410
FXUS63 KLOT 182129
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
429 PM CDT Wed Jun 18 2025

.KEY MESSAGES...

- Severe weather threat has ended, though the threat for
  isolated thunderstorms and localized flooding continues in
  portions of the area into early this evening.

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

&&

.MESOSCALE...
Issued at 429 PM CDT Wed Jun 18 2025

The band of intense convection has shifted well east of the area
into eastern Indiana. Center of the MCV is still spinning slowly
eastward across our CWA late this afternoon. Some weak instability
in advance of this MCV has allowed for some additional isolated
storms to develop over northwest Indiana, but weaker instability
and weaker low level shear is resulting in minimal severe threat
from this activity and Tornado Watch 434 has been cancelled.

On the northwestern side of this MCV`s circulation there is an
area of moderate to borderline heavy rain will continue to spread
east-northeastward across mainly southern portions of the Chicago
metro area and into Lake and Porter Counties in Indiana through
sunset. Rainfall rates with this activity seem pretty unlikely to
result in a significant flood threat, so unless radar trends
reverse course, we will likely be able to cancel the flood watch
prior to the 8 PM scheduled expiration time.

Finally, a narrow band of convection across our northwestern CWA
could persist as it pivots slightly eastward over the next couple
of hours. There have been numerous reports of funnel clouds and a
couple of brief landspouts with this line closer to the
Mississippi River where conditions appear more favorable. Given
the developing updrafts on/near the boundary up there, can`t rule
out funnel clouds and perhaps a brief landspout with this activity
for the next hour or two. Have issued an SPS to cover this threat
in the near term. These showers have been producing narrow
corridors of heavy rainfall rates. The very small footprint of
the heavier rates should generally limit the flooding threat vs.
what has occurred just over the WI border in portions of Rock and
Walworth Counties.

- Izzi/Castro

&&

.DISCUSSION...
Issued at 403 AM CDT Wed Jun 18 2025

Through Thursday:

Main forecast concerns in the near term are thunderstorm trends
and severe weather potential across the area today, along with
the potential for locally heavy rainfall amounts. SPC`s Day 1
outlook has expanded the enhanced risk a little farther north
into the southern parts of the Chicago metro area, generally
south of where last evenings convection has tracked.

Early morning water vapor imagery depicts a synoptic mid-level
short wave trough upstream (roughly through the Missouri River
valley), likely convectively-augmented by the MCS/MCV over far
southeastern KS. A couple of smaller MCVs were evident in
regional radar data as well, one over central NE, another over
northeast IA, and one lifting northeast out of southeast WI
which brought our evening storms. Expectation is that there will
be a relative lull in shower and isolated thunderstorm activity
here early this morning in the wake of this departing MCV,
before thunderstorm chances increase again mid-late morning
ahead of the approaching short wave and embedded KS MCV.

While still depicting more spread than desired, guidance is in
general agreement in developing an associated surface low
pressure wave across the forecast area this afternoon, backing
low-level flow. Meanwhile, mid-level winds are forecast to
increase to 40-50 kts along the southern periphery of the MCV,
across central IL/IN and the southeast portion of the WFO LOT
cwa. Warm and humid low- level air mass (low-mid 80s temps upper
60s dew points) is forecast to support 1500-2000 J/kg of
MLCAPE, with 50 kt deep layer bulk shear expected to support
organized convection in the form of clusters and line segments,
and upscale growth into a more solid squall line appears
possible across the southeastern parts of the forecast area this
afternoon. A little bit of a warm layer in the 500-400 mb layer
per forecast soundings weakens mid-level lapse rates somewhat
aloft, with damaging wind gusts the primary expected severe
weather threat. Backed low-level winds near/south of the low
track will also support a tornado threat as well.Precipitable
water values around 2" will also support the potential for
torrential rainfall with storms during the midday and afternoon
hours, with locally heavy rainfall amounts in excess of 2"
possible.

