overnight light and
variable winds will become southwesterly by tomorrow afternoon.
Delia
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&
$$
WFO LSX

NWS LSX Office Area Forecast Discussion