Summit-Argo, Illinois Weather Forecast Discussion

571
FXUS63 KLOT 130550
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1250 AM CDT Sat Sep 13 2025
.KEY MESSAGES...
- Showers and storms are expected to move across the area
  primarily after daybreak Saturday and linger into early
  afternoon.
- There is a low chance (5% or so) of flash flooding or severe
  weather at any given location tomorrow morning.
- Temperatures tomorrow and Sunday have trended cooler,
  especially close to Lake Michigan.
- Otherwise, a generally dry and warm pattern will prevail for
  much of the upcoming week.
&&
.UPDATE...
Issued at 823 PM CDT Fri Sep 12 2025
Close attention has been toward Iowa and Minnesota, and now
Wisconsin, as showers and storms are gradually moving eastward.
IR satellite imagery has coldest cloud temps in north and
western Wisconsin at this time where much of the lightning
activity has been. However, much of the lightning activity is
waning. Meanwhile, scattered showers have been occurring over
eastern Iowa tonight. After the appearance of a weak MCV
rotation seen on KDVN, there was some question if that would
maintain its progression eastward. While it is still raining
out west, there has been a noticeable erosion in coverage as
this activity has approached the Quad Cities, probably in
response to encountering lower dew points and a drier air mass
aloft. Despite all of this high res guidance continues to flip-
flop run to run in trying to figure out what will transpire
tomorrow. While confidence remains low, thinking is such that
light showers have a slight chance for continuing into the
forecast area over the Rockford Metro some time after midnight.
As day break approaches, shower coverage will likely expand
with instability increasing which may allow for increasing
thunderstorm coverage developing in the morning. While areal
PoPs were slightly adjusted with this forecast update,
confidence just was not there to increase the probabilities at
this time as trends will need to be monitored through the night.
Concern for localized training convective elements remains for
tomorrow morning and into early afternoon.
DK
&&
.DISCUSSION...
Issued at 202 PM CDT Fri Sep 12 2025
Through Saturday afternoon:
Regional satellite imagery this afternoon depicts a scene more
typical of mid-summer than early September with several clusters
of storms parading around the periphery of an upper- level
ridge from the Rocky Mountains to Upper Mississippi River
Valley. Our area is positioned on the downwind axis of the
ridge, which thus far has led to quiescent conditions
characterized by mostly sunny skies, relatively low humidity
levels, and temperatures in the lower 80s. The weak wind field
is allowing for a lake breeze to move inland, leading to locally
cooler temperatures along the shoreline.
All eyes are on upstream convection stretching from near the
ND/SD/MN border region eastward across southern Minnesota. Per
RAP- analyzed kinematic fields, the areas of convection are tied
to several interacting vorticity maxima, which appear to be in
the process of merging into a coherent shortwave-length trough.
As is typical with these types of regimes, model guidance is
fairly clueless on how convection has evolved thus far. As a
result, the ensuing forecast is based moreso on conceptual
models rather than explicit model guidance. (And, as can be
expected, adjustments are all but guaranteed as trends become
established this evening).
Generally speaking, the positioning of the centerline axis of
the ridge to our west will favor the incipient shortwave to
propagate east-southeastward toward Wisconsin, northern
Illinois, and Lake Michigan overnight and into Saturday. As this
occurs, increasing 700mb flow and a touch of synoptic-scale
ascent (implied via a modest 500mb speed max) should lead to the
eastward advection and enhancement of relatively steep mid-
level lapse rates (sampled near 8.0 K/km on the 12Z OAX RAOB)
toward northeastern Iowa and southwestern Wisconsin overnight.
Accordingly, the expectation is for convection to fester
overnight while building southward through Wisconsin and
possibly into northeastern Iowa. Should convection adhere to
such an evolution, the thinking is convection will move across
northern Illinois after daybreak and continue building southward
through the area throughout the morning. Will note, the old
adage of "when in doubt, go south and west" applies in this
regime so it would not be surprising to see the highest coverage
of storms end up closer to the Mississippi River than Lake
Michigan tomorrow morning.
The exact evolution of convection tomorrow morning will be tied
to exactly how convection evolves overnight. With that said,
the west-to-east orientation of the instability axis and
northwest to southeast positioning of the upper-level flow
(including some 35 knots of convective-layer shear) suggest that
convection may evolve into a southward-propagating cluster or
two with gusty to locally damaging winds across northern
Illinois through the morning hours. Assuming this occurs, the
steady eastward advection of the EML plume atop any residual
northwest to southeast-oriented cold pool would encourage
convection to regenerate through the morning hours with a threat
for small to perhaps locally damaging hail and torrential
downpours. With 14-day and even 30-day precipitation totals
running 10 to 25% of normal, soils should be able to soak up
most rain that falls tomorrow. However, if regenerating
convection occurs over any urbanized area such as Rockford or
the Chicago metropolitan areas, there may very well be a threat
for flash flooding. Both SPC and WPC have our area in a Level 1
threat level for severe weather and flash flooding,
respectively.