Storm coverage and severe threat should decrease late this
afternoon as the low moves east of the area, though the upper
trough axis will be slow to move across the region overnight
into early Thursday morning. This will likely maintain scattered
showers and isolated thunderstorms through the overnight hours
and into early Thursday before strong subsidence develops by
afternoon. SPS has maintained a marginal (level 1 of 5) risk for
Day 2 across our far northern counties, though the potential
appears to be higher north of the IL/WI border.

Ratzer

Thursday Night through Tuesday:

Main focus through the extended period continues to be building
heat and humidity this weekend into at least early next week.

Broad upper level ridging will be building across the Rockies
and high Plains Thursday as upper troughing shifts east across
the Great Lakes. This will place the local area under a
northwesterly flow aloft ahead of the building ridge to the
west. This set up will support the passage of subtle shortwaves
through the northwest flow which may result in some isolated
shower/thunder chances, especially across far northern Illinois
later Thursday or Friday. Better chances look to be to the north
so most if not all areas may remain dry.

A closed upper low upstream over the west coast is progged to
further amplify the downstream ridge and push it eastward into
the weekend. This will bring in much warmer air by Saturday with
a breezy southwesterly low level flow setting up across the
area. With such an amplified pattern, the ridge axis will extend
well north of the area by Sunday, which combined with the
fairly northerly position of the base of the western trough
would keep the favored track for showers and thunderstorms north
of the area through the weekend. Given that we`re still a few
days away from this pattern shift, we`ll have to monitor how
the amplitude of the upper features evolves over the next few
days to see if the signal for convection to be favored to the
north persists. The question going into early next week is how
quickly the amplitude decreases and when a favored track for
thunderstorms returns to the area as well as how quickly the
heat abates.

As far as temperatures, 850 mb temps are expected to warm into
the 21-23C range for the weekend. Breezy south to southwest
surface winds will support strong warming with highs expected to
reach the mid 90s for most areas both Saturday and Sunday, and
likely into Monday and perhaps Tuesday as well. Dewpoints are
forecast to reach the lower 70s but this will be another element
to monitor as drought currently persists across the area. Rain
from today`s thunderstorms may impact this, but Thursday and
Friday will likely allow for drying out of the top layers of the
ground. Given this, and the early stages of crop growth (as
opposed to mature stages in July and August which allows for
crops to add a notable amount of water vapor to the air) there
is some concern that current dewpoint forecasts could be a bit
too high. A drier airmass would allow for higher temperatures in
the upper 90s. Ultimately, a hotter/drier scenario could net a
similar heat index as a slightly less hot/more humid scenario
with the resultant messaging of taking heat precautions being
the same. At this point, the bigger concern is the duration of
the heat as opposed to heat reaching an extreme or unusual
magnitude.

MDB/Ratzer

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1256 PM CDT Wed Jun 18 2025

Concerns:

- Thunderstorms with heavy rain this afternoon
- Messy winds through the afternoon into the early evening
- MVFR ceilings and IFR/LIFR visibility in the heavy rain

Low pressure will continue to strengthen and lift to the
northeast this afternoon. An area of showers and thunderstorms
will spread north out ahead of the low. The low will move
overhead just to the south and east of the Chicago terminals,
and likely passing over KGYY. The strongest storms will be out
ahead of the low to the east, where gusty southwest winds will
accompany the storms. For the Chicago area, storms may remain
elevated. However, the storms will have heavy rain and reduce
visibility to LIFR even at KORD and KMDW. The main window of
storms will be 19z-22z, before becoming showers with embedded
thunder.

The other challenge will be the winds. Winds will remain quite
variable given the low track moving close by, but should
maintain some easterly component. ORD and MDW also have a lake
breeze to contend with, that will also aid winds from transition
from a SE to NE, though they will likely bounce around. If any
westerly component occurs, they should be on the lighter side or
brief, with some easterly component prevailing during the
storms.

Winds will shift to north and then northwest as the low pulls
away this evening. Some showers and MVFR ceilings will remain in
place this evening, with a 40% chance of IFR.

KMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch until 8 PM CDT this evening for ILZ013-ILZ019-
     ILZ020-ILZ021-ILZ023-ILZ032-ILZ103-ILZ104-ILZ105-ILZ106-
     ILZ107-ILZ108.

IN...None.
LM...None.

&&

$$

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