599
FXUS63 KILX 151743
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1243 PM CDT Sun Jun 15 2025

.KEY MESSAGES...

- Above-normal temperatures are favored this week. Daily highs of
  85 F and overnight lows of 63 F are considered normal for the
  middle of June in central Illinois, and we will average a solid
  3 to 5 degrees above these benchmarks, particularly at night.

- Heat stress will become prevalent at times this week, but
  especially by next weekend. Recent NBM guidance suggests we will
  make a run at triple-digit heat index values both Saturday and
  Sunday.

- Mid-range guidance continues to flash signals for flooding and
  severe weather between Tuesday night and Wednesday night, though
  predictability remains low at this time.

&&

.DISCUSSION...
Issued at 346 AM CDT Sun Jun 15 2025

This week`s 15-word weather summary:

An impactful week of weather lies ahead. Heat stress, flooding,
and severe storms. Oh my.

Let`s start with the heat.

There is nothing out of the ordinary with temperatures and
humidity through Monday. The synoptic pattern between now and then
features a bit of a split-flow regime, with central Illinois
positioned in between a shortwave trough currently digging
southward across the Ozarks -- forecast to become cut off from
the main belt of westerlies as it reaches the Gulf coast -- and
additional shortwave energy that will mostly pass just to our
north. Temperatures are forecast to warm into the low-to-mid 80s
each day, per NBM guidance, with dewpoints in the mid-to-upper
60s. This fits mid-June climo to a T.

The pattern becomes notably hotter and more humid by Tuesday
afternoon as a western Gulf airmass surges up the Mississippi
Valley in between an expanding sub-tropical high across the
southeast US and a collapsing ridge over the western US. Current
NBM guidance supports late afternoon highs around 90 F, and this
seems reasonable given the pattern shift. It will be a heavy 90,
too, with dewpoints near 70. Not good for band camp.

Consensus is for temps to take a step down by Wednesday, likely
owing to the increasing signal for convective debris.
Deterministic NBM guidance suggests highs in the low-to-mid 80s
for Wednesday, and then again for Thursday behind a cold front.

The oppressive heat and humidity is likely to return Friday
onward. By then, a broad 588-mb ridge axis is modeled to push
across the Corn Belt, becoming amplified (594mb) by Saturday and
Sunday as a deep western trough emerges. Daily temperatures are
forecast to warm into upper 80s by Friday, then lower 90s by
Saturday and Sunday amid low 70s dewpoints. This all adds up to
triple-digit heat index values by next weekend. Not good for
anyone.

The signal for flooding and severe storms returns between Tuesday
night and Wednesday night, but it is not clear cut.
Predictability remains low, with mid-range global guidance all
offering a slightly different version of when and where.

The main theme is for a nocturnal MCS to develop Tuesday evening
somewhere over the Corn Belt, ahead of a robust shortwave trough
and on the nose of a strengthening LLJ. A quick glance at CAPE,
shear, lapse rates and the mean wind support healthy cold pool
maintenance deep into the overnight period as the MCS pushes east
of the Mississippi Valley. While the increasing kinematic fields
will support an attendant thunderstorm wind risk late Tuesday
night, heavy rainfall might steal the show. The NBM Mean QPF
continues to ratchet upwards with each subsequent run, now
averaging around 0.5 across the ILX CWA during the Tuesday night
period. The spread (10th to 90th percentile QPF) remains large;
generally 0"-1.3. And then, there are a few outliers that
support 3-4 of QPF with MCS activity Tuesday night.

To review, the possible outcomes at this juncture for Tuesday
night range anywhere between getting missed completely to getting
hit squarely by an MCS. The trend has been moving towards us not
getting missed, with the risks for flooding and damaging
thunderstorm winds increasing across central Illinois.

Wednesdays risk for flooding and severe storms will be highly
conditional on whatever happens Tuesday night. Either the boundary
layer will recover/destabilize Wednesday afternoon ahead of the
impending cold front, or it wont. If it does, the parameter space
is very favorable for deep/organized convection and somewhat
favorable for flooding. Any further analysis at this point is
empty, boastful talk.

Consensus is then for conditions to dry out Thursday onward. Rain
chances are not zero during the late week period, but with the
amplified ridge building over the southeast US, showers and storms
should have a tendency to get deflected to our north and west.

MJA

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1243 PM CDT Sun Jun 15 2025

A robust diurnal Cu field has developed across central Illinois,
resulting in MVFR ceilings at all terminals except KPIA. Based
on satellite trends, NAM forecast soundings, and the latest
HRRR/RAP guidance, have maintained MVFR for a couple more hours
before all sites gradually improve to low VFR after 20-22z. The
diurnal Cu will scatter toward sunset: however, BKN-OVC high
clouds will persist through the night. Given partial clearing,
light winds, and a moist boundary layer...there is at least a
weak signal for patchy fog once again late tonight. Most CAMs
do not show reduced visbys: however, the GFSLAMP has been
consistently hitting areas along/south of I-72. Have therefore
dropped visbys to 5-6 miles at KSPI/KDEC/KCMI overnight into
early Monday morning. After that, NAM soundings and Cu-rule
suggest SCT-BKN clouds with bases around 2500ft will re-develop
after sunrise. Winds will initially be NE at 5-10kt today, then
will become SE by Monday morning.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...
AVIATION...Barnes