Convection is expected to gradually decay in the early
afternoon as the 700mb flow becomes more parallel to the
orientation to the long axis of any lingering cold pool. Now,
any sunshine atop locally pooled dew points on either side of
residual outflow boundaries may provide just enough low-level
instability for an isolated storm or to develop during the
afternoon and evening hours. Provided this occurs, strongly
veered low-level wind profiles in the proximity to the outflow
boundary would support supercell structures with an attendant
threat for all severe hazards. With the expectation for quite a
bit of residual cloud cover through the afternoon, suspect the
threat for the redevelopment of storms in the late afternoon is
low, but this will be something to monitor.
When taken altogether, felt there is enough of a conceptual
signal to increase PoPs to the mid-range chance (30-50%) range
tomorrow morning with low-end chance (20-30%) values lingering
through the evening. Of course, the evening and midnights shift
will be positioned nicely to adjust PoPs upward or downward
based on upstream trends.
With the expectation for a fairly high coverage of storms and
cloud cover through early afternoon, felt comfortable dropping
the forecast high temperatures to the upper 70s to lower 80s,
coolest near the Lake Michigan shoreline. Of course, if
convection and associated cloud cover ends up failing to develop
as expected or largely misses our area, highs could very well
be some 10 degrees warmer than advertised.
Borchardt
Saturday night through Friday:
Given the expected timing of the shortwave disturbance(s) and
associated convective footprint during the day on Saturday,
Saturday night should end up primarily dry. With that said,
festering isolated to widely scattered showers and storms are
possible, mainly southeast of I-55. Due to light/calm winds
Saturday night, we`ll likely need to monitor for patchy fog
development (primarily outside of Chicago) overnight night into
Sunday morning if skies clear out. This would particularly be the
case for any areas that receive a soaking rain on Saturday. Dry
and warm conditions will then prevail through much of the upcoming
week as high amplitude upper level ridging sets up across the region.
While generally warmer conditions are expected to persist, model
guidance has continued with the theme of onshore flow through
much of the day on Sunday in the wake of the Saturday system
paired with a reinforcing lake breeze in the afternoon. This would
result in less warming into portions of northeast Illinois and
northwest Indiana. While upper ridging will be building over the
area (supporting broad low-to-mid level warm advection),
maintained cooler daytime temperatures with upper 70s to mid 80s
expected across far northeast Illinois and northwest Indiana
(coolest along the lakeshore). With that said, confidence in how
far inland the cooler air reaches remains lower and have
accordingly maintained upper 80s to lower 90s for areas south and
west of a Rockford to Joliet to Valparaiso line for now.
Long-range guidance begins to diverge a bit heading into the
work week on the position and strength of the various upper
level features. The EPS indicates a pseudo-Rex Block pattern may
set up across the eastern half of the CONUS (upper high over
the Great Lakes and upper low over the Southeast) which favors
warm and dry conditions prevailing in the local area for the
majority of the workweek. The GEFS also has high amplitude
ridging across the region but without a closed upper low feature
which allows the eastern periphery of the western trough and any
embedded shortwaves to get closer to the region which could lead
to an earlier return to shower chances. For now this update leans
toward the EPS solutions and maintains a dry forecast Sunday
through Thursday with low shower chances then returning Thursday
night onward (20-30%).
High temperatures will be mainly in the mid 80s to lower 90s the
rest of the week for much of the area. High pressure over the
Great Lakes and points east will favor easterly winds reinforced
by lake breezes during the daytime and the lake cooling footprint
extending a bit farther inland. Highs near Lake Michigan should
top out in the mid 70s to lower 80s. Dew points will likely also
mix out, keeping humidity levels in check and leading to
comfortable overnight conditions during the increasingly long mid
September nights.
Castro/Petr
&&
.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1250 AM CDT Sat Sep 13 2025
- A period of SHRA/TSRA expected Saturday morning
Showers and thunderstorms have redeveloped across portions of
west central Wisconsin and continue to back build along a
northwest to southeast oriented axis. This activity is expected
to gradually expand and shift southeastward early this morning,
eventually bringing a period of showers and thunderstorms to the
terminals through the morning hours.
Current showers/storms are not making a lot of forward progress
just yet so it is still difficult to track when they may reach
the local area. As a result, opted to not make too many changes
to the inherited TAFs (which were based on model data and
timing of larger scale features) and will plan to handle
additional updates tactically. In addition to lightning and
gusty winds, heavy rain will lead to VSBY reductions to low-end
IFR to potentially LIFR under any of the stronger storms. In the
wake of the morning storms there remains a 20% chance for
isolated thunderstorms to linger later into the afternoon, still
too low for a formal TAF mention for now.
Winds overnight will be generally light out of the southeast
between 5-10 kt. Confidence in directions through the daytime
hours remains low due to potential influence from the morning
convection and position of any remnant outflow boundaries. There
is the possibility that winds return to a SSW direction briefly
during the afternoon before returning to a southeast direction
toward sunset. Winds then turn light and variable after sunset
through early Sunday morning.
Petr
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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NWS LOT Office Area Forecast Discussion