textproduct: Central Illinois

KEY MESSAGES

- Periods of thunderstorms will track across central Illinois today, bringing a marginal risk (Level 1 of 5) for 60 mph wind gusts and golf ball-sized hail.

- Slow-moving storms will bring a risk of localized flooding today, producing isolated rainfall totals of 3 to 4 inches in east-central or southeast Illinois. - A separate, much stronger storm system will bring a threat of organized, high-impact severe weather to the area in the Monday-Tuesday timeframe.

- A moderate heat risk will persist through Tuesday, particularly impacting individuals sensitive to high temperatures or those lacking sufficient cooling and hydration.

DISCUSSION

Issued at 243 AM CDT Sat May 16 2026

A complex mesoscale environment is unfolding across the Midwest this morning. Regional radar mosaics highlight a pair of outflow boundaries currently sagging into Central Illinois, downstream of decaying convection over Iowa. To the west, a pair of shortwave troughs positioned over north-central Missouri and southeast Nebraska are progged to traverse the local area later this morning into the early afternoon. Supported by a modest low-level jet (LLJ), these features will encounter a sufficiently buoyant airmass characterized by MUCAPE values in the 2500-3000 J/kg range.

Expect convection to blossom across portions of central Illinois later this morning and throughout the afternoon. While the ripples in the flow may provide some local enhancement to deep-layer shear, values are generally expected to remain around 30 kts or less. This should limit the potential for widespread organized severe weather; however, a Marginal Risk (Level 1 of 5) remains in place. The primary threats with the more robust cores will be isolated damaging wind gusts up to 60 mph and large hail up to the size of golf balls.

Of additional concern is the potential for localized hydro issues. With slow-moving convective outflows and a high-moisture environment, training or backbuilding cells could produce heavy rainfall. The 00z HREF LPMM QPF indicates a signal for 3-4 inch totals, particularly over east-central or southeast Illinois, a trend supported by recent HRRR runs.

Central Illinois will remain firmly entrenched in the warm sector through the remainder of the weekend. Humid conditions will persist with temperatures climbing into the mid-80s. While zonal mid-level flow and subtle shortwaves may trigger isolated to widely scattered showers and storms, the lack of robust deep- layer shear (generally less than 30 kts) will keep the severe threat disorganized and low-impact.

The synoptic focus shifts significantly toward the Monday- Tuesday timeframe as a potent upper-level wave pivots out of the Great Basin and lifts into the Northern Plains. An associated cold front will sweep into the Mid-Mississippi Valley, encountering a highly unstable environment with MLCAPEs exceeding 2500 J/kg. Unlike the weekend activity, this system will be accompanied by a notable increase in kinematic support, with deep-layer shear increasing into the 30-40 kt range. This environment is conducive to organized, high-impact severe weather, and this period will need to be monitored closely for potential upgrades in risk categories.

Model consistency begins to diverge slightly by mid-week regarding the timing of the frontal passage. Should the front slow, lingering moisture and instability could support re- development on Wednesday, likely confined to areas along and south of the I-70 corridor. By Thursday and Friday, a transition to a cooler and more stable post-frontal regime is anticipated as high pressure begins to build into the region. Mid-range deterministic guidance does show semblance of another frontal passage in the Friday-Saturday timeframe, but moisture return ahead of the front looks too poor to support appreciable rain chances then, at least for now.

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/

Issued at 1246 AM CDT Sat May 16 2026

Scattered tsra expected to develop after 10Z-12Z, continuing through morning hours, and have included PROB30 for tsra with MVFR vsby for this activity. While not included in TAFs, briefly worse conditions possible in storms. Predictability of storms later in the day is too low for mention in TAFs, but there remains a low chance for storms the remainder of the 24 hour TAF forecast period. Low level wind shear will continue through much of the night from KSPI-KBMI eastward due to a SW 40 kt wind aloft. Otherwise, surface winds S 10-12 kts overnight, with gusts to around 20 kts developing by 18Z for most of the area. Gusts decreasing after 00Z.

ILX WATCHES/WARNINGS/ADVISORIES

None.