textproduct: Central Illinois

KEY MESSAGES

- Widespread rain will occur on Sunday. A Flood Watch has been issued roughly from Macomb to Danville and southward, where the NBM indicates a 30-50% chance of rainfall amounts exceeding 3". - Severe storms are possible on Sunday, with a Slight Risk (level 2 of 5) along and south of a Jacksonville to Charleston line, and a Marginal Risk (level 1 of 5) to the north.

DISCUSSION

Issued at 249 AM CDT Sat Jun 20 2026

Regional mosaic shows a few showers persist north of I-72/Danville as of 230am/0730z Sat, driven in part by weak FGEN. Kept some low PoPs in the forecast through 7-8am for this activity. Otherwise, no weather impacts are expected on Sat with partly cloudy skies and highs in the upper 70s to low 80s. Much of the forecast attention remains on Sunday's system and the potential for both heavy rain and severe storms.

*** FLOOD WATCH IN EFFECT SUNDAY ***

A shortwave currently located over the Intermountain West is set to emerge today and aid convective development across NE/KS, leading to an MCS approaching IL from the west by Sun morning. SW winds will feed rich moisture into the system, with very high PWATs exceeding 2". There is still uncertainty as to the exact track of the MCS Sun AM, which impacts both the heavy rain and severe storm forecast. Other synoptic scale sources for ascent, such as DPVA from the approaching low and isentropic upglide near the warm front, will result in widespread rain during the day Sun.

Unseasonably strong wind fields may keep individual convective elements progressive, but with an east-west motion to the larger system and a roughly east-west oriented front, the expectation is a corridor of 2-3"+ rainfall will occur. The latest NBM probs of exceeding 3" of rain are maximized roughly along a Quincy/Macomb to Danville/Paris corridor (30-50%), which is similar to where the highest HREF LPMM values occur. The HREF shows potential for localized amounts in excess of 4 inches.

In addition to the heavy rain signal, soils remain saturated with 50- 70% soil moisture (0-100cm) per NASA SPoRT, area rivers and streams are swollen with numerous rivers nearing or into minor flood stage currently, and many fields still have standing water after the previous rain event. This results in lower than normal rain needed for flash flooding, with 3-hr FFG less than 2" across much of the area. The National Water Model depicts a 25-50% chance of rapid onset flooding across portions of central IL.

Collectively, these considerations were enough to warrant an upgrade to a Moderate Risk (level 3 of 4) of excessive rainfall for portions of the area on Sunday, and a Flood Watch was issued from Fulton to Logan to Vermilion counties southward. Considered including another tier of counties northward (to encompass Peoria and Bloomington- Normal), but opted to keep the watch confined closer to the highest NBM/HREF probs for now. Watch expansions may be needed pending evaluation of future guidance. The Flood Watch expires at 06z Mon (1 am), by which point most, if not all, of the precip should be south/east of the forecast area.

*** CONDITIONAL SEVERE STORM THREAT SUNDAY ***

Due to seasonably high wind shear (~40 kts) associated with the system on Sunday, there remains a conditional, but low confidence, severe storm risk. Confidence is lower due to AM storms associated with the incoming MCS, and uncertainty as to what degree the atmosphere recovers during the afternoon. If these morning storms produce an outflow boundary, and sufficient airmass recovery occurs, a localized corridor of greater tornado potential could develop. Based on the slight southward trends in models, the Slight Risk (level 2 of 5) remaining south of I-72 (with a Marginal Risk to the north) seems appropriate. It's worth noting there are still outlier solutions, such as the 00z NAMNest, which track the sfc low much further north and would subsequently bring the severe risk northward. However, that particular solution seems unrealistically weak with the morning MCS. Ultimately, the severe storm outlook will remain fuzzy until we see how storms evolve Sunday morning.

*** REST OF THE WEEK ***

The upper pattern will generally be characterized by ridging over the Four Corners region and broad troughing over the upper Midwest/New England. Monday and Tuesday are expected to be mostly dry in the wake of our departing low, but NW flow aloft will guide additional waves towards the region during the latter half of the week. In response, the NBM has occasional low (20-40%) precip chances from Wed onward. Temps remain below normal, but there are signs that summer-like temps return at the end of June. The NBM probs of temps above 90 degF is essentially 0% area- wide through Sat Jun 27, but rises to 30-50% by Tues Jun 30.

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/

Issued at 1231 AM CDT Sat Jun 20 2026

Scattered -shra look to develop overnight along a weak frontal boundary near KSPI-KDEC-KCMI, and have included PROB30 for -shra from 09Z-13Z as a result. Nevertheless, VFR conditions are expected to continue with minimal chances for any MVFR cigs/vsbys in this area. Winds light and variable, becoming WNW 7-11 kts behind the frontal boundary by around 16Z. Winds decreasing to light and variable again after 00Z.

ILX WATCHES/WARNINGS/ADVISORIES

Flood Watch from Sunday morning through late Sunday night for ILZ036-040>057-061>063-066>068-071>073.