451
FXUS63 KLOT 182355
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
655 PM CDT Fri Jul 18 2025

.KEY MESSAGES...

- There is a chance of thunderstorms on Saturday, some of which
  could be strong to severe, though uncertainty is high in the
  specific details.

- Several rounds of thunderstorms may occur early next week,
  with strong/severe thunderstorms possible along with
  torrential rain and localized flash flooding, particularly
  south of I-80.

- Dangerous heat and humidity possible next Wednesday and
  Thursday.

&&

.DISCUSSION...
Issued at 305 PM CDT Fri Jul 18 2025

Through Saturday Night:

The bottom line up front for Saturday`s convective forecast is
there are no significant changes in thinking from our previous
forecast package issuance early this morning (Friday). There`s
too much uncertainty to offer much in the way of specific
details, so we`ll continue to highlight some of the (still wide
range) of plausible outcomes. Another suggestion for those
assessing the latest high resolution convective allowing model
data is to not necessarily put too much stock in any particular
solution being offered, as these models are forecasting
convection and the subsequent effects of ongoing and prior
convection that mostly haven`t materialized yet as of Friday
afternoon.

Most of this evening and early overnight will be quiet and
pleasant with weak surface high pressure over the area, so there
are no concerns for outdoor activities.

The forecast then becomes quite challenging overnight through
Saturday. An embedded wave within overall zonal mid-level flow
across the northern Great Plains and Great Lakes is loosely
phasing with an impulse originating from the northern periphery
of the southwest monsoon today. Effects from this wave and
convection over the northern Great Plains tonight will have
substantial implications on the potential for severe convection
into our area on Saturday. Four general (somewhat more
simplified) scenarios still appear possible at this time.

1) Upstream convection that is expected to develop over
southern Minnesota this evening tracking southeast with a
veering 40-50 knot LLJ into eastern Iowa and southern Wisconsin
overnight. Convection could congeal into a loosely organized
MCS given modest effective shear, but the thermodynamic
environment will become less favorable with eastward extent.
This would bring outflow-dominant convection into portions of
northwest and north central Illinois toward or after daybreak,
with continued weakening over northern Illinois through the late
morning and early afternoon (showers and perhaps embedded
storms with isolated pop-up convection). Gusty winds would be
possible during this time. Then, depending on how far south the
outflow tracks, an incoming cold front will interact with a
moisture rich airmass and seasonably strong mid-level flow to
produce renewed upscale growth of convection over eastern Iowa
or far western Illinois during the afternoon. This would then
potentially yield at least a broken line of convection or
possibly an organized MCS into central Illinois and Indiana and
by late afternoon, clipping the southern CWA with a risk for
damaging winds.

2) Capping breaks to the southwest of the expected convection
over southern Minnesota, allowing a more-organized MCS to
develop across western Iowa late this evening. This MCS would
then possibly encroach on the northwest half or western half of
the forecast area around or shortly after sunrise. Gusty winds,
potentially damaging, would be possible across at least the
western half of the CWA through mid-morning before renewed
strengthening early afternoon past the diurnal convective
minimum. In this case, the overall potential for severe
convection may end up being highest toward northwest Indiana.
The effective outflow would then limit or prevent additional
convection before the cold front reaches the area Saturday
evening.

3) Upstream convection tonight does materialize and become
organized into an MCS, but dives farther southwest (into west
and southwest CWA and points west-southwest) and weakens quickly
during the morning. This would maintain a play for robust
convection/potentially severe, to develop ahead of the cold
front mid-late Saturday afternoon.

4) The convection well to the northwest tonight fails to
organize, with little impact on the forecast area Saturday
morning. The cold front would become the main driver for
convection over the area mid-afternoon and early evening, with
some surface-based capping remaining. The modest shear noted
above would support organized convection, with initial storm
motion favoring clusters or semi-discrete supercells with
damaging winds and hail quickly transitioning into multicell
segments with damaging winds.

The first scenario still appears more likely based on the
overall pattern and more-favorable thermodynamic environment
remaining south and west of the forecast area. This would mean
that much of the CWA could be split by the convective severe
threat on Saturday. With that said, thorough mesoscale
assessment will be needed through tonight to ascertain whether
one of the other scenarios becomes more favorable.

