485
FXUS63 KLOT 171941
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
241 PM CDT Tue Jun 17 2025

.KEY MESSAGES...

- Periodic bouts of scattered showers and t-storms tonight
  through Wednesday evening. Some risk for severe t-storms,
  particularly during the daytime hours Wednesday.

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

&&

.DISCUSSION...
Issued at 240 PM CDT Tue Jun 17 2025

Through Wednesday Night:

Tricky forecast in the short term with a lot of the key forecast
details dependent on convective influences.

Shortwave trough moving slowly eastward across IA and northern
MO is still expected to lead to scattered showers and
thunderstorms developing over eastern IA south into northeast MO
late this afternoon. This activity is expected to spread
eastward into and possibly across our CWA this evening. Weak
shear, nocturnally stabilizing boundary layer, and no
meaningful low level jet raise doubts as to how well this
convection will be able to maintain itself as it moves into and
across our CWA this evening. Still seeing an impressive QPF
signal in HREF, so have maintained some fairly high POPs this
evening despite the uncertainties. Weak mid level lapse rates
and lack of shear really suggest there should be very little
severe threat by the time any storms reach our area this
evening.

There appears to be another subtle shortwave over eastern
Nebraska which most guidance either loses or merges with the
lead shortwave over IA. Given this upstream wave, it is hard to
rule our some scattered showers and thunderstorms lingering even
past midnight, but again weak lapse rates and bulk shear really
suggests if there is any convection overnight it would likely
be scattered/unorganized.

The main larger scale synoptic upper trough is expected to move
across the region Wednesday. Given the expected widespread and
intense convection across the central and southern Plains this
afternoon and tonight, it is quite plausible that the synoptic
shortwave trough and associated surface low could become
convectively augmented. This leads to lower than average
forecast confidence for Wednesday.

There is a decent signal in guidance that showers and
thunderstorms, rooted above a more stable boundary layer, could
move across portions of our CWA during the morning hours
Wednesday. Given the weak mid level lapse rates, the primary
hazard with this convection would likely be torrential
downpours with the potential for locally heavy rainfall.

The strongest signal in guidance now is for destabilization to
occur mainly east of our CWA tomorrow, where SPC has the "enhanced"
(level 3 of 5) severe risk. In the absence of convection and
with some breaks in cloud cover, there could be a window of
opportunity for boundary layer to destabilize tomorrow
morning/early afternoon over our eastern/southeastern CWA. If
this were to occur before the stronger forcing shifts east,
there would be a threat for severe thunderstorms in our CWA for
a couple of hours late morning/early afternoon Wednesday.
Seasonably strong deep layer shear could support supercells with
even some tornado threat given strong low level shear with the
synoptic low level jet. Severe threat in our CWA is conditional
on destabilization occurring prior to better forcing shifting
east by early-mid afternoon.

- Izzi

Thursday through Tuesday:

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 will push over the west coast further
amplifying the ridge ahead of it and pushing 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 several 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 Saturday and Sunday and probably
Monday too. Dewpoints are forecast to reach the lower 70s but this
will be another element to monitor as drought currently persists
across the area. Any new rain received today and tomorrow 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

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1251 PM CDT Tue Jun 17 2025

Key Aviation Messages for the current TAF period:

- Chances for thunderstorms this evening. Low confidence in
  timing and coverage.

- Additional chances for thunderstorms midday through mid
  afternoon Wednesday.

No major forecast changes from the previous forecast. Breezy
southwest winds will continue across the terminals through the
afternoon. Have maintained the PROB30 mention for thunderstorms
early this evening as we continue to monitor a disturbance
approaching from southeast Iowa and northeast Missouri.
Instability is in place so do expect at least scattered
thunderstorm development later this afternoon as this
disturbance arrives which would work eastward towards the
terminals early this evening. Debated adjusting to a TEMPO for
TS but confidence is not all that high, especially when it comes
to coverage. May need to adjust the timing slightly later and
the later portion of the PROB30 period may yield the best chance
for storms to occur. A lull in activity is expected late
tonight and early Wednesday morning before a cold front
approaches closer to mid day bringing another window for
thunderstorm activity into mid Wednesday afternoon. South winds
Wednesday will shift west-southwest with the frontal passage
later in the afternoon and MVFR ceilings look to be in place
behind the front as well.

MDB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.

LM...None.

&&

$$

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