Niantic, Illinois Weather Forecast Discussion

665
FXUS63 KILX 230654
AFDILX
Area Forecast Discussion
National Weather Service Lincoln IL
154 AM CDT Tue Sep 23 2025
.KEY MESSAGES...
- Areas of dense fog will reduce visibility to a quarter mile or
  less this morning, especially across portions of east central
  and southeast Illinois.
- An unsettled weather pattern will result in additional chances
  for showers and storms through Wednesday. A few isolated pockets
  of 2 inches of rain will be possible this afternoon and evening
  in slow moving storms north of I-72. There is a 50-60 percent
  chance for a more widespread area of 2+ inches of rain south of
  I-72 late tonight through Wednesday evening.
- A stretch of dry weather returns Friday through at least early
  next week with temps returning to above normal (highs in the mid
  80s).
&&
.DISCUSSION...
Issued at 149 AM CDT Tue Sep 23 2025
Slow moving upper low that has been anchored over the Great Lakes
will continue to result in unsettled weather through at least
Thursday across central Illinois.
Elongated upper trough that stretches from the Canadian Maritimes
back into the Upper Midwest will make little southward progress
today, while at the surface, an attendant trough is expected to
inch into the Illinois River Valley this afternoon. A damping H5
shortwave trough is expected to move across the mid Mississippi
Valley late this afternoon into the evening, while coupled jet
streaks with favorable right entrance and left exit regions will
accompany this feature and overspread central Illinois. Synoptic
scale forcing for ascent associated with these features will
result in the next round of scattered showers and storms moving
across central Illinois later today with some additional
development along the aforementioned surface boundary. Modest
instability is expected across central Illinois with the GFS
suggesting only up to around 500 J/kg MLCAPE will be present this
afternoon. Favorable deep layer shear may be present under the
aforementioned jet streaks to our north and areas south of I-72,
but weak shear is present in between for much of central IL north
of I-72. HREF joint probs for exceeding 500 J/kg CAPE and 30kt
deep layer shear peaks around 40-60 percent south I-72 and around
70% south of I-70. This appears to be the area where parameter
space is most favorable for severe weather, albeit only marginally
so. Where the deep layer shear is much weaker near the Illinois
River Valley, slow storm motions are expected with some pockets of
locally heavy rain possible. HREF LPMM suggests isolated totals
of 2-4 inches of rain will be possible north of I-72.
Tuesday night into Wednesday, main upper trough axis is finally
expected to push across central Illinois with a weak surface low
tracking across southern Illinois. Strong isentropic ascent will
begin to overspread central Illinois ahead of the low Tuesday
night and will persist across portions of central Illinois into
Wednesday morning. Heavy rain appears possible near the surface
low track, but expect a sharp gradient in rainfall amounts across
portions of central Illinois on the northern edge of this system.
NBM probabilities for exceeding 2 inches of rain in 24 hours
ending Wednesday evening peaks south of I-70 in the 50-60 percent
range, but drops off to around 20 percent or less north of I-70.
NBM 50th percentile QPF for this same time range shows the one
inch contour running roughly Taylorville to Terre Haute, and
ranges from less than a tenth of an inch near Galesburg to around
2.5 inches near Lawrenceville.
Thursday will be the transition day between the wetter pattern
we`ve been under to another stretch of dry and warmer weather as
a strong upper ridge builds back across the middle of the country
through the weekend and into next week. Thursday will see a few
lingering showers in the low level cyclonic flow and weak diurnal
instability. Forecast sounding show strong capping around 700mb
which will limit the depth of any diurnally favored showers and
keep the overall thunder threat low. Dry weather should prevail
Friday into early next week, while temps warm back above normal
with highs in the mid 80s under the influence of the strong upper
ridge.
Deubelbeiss
&&
.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1218 AM CDT Tue Sep 23 2025
A complex TAF forecast exists for the 06z TAFs. Regional radar
shows a line of storms extending from near the Quad Cities into
far southeast IA, moving southeast. A weakening trend is expected,
but did add a VCSH mention at KPIA/KSPI for a few hours.
The next concern is dense fog, which is most likely southeast of
the I-72 terminals. Maintained visbys below 1 mile at those
terminals. Considered adding a TEMPO for 1/4 mile vis, but opted
to wait and monitor the fog evolution on satellite over the next
few hours.
The remnant boundary from the storms currently approaching from
the northwest could serve as a source of t-storm development this
afternoon. A PROB30 -TSRA group is included at all terminals due
to this. By the evening, a more robust wave moves in from the
southwest, resulting in widespread -SHRA and embedded -TSRA
overspreading the area after 23-00z. Ceilings will lower
overnight, with MVFR ceilings possible between 00-06z but becoming
more likely after 06z.
Erwin
&&
.ILX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 9 AM CDT this morning for ILZ054>057-
061>063-066>068-071>073.
&&
$$

NWS ILX Office Area Forecast Discussion