127
FXUS63 KILX 180813
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
313 AM CDT Mon Aug 18 2025

.KEY MESSAGES...

- A few more days of heat and humidity, with peak heat indices of
  100 to 105 degrees again today. Some relief is expected by
  Wednesday (highs in the 80s).

- Scattered thunderstorms are possible from late this afternoon
  into tonight (30-50% chance), mainly along and north of
  I-72/Danville. Some of the storms could be strong, with isolated
  damaging winds and locally heavy rainfall. Storm chances expand
  to the entire area on Tuesday.

&&

.DISCUSSION...
Issued at 311 AM CDT Mon Aug 18 2025

*** THROUGH TONIGHT ***

A stalled cold front/outflow boundary was draped from near
Galesburg to Lincoln to Paris as of 1am/07z. An upper ridge
remains centered over the southern Plains/mid-MS Valley and will
continue to retrograde west during the first few days of the week.
Elsewhere across the region, a shortwave tracking around the
ridge has helped kick off storms across eastern SD/NE while low-
level WAA has led to a corridor of scattered storms along the
MN/IA border eastward into far N IL (as of 1am/07z). The point of
mentioning these regional conditions is two-fold. First, pressure
falls to our north associated with the wave aloft and/or a remnant
MCV that tracks east seem likely to keep the front near its
current location or lifting slightly northward through the day.
Second, similar to previous days, the mesoscale convective
evolution remains low confidence.

The implication of the boundary lifting northward is it would
allow heat indices to climb near advisory criteria (100-105
degF). 18.00z HREF mean has low probabilities for exceeding heat
indices of 104- 105 deg, but the HREF mean also appears to be
underdoing the expected dewpoints by several degrees. Given that
this would be the third straight day of near or above heat
advisory criteria of 105 degrees, opted to be generous with the
advisory and expanded it to include the entire ILX CWA. The lowest
confidence area is east of I- 39/north of I-74. In this area, it
would not be a surprise if peak heat indices stayed just below 100
degrees.

As noted above the convective evolution remains low confidence,
but some general trends are apparent in probabilistic data. A
stray shower/storm can`t be ruled out today thru mid-afternoon
(10% chance), then chances start to ramp up late afternoon into
the evening (20% along the I-72 corridor, 40-50% along the I-74
corridor). As instability remains quite high, isolated, briefly
severe storms can`t be ruled out, but weak shear should preclude a
more organized severe storm threat. A marginal risk (level 1 of
5) of severe storms includes northern portions of the forecast
area. The other concern will be locally heavy rainfall/isolated
flash flooding due to high PWATs (near/over 2"), sufficient warm
cloud layer depth (>10 kft), and slow storm motions (cloud layer
mean wind less than 10 kts). 18.00z HREF LPMM has a isolated
pockets of 3+ inches of rainfall through 12z/7am Tues. WPC issued
a slight risk (level 2 of 4) of excessive rainfall from Fulton to
Vermilion counties and northward, which is well-aligned with where
the highest PoPs are in our current forecast.

*** TUESDAY AND BEYOND ***

Conditions are expected to remain unsettled on Tues as the cold
front slowly shifts south across the region. The latest NBM keeps
peak heat indices near 100 on Tues, and if that verifies it would
warrant keeping a heat advisory in effect as its the 4th straight
day of 100+ heat indices for much of the area. However, with
uncertainty in the positioning of the front and convective chances
through the day (30-50%), was not going to extend the Heat
Advisory into Tues at this time.

Precip could linger south of I-72 on Wed (20% chance) depending on
how quickly the front shifts south. Temps should be cooler, with
highs in the 80s (perhaps the low 80s north of I-72/Danville). By
Wed, the synoptic pattern will feature the retrograded upper ridge
over the Four Corners, a sfc high over the upper Midwest, and
Hurricane Erin about 300 miles east of the Outer Banks of NC.

The extended forecast continues to appear free of hazardous
weather. Global models are in good agreement that a deep upper
low will track across Canada late in the week, nearing the
northern Great Lakes by the weekend. This is expected to send
another cold front across the Midwest, providing a reinforcing
push of cooler air. Wouldn`t rule out some rain from this cold
front, but the current model blend keeps PoPs below 10%. In fact,
the blend keeps PoPs below 10% from late Wed through early next
week. If rain does occur with the weekend front, I don`t expect it
to amount to much as the front appears quite progressive and
ensemble mean PWAT values are around 1.3", which is near the 50th
percentile for late August but below what we`d expect to see in
advance of a cold front. Following the weekend FROPA, seasonably
cool/dry conditions are forecast, with highs in the 70s (50-80%
chance for Sun-next Mon) and afternoon dewpoints in the 50s (!), a
welcome shift after a prolonged stretch of summer heat and
humidity.

Erwin

&&

.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1156 PM CDT Sun Aug 17 2025

Thunderstorms clusters north of the local area should stay that
way tonight, though can`t rule out a stray storm that would impact
a terminal into early morning. Probabilities less than 20% so did
not include mention. MVFR to IFR ceilings are expected to develop
to our northwest over the next few hours then expand southeast
through sunrise. KPIA-KBMI have the best chances of seeing the
lower ceilings so added a tempo group for this. Patchy MVFR
visibilities are likely area-wide due to low temp/dew point
spreads for a few hours around sunrise. Scattered thunderstorms
are forecast to develop this evening ahead of an advancing cold
front. Due to potential for sparse coverage, and timing
uncertainty, kept mention to PROB30 for now.

25

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 8 PM CDT this evening for ILZ027>031-036>038-
040>057-061>063-066>068-071>073.

&&

$$