Waterloo, Illinois Weather Forecast Discussion

309
FXUS63 KLSX 211121
AFDLSX
Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
621 AM CDT Sun Sep 21 2025
.KEY MESSAGES...
- Rain chances return this evening and peak overnight at 80% for
  much of the area. Thunderstorms late this afternoon and evening
  will be capable of gusty winds around 45 mph and locally heavy
  rainfall.
- Our active weather pattern will continue through at least Wednesday,
  with several additional chances for rain for portions of the
  area through then.
&&
.SHORT TERM... (Through Monday Night)
Issued at 248 AM CDT Sun Sep 21 2025
Water vapor imagery this morning shows a trough digging across the
Northern Plains, with SPC Mesoanalysis depicting a mutli-pronged low-
level jet across the Plains. This jet is feeding several areas of
convection, including those across Kansas and Oklahoma. As the day
progresses, the trough will deepen further and decrease heights
across the Middle Mississippi Valley as a subtle shortwave enters
the region. Some guidance wants to develop an MCV out of the ongoing
convection over Kansas, with the steering flow supporting it
generally heading toward and into the CWA this evening. Whether or
not this is a part of the disturbance seen in guidance passing
through the Midwest this evening has only minor impacts on the
forecast, as either way, it will support another round of scattered
showers and thunderstorms that will form during the very late
afternoon and evening just west of the CWA that will track east-
northeastward.
There is a narrow window in which a storm or two could become strong
to severe late this afternoon and early evening. Instability will be
decreasing as the sun sets, but shear will be increasing as flow
aloft strengthens slightly thanks to the trough and the low-level
jet ramping up. If an MCV does indeed form, this would support a
local enhancement of shear, further aiding in organizing storms.
Steep low-level lapse rates and inverted "V" soundings support an
isolated damaging wind threat. Again, the window for these
parameters to come together just right for the local environment to
be conducive for strong to severe storms is very narrow and also
partially dependent on the development of an MCV from convection
that has only recent formed. Therefore, confidence in it occurring
remains quite low, and we will forgo publicly messaging the Day 1
Marginal Severe Weather Outlook unless confidence increases in an
organized severe thunderstorm threat.
The low-level jet will remain focused over the region through the
night, leading to an expansion of convection through the evening
into the overnight hours. This will be the greatest chance in some
time for the area to see widespread rainfall, with the 00z HREF
showing a 70-90% probability of the entire CWA seeing measurable
rainfall overnight. There may be some storm training earlier in the
evening when convective cores are at their strongest, leading to a
low threat of isolated flash flooding. However, ongoing drought will
likely mitigate this threat. As the low-level jet veers and the
trough axis swings through the region, rain will exit the CWA
eastward Monday morning. Subtle disturbances aloft rippling through
the flow around the base of the trough through the day Monday will
keep a low chance for isolated to scattered convection going.
Elmore
&&
.LONG TERM... (Tuesday through Saturday)
Issued at 248 AM CDT Sun Sep 21 2025
Tuesday morning, guidance consensus is that the Middle Mississippi
Valley will be beneath quasi-zonal flow aloft as a shortwave digs
equatorward out of the Northern Rockies and into the Central Plains.
This shortwave is expected to impact our local weather, but to what
extent remains uncertain due to spread among guidance and run-to-run
variability in the phasing of the shortwave. The general expectation
is that the shortwave will slowly move eastward toward the Lower
Midwest or Mid South. As this occurs, a majority of solutions show a
surface low deepening and tracking eastward along a cold front
sagging southward through the central CONUS. Given the variability
in the phasing of the shortwave among guidance, the strength of the
surface low and its track vary as well. Where it does track,
confidence is high in a soaking rainfall. Northern solutions favor
our CWA for this rainfall, while more southern solutions may skirt
our southeastern Missouri with the more persistent rainfall at best.
Currently, a majority of solutions favor the latter scenario, which
is reflected in the current forecast.
Starting Wednesday, guidance has been consistently having the
aforementioned shortwave interacting with additional disturbances
over the Midwest to develop a cutoff low that will meander somewhere
over the eastern half of the CONUS. While cutoff low behavior is
notoriously difficult to forecast at extended lead times, the
majority of guidance over the last few days worth of initializations
has had this cutoff either over the CWA or just to the east.
Climatologically, this favors relatively cooler temperatures and at
least a period or two of rain chances. Indeed, ensemble means
support diurnal temperatures at or just below seasonal normals
(highs in the mid to upper 70s, lows in the mid to upper 50s), as
well as daily chances for lighter rainfall (hundredths to potentially
a couple of tenths) through the end of the week.
Elmore
&&
.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 611 AM CDT Sun Sep 21 2025
Fog has once again developed across the area this morning, and
while some of the local terminals have experienced earlier impacts,
a slight uptick in winds and warming temperatures is expected to
mitigate this threat further. Our attention then turns to this
afternoon and evening when another round of scattered showers and
thunderstorms are expected to move across the area. While this is
the most widespread chance for rain in the area in a while, the
scattered nature of the convection decreases confidence in exactly
when and where convection will be during the forecasted time
period. The PROB30 groups represent the window in which confidence
is greatest in impacts, but impacts could linger longer than
indicated or be broken up into multiple windows.
In the wake of the evening rain, ceilings are expected to
decrease, with low MVFR expected by the end of the period.
Ceilings for some of the local terminals may get as low as high
IFR; however, the lead time prevents confidence from being high
enough to include in the TAFs right now.
Elmore
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until 9 AM CDT this morning for Knox MO-
     Monroe MO-Shelby MO.
IL...None.
&&
$$
WFO LSX

NWS LSX Office Area Forecast Discussion