Spring Grove, Illinois Weather Forecast Discussion

738
FXUS63 KLOT 131706
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1206 PM CDT Sat Sep 13 2025
.KEY MESSAGES...
- Isolated storms may continue to develop across the area into
  the afternoon and evening hours. The strongest may produce
  damaging hail up to 1 inch in diameter.
- Generally warm and dry conditions are expected early-to-mid
  next week.
- Turning more seasonable toward the end of the week with
  periodic shower and storm chances returning (20-40%).
&&
.UPDATE...
Issued at 842 AM CDT Sat Sep 13 2025
A complex of storms continues to work southward across the area
this morning, accompanied by a notable 2-4 mb/15 min pressure
rise and localized wind gusts of 45-50 mph (peak of 51 mph
measured at Gary Airport). Radar and pressure data have actually
showed the hallmarks of an undular bore tied to the complex of
storms, which is not surprising given the instability feeding
the complex is originating decidedly above a near-surface stable
layer. The complex will continue to move southeastward at
around 35 mph through the next 1-3 hours.
While forecast westerly 925 to 850mb flow is expected to taper
over the coming hours, west to northwesterly winds at 700mb are
in the process of intensifying as confirmed by some 45 to 50 kt
of flow sampled at about 4km sampled by the MKX VWP. Such flow
will transport a plume of steep mid-level lapse rates (sampled
near 8 K/km above 700mb at the 12Z DVN RAOB) eastward atop the
residual bore/cold pool through the morning and early afternoon
hours. While the strongest flow at 700 mb may tend to be above
the depth of the residual bore/cold pool, at least some
convergence/forced isentropic ascent carries a threat for new
storms to regenerate this morning along a northwest to southeast
axis. Pin-pointing exactly where this will occur is tricky, but
have been eyeing persistent radar echoes streaming from La
Cross, WI to Crystal Lake, IL as a proxy for where redevelopment
may be favored. If and where storms redevelop will be a threat
for continued downpours and lightning strikes, though favorable
convective-layer shear of around 35 kt may afford a few spits
of hail, as well.
All things considered, the inherited gridded forecast appears
to be in excellent shape with only cosmetic changes needed to
account for exact placement of the complex of storms.
Updated products will be sent soon.
Borchardt
&&
.DISCUSSION...
Issued at 333 AM CDT Sat Sep 13 2025
Through Sunday:
A shortwave trough is pivoting across south-central WI as of
230 AM with a weak surface low noted near Green Bay. Associated
with these features is a broad area of showers and a developing
axis of thunderstorms near the Wisconsin Dells. The shortwave
will continue to pivot across WI and into northern IL this
morning as a warm front near the Mississippi River lifts into
the area which will allow the atmosphere to moisten and
destabilize. Therefore, the area of showers and thunderstorms is
expected to remain intact as they move through northern IL and
eventually northwest IN between 5 AM and midday (earliest near
the IL-WI line, latest NW IN). Given that the showers and storms
will be moving parallel to the aforementioned warm front,
conditions remain favorable for training convection to establish
across the area particularly east of a Belvidere to Valparaiso
line. With PWATs expected to peak around 1.5-1.7 inches, these
showers and storms will be efficient rainfall producers (rain
rates on the order of 1-2 in/hr) which could lead to localized
instances of flooding especially in urban areas. Furthermore,
there is also 25-30 kts of effective shear associated with these
showers and storms which could support a stronger storm or two
as instability tries to build after daybreak. If a stronger
storm does materialize the main threat will be locally gusty
winds and hail.
Heading into this afternoon, the shortwave is expected to be
pivoting into eastern IN and western OH which should allow the
coverage of showers and thunderstorms to diminish as forcing
wanes. However, the warm front is still expected to be lifting
across northeast IL and northwest IN which may allow scattered
showers and storms to linger through early to mid-afternoon if
sufficient instability remains in place. Forecast soundings this
afternoon don`t look overly impressive from an instability
standpoint so confidence on the coverage of showers and storms
remains low (say a 20-30% chance at this time). That said, any
shower/storms that do develop will still be capable of heavy
rainfall and possibly gusty winds and/or hail given the humid
airmass and 35 kts of shear present. Despite the rain,
temperatures are still expected to be on the seasonably warm
side today with highs in the low to mid-80s inland and mid to
upper 70s near the lake.
