700
FXUS63 KLOT 191406
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
906 AM CDT Thu Jun 19 2025

.KEY MESSAGES...

- Scattered showers/isolated thunderstorms end by mid-morning
  today, though could see an isolated storm or two in the
  vicinity of the lake breeze late this afternoon in NE IL and
  NW IN. Additional scattered thunderstorm potential exists
  Friday.

- Stretch of hot and humid conditions with peak afternoon heat
  indices possibly over 100 degrees this weekend into early next
  week.

- Chances for rain return middle of next week

&&

.UPDATE...
Issued at 906 AM CDT Thu Jun 19 2025

Recent satellite imagery depicts clearing skies from northwest
to southeast across the area in the wake of a departing upper-
level shortwave. Current temperatures around 70 will hence be
poised to rise 10 to 15 degrees by this afternoon leading to
highs in the lower 80s.

As a surface high pressure system currently centered near the
KS/MO/OK/AR border regions shifts southeastward this afternoon,
surface winds across the Great Lakes will modestly back
westerly. Meanwhile, winds over Lake Michigan will turn south to
southeasterly in response to the thermal gradient along the
shoreline. The net result should be the development of a low-
level confluence zone along the shoreline. With forecast
soundings depicting largely uncapped temperature profiles by
mid-afternoon, a pocket of dry air above 850mb in tandem with
mid-level subsidence may act to effectively squash any
convective attempts this afternoon. With that said, if any
updraft were to become established, effective shear on the order
of 25 to 30 kt and relatively low freezing levels near 12kft
would encourage storm- scale organization with attendant threats
for gusty winds and hail. At this point, this seems to be a
fairly low- probability scenario (20% chance a storm forms,
followed by a 5% chance it becomes severe, effectively leading
to a 1% joint probability for a severe storm. That`s not zero,
but certainly quite a bit lower than yesterday).

Updated products will be sent soon.

Borchardt

&&

.DISCUSSION...
Issued at 356 AM CDT Thu Jun 19 2025

Through Friday Night:

Early morning GOES vapor imagery depicts a mid-level trough
centered across the western Great Lakes and Midwest. Scattered
showers and a few isolated thunderstorms persisted per regional
radar mosaics, mainly associated with two sheared mid-level
vorticity axes, extending as far west as southwest WI/northwest
IL. These scattered showers and isolated storms will continue to
shift east across the region over the next several hours,
exiting the forecast area by around mid-morning. Guidance
indicates substantial subsidence developing in the wake of the
departing trough this afternoon, with 50-60 meter 500 mb height
rises noted. This, combined with a modest decrease in low-level
moisture, would appear to be at least somewhat unfavorable for
any diurnal convective redevelopment today. However, several
CAMs depict isolated to widely-scattered convective cells during
peak diurnal heating later this afternoon across far northeast
IL and northwest IN, in a region of weak convergence in the
vicinity of a weak lake breeze boundary (which looks to stay
near the IL shore then extend southeast into IN). Even through
forecast soundings do show some subsident warming in the 600-700
mb layer, afternoon thermodynamic profiles suggest little/no
capping with perhaps 500-700 J/kg of MLCAPE for a time. Surface
dew points are likely too high and thus convection likely
overdone in the 4km NAM, ARW and NSSL WRF guidance, though can`t
rule out a couple of isolated late afternoon SHRA/TSRA similar
to the drier HRRR output. Have included a slight (15%) pop
within the aforementioned convergence zone for this late this
afternoon.

Any isolated showers or storms should dissipate quickly by
sunset this evening, with dry and quiet conditions expected
overnight. Well to our northwest however, guidance has trended
more bullish with the development convection across the eastern
Dakotas/Minnesota which several models develop into an organized
MCS. Several CAMs propagate this MCS southeast into the upper
Mississippi Valley early Friday morning, potentially moving into
northwest/northern IL after sunrise in a decaying form. While
confidence is not extremely high in this, did add chance pops
for thunderstorms across northern IL Friday morning. SPC`s Day
1/2 severe outlooks include far northern IL along the southern
periphery of the greater severe threat across MN/WI for this
possibility, with wind the main concern if the MCS were to
maintain itself this far southeast.

