Murphy, Missouri Weather Forecast Discussion

041
FXUS63 KLSX 221921
AFDLSX
Area Forecast Discussion
National Weather Service Saint Louis MO
221 PM CDT Mon Sep 22 2025
.KEY MESSAGES...
- Patchy dense fog is expected late tonight into Tuesday morning, mainly
  across portions of south-central Illinois and river valleys in
  east central and southeast Missouri.
- Another round of widespread showers and thunderstorms is
  forecast Tuesday afternoon through Wednesday, providing
  additional relief to those areas in drought.
- Mostly dry weather is forecast Thursday through the weekend
  along with a warming trend in temperatures.
&&
.SHORT TERM... (Through Late Wednesday)
Issued at 220 PM CDT Mon Sep 22 2025
A few isolated showers are possible through late this afternoon
across portions of southeast Missouri and southwest Illinois.
Otherwise, look for dry weather overnight tonight as a midlevel
shortwave ridge moves into the mid-Mississippi Valley. Fog is a
concern tonight as at least partial clearing is expected. Given
light winds, high dewpoints, and recent rainfall in many areas,
this is a good recipe for fog. This would be particularly true in
areas where the cumulus/stratocumulus dissipates last. This is
most likely to be across parts of south-central Illinois. Patchy
dense fog is possible in this part of the area in addition to
southeast and east- central Missouri river valleys. The main
source of uncertainty is cloud cover as model RH plots suggest
increasing cirrus advecting in from the west overnight, and
possibly some convective blowoff clouds entering northeast
Missouri as well. Therefore did not issue a dense fog advisory
yet, but we may need one for parts of the area if confidence in
more widespread dense fog materializes.
Attention then turns upstream as a robust midlevel shortwave trough
moves into western Missouri Tuesday morning. Mid/upper level forcing
for ascent will increase downstream of this feature concurrently
with increasing low-level moisture convergence. Divergence aloft
will also increase during the day on Tuesday beneath the left-exit
region of a 70+ knot upper-level jet streak across the south-central
Plains. There are even some hints at jet coupling between this jet
streak and the right-entrance region of a weaker jet streak across
the western Great Lakes. Widespread showers and some embedded
thunderstorms will be on the march eastward during the day, entering
central Missouri first around 1500 UTC. By 2100 UTC, showers and
thunderstorms should make it to around the Mississippi River and
then overspread our Illinois counties by 0000 UTC Wednesday. There
may be a brief temporal/spatial window for a strong thunderstorm or
two as storms approach the Mississippi River mid/late afternoon.
Joint probabilities from the 12Z HREF for at least 1000 J/kg of
SBCAPE and >30 knots of deep-layer shear are in the 30-40% range. A
couple of multicellular clusters may be possible along with a
transient supercell structure. However, midlevel lapse rates suggest
any potential for near-severe hail would have to come via a rotating
updraft/supercell. What is also different is that low-level lapse
rates will not be as steep as we have seen over the past several
days. Therefore, DCAPE values are more modest so the threat for
damaging wind gusts appears low. Finally, guidance shows thickening
mid/upper level cloudiness out ahead of any convection, which should
prevent instability from getting high enough to cause problems.
Simulated reflectivity from CAMs do not really have any semblance of
robust deep convection in the area, which makes sense given some of
the limiting factors described above.
Locally heavy rainfall is possible for Tuesday afternoon/night due
to efficient rainfall in heavier showers/thunderstorms and the
fairly long duration event upcoming. Precipitable water values are
expected to approach 1.75" in southern sections of the area, or just
above the 95th percentile of climatology. The probabilities for at
least 1" of rain on the LREF are highest (60-90+%) along/south of I-
44/I-70 in Missouri and Illinois respectively. There probably will
be some pockets of higher totals. The 12Z HREF LPMM has some
locations in the 2-3" range, which would be similar to what occurred
overnight last night in parts of southeast Missouri. Any threat for
flash flooding remains pretty low given the ongoing drought and dry
soils. The earlier round of rain last night had very little runoff.
This round should have more runoff, but the soils should still be
plenty dry enough to absorb quite a bit of additional rainfall. The
bottom line is that this rainfall should be very beneficial and
provide further relief to areas experiencing drought.
Widespread rain showers and embedded thunderstorms will continue
Tuesday night as moisture convergence on the nose of a 30-35 knot
low-level jet increases overnight. By late Tuesday night into
Wednesday morning, the overall organization/coverage of rain showers
should decrease as the stronger low-level moisture convergence moves
into the Ohio Valley and mid/upper level ascent decreases behind the
departing lead shortwave. The threat for scattered light rain
showers will continue though much of the day on Wednesday along/near
a low-level trough axis/convergence zone. A secondary midlevel
shortwave will also move through Wednesday afternoon across
southeast Missouri/southwest Illinois. This feature should
weaken/shear out with time, but help provide a bit more ascent ahead
downstream and possibly a bit more widespread shower activity as
well.
Gosselin
&&
.LONG TERM...  (Wednesday Night through Next Monday)
Issued at 220 PM CDT Mon Sep 22 2025
Ensemble guidance has shifted a bit quicker with the evolution of
the mid/upper level closed low later this week. The low is expected
to move into the Ohio Valley almost 24 hours earlier than it looked
yesterday (Friday compared to Saturday). This means a shift to drier
conditions with moderating temperatures should begin on Friday and
continue through this upcoming weekend. The only rain chances in the
extended portion of the forecast come on Thursday as there probably
will be some isolated to widely scattered showers near the closed
low. The best chances (20-30%) will be Thursday afternoon mainly
across southeast Missouri and southwest Illinois.
High temperatures Thursday are expected to be mainly in the mid 70s,
or a couple degrees cooler than normal for the date. Rising
mid/upper level heights and increasing low-level warm air advection
mean a return to above-normal readings for the weekend with highs
back into the low to mid 80s along with plenty of sunshine.
Decreasing cloudiness also means cooler overnight lows though too.
Lows starting Friday morning should be mainly in the low to 50s,
which would be right above average for late September.
Gosselin
&&
.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1228 PM CDT Mon Sep 22 2025
Stratus/stratocumulus should rise and scatter out a bit by late
afternoon, with VFR conditions then forecast through the late
evening/early overnight hours. Primary concern late tonight into
Tuesday morning is on fog development. Fog should be most
prevalent in south-central Illinois and east central/southeast
Missouri river valleys. IFR visibilities are expected, with the
lowest most likely at KSUS. Visibilities should gradually improve
Tuesday morning as mid/upper level clouds increase ahead of the
next disturbance. Chances of showers/thunderstorms should hold off
until later Tuesday.
Gosselin
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&
$$
WFO LSX

NWS LSX Office Area Forecast Discussion