Grafton, Illinois Weather Forecast Discussion

714
FXUS63 KLSX 211945
AFDLSX
Area Forecast Discussion
National Weather Service Saint Louis MO
245 PM CDT Sun Sep 21 2025
.KEY MESSAGES...
- Widespread showers and thunderstorms are forecast through the
  early overnight hours. A few storms through mid evening may be strong
  with wind gusts around 45 mph and locally heavy rainfall also
  possible.
- Additional rounds of showers and a few thunderstorms are
  expected through midweek, providing beneficial rain to drought-
  stricken areas.
&&
.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 237 PM CDT Sun Sep 21 2025
GOES-16 satellite imagery shows an MCV across northeast Oklahoma
moving northeast. Ahead of this feature, scattered thunderstorms
have developed across south-central Missouri. This activity is
expected to become more widespread this afternoon and into the
early evening as it moves northeast. Similar to the past couple of
days, a few storms may be strong with gusty winds the main
threat. Weak midlevel lapse rates will help prevent too much
instability from developing, with the RAP only showing about 1000
J/kg of MLCAPE into early this evening. Deep-layer shear will be
modest, even with the presence of the MCV and approach 20 to
perhaps as high as 25 knots. Steep low-level lapse rates will be
in place however, which could lead to some gusty winds. This
threat should diminish by late evening as instability wanes after
sunset.
Widespread showers and weak thunderstorms are expected to continue
through the overnight hours as moisture convergence increases on the
nose of a 25-30 knot low-level jet. Most of this rain should occur
across east central and southeast Missouri as well as south-central
Illinois. Widespread rainfall totals of 1.0-1.5" are expected in
these areas through Monday morning. Given the high precipitable
water values expected (>1.5"; >90th percentile of climatology) and
the long duration of the event, some isolated totals of 2-3" are
certainly on the table similar to the LPMM of the HREF. While
very isolated short-term hydrological issues could arise, the
very dry soils/high flash flood guidance suggest this rain will be
mainly, if not completely, beneficial. These areas have been the
driest since early/mid July, with many locations in moderate (D1)
to severe (D2) drought conditions.
The widespread showers and a few thunderstorms will continue Monday
morning across southeast Missouri and southwest Illinois. As low-
level moisture convergence weakens and shifts further southeast, the
coverage should lessen. Only scattered showers and a few
thunderstorms are expected by afternoon in southeast
Missouri/southwest Illinois, with areas further to the north and
west expected to stay dry.
Temperatures are forecast to be cooler on Monday compared to this
afternoon, mainly due to increased cloud cover. Highs are expected
to be mainly in the upper 70s, or pretty close to normal for the
date.
Gosselin
&&
.LONG TERM...  (Monday Night through Next Sunday)
Issued at 237 PM CDT Sun Sep 21 2025
(Monday Night)
A brief period of quiet and dry weather is expected on Monday night
as the mid-Mississippi Valley sits beneath a shortwave midlevel
ridge. Low temperatures are expected to be in the low to mid 60s
beneath a partly to mostly cloudy sky.
(Tuesday - Wednesday)
Multiple rounds of rain and embedded thunderstorms are expected for
midweek. Multiple strong midlevel impulses are expected with the
first coming through Tuesday afternoon/evening. The mid-Mississippi
Valley will also be sitting beneath the left-exit region of a 80+
knot upper-level jet streak across the south-central Plains. Low-
level moisture convergence strengthens Tuesday afternoon/evening as
well. Long story short, there is a strong synoptic signal for
widespread rain especially Tuesday afternoon/evening. The strongest
signal for the heaviest rain is across southeast Missouri and
southwest Illinois. This is where 24-hour probabilities for >1" of
rain are highest, generally in the 30-50% range from the LREF.
Precipitable water values remain above the 90th percentile during
this time period, but the threat for flash flooding should be low
given more of a lack of convection. Widespread significant rainfall
will however provide another dent to the drought. This is also
the period where the EFI is showing values of 0.7-0.8, showing
good ensemble agreement with the EPS suite for a significant
rainfall event.
A secondary midlevel shortwave trough will move through on
Wednesday, inducing surface cyclogenesis across the area. This
activity looks to be a bit more scattered and less significant than
the earlier round as low-level moisture convergence is displaced
east of the area, with enhanced divergence beneath left-exit
region of the upper-level jet streak further northwest.
Temperature wise, cooler daytime highs are expected due to a lack of
solar insolation and high rain chances both days. Highs are only
expected to be in the mid to upper 70s, though some locations
probably will stay in the low 70s depending on the timing of the
rain.
(Wednesday Night - Next Sunday)
Ensemble data is in good agreement that the a deep, closed low will
form across the Mississippi Valley by Wednesday night. Individual
vorticity maxima are forecast to rate cyclonically around the closed
low, providing plenty of cloud cover and continued chances for
showers, and perhaps a few rumbles of thunder. There is some
uncertainty on how long the closed low will remain quasi-stationary,
but the preponderance of members from the grand ensemble suggest it
will linger in/just east of the area through Saturday. Three of the
four clusters, or about 71% of the members from the grand ensemble
show this scenario. A minority are quicker with its ejection into
the Ohio Valley. If this occurs, a faster transition to dry weather
with warming temperatures would occur. However, as it stands now, it
should stay close enough for persistent isolated to scattered shower
activity at least through Friday, and perhaps into Saturday for
parts of southern Illinois. Temperatures are expected to be in the
70s through Saturday, before warming back to near the 80 degree mark
on Sunday as mid/upper level heights rise behind the departing
closed low.
Gosselin
&&
.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1236 PM CDT Sun Sep 21 2025
There is an increasing threat for showers and thunderstorms this
afternoon through this evening. Widespread showers and
thunderstorms should impact all the terminals, but linger longer
for the central Missouri and metro St. Louis sites. Stronger
storms will be capable of gusty winds and low visibilities in
thunderstorms. The threat will gradually push off to the southeast
late tonight into Monday morning. Guidance has backed off
significantly on lower ceilings overnight tonight into Monday
morning. The best chance is at KUIN so have a BKN025 group in
starting at 1200 UTC Monday. Further southeast, there was too much
uncertainty in seeing any MVFR ceilings to include.
Gosselin
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&
$$
WFO LSX

NWS LSX Office Area Forecast Discussion