How the rest of Saturday evening and night plays out will be
substantially modulated by the effects of prior convection with
respect to outflow/effective front position. Some CAMs indicate
training convection firing just north of the effective frontal
boundary across central IL and central IN. This particular
scenario would entail a potential flash flood threat within the
CWA, mainly near and south of the Illinois and Kankakee River
Valleys (where our highest PoPs/50-60% are depicted) Saturday
night. There`s an alternative scenario where the effective front
gets shunted far enough south to force showers and storms
safely south of the CWA with time.

Kluber/Castro


Sunday through Friday:

Primary forecast concerns are thunderstorm chances/timing and
building heat/humidity midweek.

A frontal boundary is expected to be across the southern cwa or
central IL Sunday morning and its possible thunderstorms will be
ongoing Sunday morning across these areas. The main threat
during this time would be torrential rain and possible localized
flooding, as alluded to above. This activity would then shift
southeast and dissipate into the mid/late morning. With the
boundary nearby, there would remain a chance of showers and a
few thunderstorms Sunday afternoon into Sunday evening, but much
of northern IL may remain dry on Sunday and have trimmed pops
across the northern cwa. Northeasterly winds will bring in an
other period of cooler temps, perhaps only reaching the upper
70s for northern areas and near Lake Michigan, though dewpoints
may still be in the 60s for much of this time period. The
northeasterly winds may be strong enough to build waves into the
2-4 foot range at Lake Michigan beaches, resulting in a
moderate swim risk but still some uncertainty for how strong the
winds will become.

The upper ridge is still expected to build into the region
Monday night into Tuesday. This time period was looking to be a
concern for possible thunderstorms, along with heavy rain, but
much of the models and ensembles are now rather dry during this
transition with really only the NAM showing any convection.
Blended pops have followed these trends and are now either dry
or just low slight chance pops. Made no changes, but this time
period will need to be monitored. The lack of precipitation may
allow highs on Tuesday to reach 90 for most areas.

No real changes for midweek, Wednesday is still looking like
the hottest day with highs in the mid 90s and dewpoints in the
upper 70s/near 80. Mature crops will likely push some dewpoints
into the lower 80s. Peak heat indices on Wednesday could be in
the 110-115 range. And with these conditions, the atmosphere
should be sufficiently capped for dry conditions.

Thursday will also be hot and humid with highs at least in the
lower 90s and dewpoints still in the upper 70s/near 80. A cold
front will slowly be moving south across the western Great Lakes
Thursday into Thursday night. Still quite a bit of uncertainty
regarding when this front moves into the local area and how long
it persists, as it may slow/stall. The net effect is that the
best precip chances next week may end up being at the end of
the week. cms

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 655 PM CDT Fri Jul 18 2025

Aviation forecast concerns for the 00Z TAFs:

- TSRA potential Saturday. Timing and coverage continues to be
  of low confidence.

Early evening surface analysis shows an area of weak low
pressure over eastern South Dakota, with a warm front extending
southeast through western IA into MO and far southern IL. To the
east, weak high pressure was centered over the eastern Great
Lakes, drifting slowly to the east. In the near term, light east
lake breeze winds will linger across ORD/MDW/GYY early this
evening, before eventually becoming light south by predawn
Saturday morning as the surface high continues to move off to
the east and the warm front to our south slowly evolves
northward. The surface pressure gradient is weak enough that
winds may just go light and variable for a time overnight.

Scattered TSRA were developing along the SD/MN border early this
evening in the vicinity of the weak surface low/warm front.
While CAM guidance continues to show considerable spread in the
details, the general expectation is that these storms will
continue to develop into southern MN/IA and western WI
overnight, which would then move east-southeast into northern
IL after sunrise Saturday morning. This would likely be during a
period of diurnal convective weakening, especially if a
consolidated cold pool/outflow boundary does not develop or is
weak. The best timing for these storms to impact our terminals
looks to be roughly 11-17Z at RFD, and in the 14-19Z time frame
for the Chicago sites, though again confidence remains low with
coverage and exact timing. Most of the CAM guidance suggests
relatively little/no significant coverage later into the
afternoon, thus have maintained dry TAFs beyond prob30s for the
above windows for now. If morning activity is of lower coverage,
and there is no significant outflow however, the chance of TS
later into the afternoon can`t be ruled out.

Most guidance is in agreement that winds will turn west-
southwest and may be slightly breezy during the afternoon,
eventually turning more northwest Saturday night ahead of an
approaching cold front.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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