Showers and storms will finally begin to taper by this evening
as the front pushes east and mid-level ridging begins to build
into the area. With skies expected to clear out through the
night and the lingering humidity from today`s rain, conditions
will become favorable for fog to develop areawide overnight.
While intensity of fog remains uncertain, pockets of dense fog
cannot be ruled out especially late tonight into early Sunday
morning. Any fog that does develop tonight will erode by mid-
morning on Sunday leaving us with another mostly sunny and
seasonably warm afternoon. That said, persistent easterly winds
on Sunday will keep high temperatures in the mid-70s to lower
80s near the lake with mid-80s forecast inland despite the
continued warm advection. Though, localized upper 80 to near 90
degree readings may be seen near and west of I-39 closer to the
ridge axis.
Yack
Sunday Night through Friday:
The upper level pattern next week will feature high amplitude
ridging across the Great Lakes paired with a closed upper low
in the Southeast setting up a pseudo-Rex Block pattern across
the eastern CONUS. This should keep the various western CONUS
shortwave troughs and associated showers and storms mainly west
of the region through the first half of the upcoming week. Long-
range guidance continues to diverge slightly on the position and
strength of the upper low which has some implications for the
degree of warming each day and timing of the onset of more
unsettled weather later in the week. The GEFS has the upper low
closer to the coast allowing much warmer air to lift into the
region paired with an earlier breakdown of the ridge by midweek
and return to shower chances. Meanwhile, the EPS maintains a
further west position of the upper low (closer to the more
classic Rex Block pattern) which slows the breakdown of the
ridge and delays the arrival of precipitation to later in the
week. This still favors warmer (albeit cooler than the GEFS) and
dry conditions prevailing in the local area for the majority of
the workweek.
With all of that said, there are no major changes in
expectations through the upcoming work week with the going
forecast favoring the EPS solutions with dry and generally warm
conditions expected well inland of Lake Michigan through
midweek. Expect highs in the 80s to potentially near 90 toward
western and central Illinois. Light surface flow will allow
daily lake breezes to surge well inland each afternoon which
should hold highs in the 70s along the lakeshore with falling
temperatures in the afternoon expected for inland areas in its
wake.
The upper ridge/blocking pattern begins to breaks down toward
the end of the week with the western trough axis gradually
shifting toward and over the area into next weekend. This will
lead to increasing shower and thunderstorm chances Thursday
night onward; however, there is not a strong signal for more
organized convection or heavy rain at this time. In tandem,
temperatures will trend cooler and more autumn-esque with highs
in the 70s and lows in the 50s.
Petr
&&
.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1206 PM CDT Sat Sep 13 2025
In the wake of a complex of storms earlier this morning, a broad
baroclinic zone is draped from RFD to RZL. Strong 700mb winds
continue to advect a plume of steep mid-level lapse rates atop
the baroclinic zone, which may support the development of a
stray shower or storm may develop at any time this afternoon and
evening. (Current thinking is that the area to watch is the
narrow band of radar echoes streaming southeastward from
southern Wisconsin). At this point, the chance of a new storm
developing at any given terminal is assessed to be 20% or lower,
so will withhold formal mention in the outgoing TAF. The threat
of storms should shift east of the terminals after sunset.
Surface winds are expected to be generally south to
southeasterly through early afternoon, though occasional bouts
of southwesterly winds (wind direction 200 to 180) cannot be
ruled out. Later this evening, winds are expected to turn
easterly owing to influences of Lake Michigan. An easterly
direction is then preferred through the remainder of the TAF
period.
Finally, will have to watch for patches of fog after midnight
tonight and especially toward daybreak Sunday. Will introduce
TEMPO groups for 3 SM BR at DPA/RFD where confidence is
currently highest in fog development.
Borchardt
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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NWS LOT Office Area Forecast Discussion