242
FXUS63 KLOT 150857 CCA
AFDLOT

Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
357 AM CDT Sun Jun 15 2025

.KEY MESSAGES...

- Temperatures will continue to increase into mid week with
  highs potentially 10 degrees above normal

- Multiple rounds of thunderstorms may occur in the general
  region Tuesday through Wednesday evening. Some storms may be
  severe, with locally heavy rainfall, particularly on
  Wednesday.

- A signal persists for the warmest and most humid air of the
  season to arrive next weekend.

&&

.DISCUSSION...
Issued at 341 AM CDT Sun Jun 15 2025

Through Monday:

There is a cluster of showers and storms over much of northeast
Iowa as seen on KDVN`s radar. However, not only has the
lightning trend been down over the last hour or so, the overall
movement is following a weak north-south instability axis
leading the thinking that it should not impact the western
portions of the CWA. There was a smaller group of storms that
was in southeastern Wisconsin, but it completely eroded away as
it entered Jo Daviess County.

As an upper level ridge builds over the desert southwest,
fairly weak flow is expected to develop over northern Illinois
and northwest Indiana. With a drier air mass and limited to no
forcing, PoPs were scaled back through Monday afternoon. Perhaps
the stuff in southeastern Minnesota/northern Iowa sends a weak
outflow that could develop some sprinkles, but there was not
enough confidence in time, location, and coverage to have formal
mention of it in the forecast. Hi-res models are still
suggesting an upper level wave to move from Iowa toward northern
Illinois (with an associated weak surface low into Wisconsin)
on Monday. However, models have slowed the progression and kept
the highest chance PoPs north and west of the forecast area
(where the Storm Prediction Center has a level 1 out of 5 threat
of severe weather). Perhaps with diurnal heating, there could
be some showers/storms that develop over the Rockford Metro,
but it looks more like that the surface boundary and better
forcing does not arrive until Monday night. Lastly, models do
keep a well of better moisture along an instability axis in
Central Illinois/Indiana. Guidance is suggesting that that
boundary could inch northward on Monday afternoon enough to
introduce some low end (less than 20 percent) chance PoPs around
and south of US-24 during the afternoon.

Winds will remain out of the east-northeast through today. This
will help keep the Lake Michigan shoreline cool, once again, in
the low to mid 70s. Meanwhile, as a warmer air mass moves in,
temperatures are expected to reach the low to mid 80s for the
remainder of the forecast area. As winds move to the southeast
on Monday afternoon, better warm air advection can help drive
high temperatures into the upper 80s as the summer heat up for
the area is on! (more on that in the discussion below)

DK

Monday Night through Saturday:

Global deterministic and ensemble guidance remains in good
agreement in their depiction of a more zonal upper level flow
pattern developing from the Plains to the northern Great Lakes
by Monday evening. A notable low-amplitude short wave is progged
to track north of the forecast area overnight, with a surface
low pressure reflection passing in the vicinity of the north
shore of Lake Superior late. The best low-level return flow
appears to set up from the Central Plains to the upper
Mississippi Valley and northern Lakes region, with the primary
MUCAPE axis indicated from southwest IA into MN and northern WI.
Forecast soundings indicate increasing capping farther east
into northern IL overnight, suggesting minimal organized
convective potential into the WFO LOT cwa. Can`t completely rule
out some decaying showers/storms approaching northwest IL late
however, and have maintained some slight chance (15-20%) pops
from RFD northwest after midnight.

Southwesterly low-level flow looks to gradually become better
established into northern IL on Tuesday into Tuesday night, with
associated strengthening low-level theta-E advection working to
push surface temperatures to around 90 and surface dew points
into the upper 60s/near 70 by afternoon especially across the IL
portion of our forecast area. While forecast soundings indicate
a warm capping inversion in the 800-700 mb layer Tuesday
afternoon, there is guidance support for a low-amplitude,
somewhat convectively-enhanced short wave approaching from IA
during the afternoon/early evening hours. This would likely help
weaken/erode the cap, increasing thunderstorm chances during
the afternoon and early evening hours. Deep layer bulk shear is
progged in the 20-30 kt range which is on the low side for much
organized convection, though the presence of up to 2000 J/kg of
conditional MLCAPE would likely support some isolated severe
wind gust/hail threat. SPC has included our IL cwa in their new
day 3 outlook.

The late Tuesday night into Wednesday period continues to be
most likely period to watch for a better organized thunderstorm
threat, as a more amplified mid-level short wave and likely
upstream MCS approaches from the west. Late night diurnal timing
would potentially favor a decreasing severe threat with this
complex as it moves into northern IL/northwest IN, 00Z guidance
comparison does show quite a bit of spread in mid-level wind
fields and resulting shear profiles. Deep, unidirectional
southwest to west-southwest flow does provide impressive
moisture transport however, with forecast p-wats exceeding 2.00"
across the area late Tuesday night into Wednesday. This raises
the threat of locally heavy rainfall amounts/flooding,
especially if weaker mid-level winds and training of cells
occurs.

Potential footprint of the convective cold pool associated with
early Wednesday storms will likely have an impact on
thunderstorm trends and severe potential across the forecast
area Wednesday afternoon. The primary mid-level short wave
trough axis is progged to still be upstream of the area later in
the day and evening, though it is conceivable that the main
focus for additional renewed diurnal convection is focused
across our southern cwa or even potentially farther south if the
previous MCS outflow maintains more stable conditions farther
north. The guidance trend toward a weaker, faster surface low
pressure wave moving northeast of the area Wednesday
afternoon/evening (and implied weaker southerly low-level wind
component) would also suggest recovery from a strong stabilized
cold pool might remain more difficult farther north of the
outflow boundary. Of course, confidence is quite low with these
mesoscale details several days out from the event.

In the wake of Wednesday night`s mid-level trough passage,
medium range models indicate that broad upper-level ridging
develops across the region. Subsidence and drying beneath the
developing ridge would support mainly dry weather conditions for
the end of the week, though a couple of minor-amplitude waves
are progged across MN/WI just to our north. The strongest of
these may briefly flatten the ridge along the IL/WI border
Friday night, before the ridge really builds into the weekend.
Widespread lower 90s appear passable with increasing humidity
levels by next weekend beneath the building ridge, with afternoon
heat indices potentially above 100 degrees.

Ratzer

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1223 AM CDT Sun Jun 15 2025

No significant aviation weather concerns for the 06Z TAFs.

A mid-level disturbance and it`s accompanying weak surface low
continue to move slowly eastward across the OH Valley early
this morning. Farther north, weak surface high pressure ridging
developing across the western Lakes. Light east-northeast winds
around the southern periphery of the surface ridge will become
variable in spots for the remainder of the early morning hours,
before becoming east-northeast near 10 kts again sunrise. Light
easterly or variable winds of 5 kt or less are expected again
Sunday night.

VFR conditions are expected through the period. Some SCT-BKN
3500 foot stratus will glance the southern Chicago metro for a
few hours early this morning before moving off to the southeast
with the departing low. A few 4500-5500 diurnal cumulus can then
be expected late morning into this afternoon, mainly away from
the more stable onshore flow off of Lake Michigan. Otherwise,
high-level cirrus will thicken a bit this afternoon and
tonight. Convection across IA should generally propagate
southward along/west of the MS river, as instability decreases
rapidly farther to the east. A few decaying SHRA my work into
far NW IL early this morning and again this evening, but
probability of them making it to RFD remains low (<20 percent)
at this time.

Ratzer

&&

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

&&

$$

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