If the decaying MCS and/or remnant outflow boundary were to
push into the forecast area Friday, redevelopment of
thunderstorms may occur during the midday/afternoon hours with
diurnal heating. At the same time however, a well-advetised
synoptic upper level ridge begins to build in earnest toward the
Mississippi River Valley. Warming aloft (noted in the 450-650
mb layer in forecast soundings) associated with the development
of the ridge may provide more substantial capping which could
limit the redevelopment of afternoon convection, especially if
the earlier outflow limits surface temps a bit. Have maintained
some slight chance (<25%) pops north of I-80 for this at this
time, though confidence is fairly low in that level of detail at
this time. Strengthening of ridge and increasing capping looks
to shift thunderstorm threat north of the area Friday night.

Temperature-wise, highs this afternoon should rebound into the
low- mid 80s (coolest into northwest IN) with some sunshine.
Solid mid- upper 80s are reachable Friday as low-level warm
advection begins to ramp up, though the potential effects of any
morning MCS outflow lends lower confidence to widespread upper
80s just yet.

Ratzer

Saturday through Wednesday:

A trough over the western CONUS is expected to deepen, further
amplifying the upper level ridge over the eastern CONUS on
Saturday. Southwesterly steering flow will continue to advect in
warmer temperatures, driving 850 mb temperatures into the 20 to
25C range over the weekend. As dew point temperatures climb
into the low 70s, it is setting up quite the hot and humid
weekend across the forecast area where heat indices will reach
into the mid 100s (low 100s in Chicago). However, there is lower
confidence in dew point trends. The NBM is coming lower than
other models, particularly for areas outside of the Chicago
Metro. If higher dew point temperatures are realized, that could
drive up heat index values above the current forecast. In
addition to the uncertainty of the dew points, ensembles are
suggesting a middle level shortwave will move from the Dakotas
toward Lake Superior and potentially send a MCS west to east
just north of the forecast area over Wisconsin. While both
Saturday and Sunday are expected to be dry, there is still
uncertainty on this storms track and its influence in northern
Illinois. For now, it was decided not to issue any excessive
heat products for the weekend.

The upper level trough out west is expected to phase with
another upper level low that drops southward over Saskatchewan
Sunday into Monday. Models are suggesting the lows aloft break
down into more of an open wave and move east on Monday into
Tuesday. The upper level ridge over the Mid-Atlantic will
gradually compress and drift slightly more over the southeast
through midweek. As the ridge sags southward, temperatures over
northern Illinois will slowly diminish from the 90s on Monday
and Tuesday down into the 80s on Wednesday.

As a new surface high pressure grows early next week in Central
Canada, a weak surface baroclinic zone is expected to develop
along a roughly Denver to Milwaukee line. There is still some
discrepancies in the models regarding the boundaries exact
location, but many of the ensembles are suggesting it could
become quasi- stationary through the middle of next week.
Meanwhile, persistent south-southwest mid level winds (from the
high over the southeast) will advect Gulf moisture northward
into this area driving precipitable water amounts up to around
two inches. With such nebulous forcing, rain rates should be
weak enough that inhibit flooding concerns for the moment. And
given the uncertainty on the exact placement of the boundary and
northern Illinois sitting just on its southern periphery, there
could be periods without rain that will need to be ironed out.
But the signal is growing that the middle of next week could be
a bit wet.

DK

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 633 AM CDT Thu Jun 19 2025

Key Aviation Messages for the current TAF period:

- Lingering showers are expected to move east of terminals by
  14Z

- Less than 20 percent chance for an isolated storm to develop
  along a lake breeze this afternoon.

VFR cigs and vis have returned to the airspace and are
maintained through the current TAF period. The last bit of
showers are still over northern Illinois and moving east-
southeast. Most terminals will start to dry out by 13Z, but high
confidence for showers to be east of the terminals by 14Z.

Drier conditions are expected today. Models are suggesting a
weak lake breeze that will remain east of airfields this the
afternoon. Confidence is low in an isolated storm developing
along it so the TAF was kept dry. Winds will remain light and
out of the northwest and slowly become west by midday. Lighter
south-southwest winds are expected this evening and through the
overnight. There is also the potential for another chance for
thunderstorms around and after 18Z on Friday, but the
probability was too low to consider it at this TAF issuance.

DK

&&

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

&&

$$

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