Flash Flood Warning: De Kalb, La Salle, Lee counties IL — 3-5 inches of rain

Early Thursday morning, the National Weather Service office in Chicago issued a Flash Flood Warning for southern De Kalb County, northern La Salle County, and southeastern Lee County at 7:51 AM CDT on September 17, 2026. The warning, valid through 11:00 AM CDT, followed confirmation from Doppler radar and automated rain gauge networks that between 3 and 5 inches of rain had already fallen across the warned area — with an additional inch of rainfall still possible as the storm system continued to move through north-central Illinois.

Flash flooding was already occurring across the region at the moment the warning was issued — not a forecast, but an active, ongoing event. Communities within the affected zone include Sandwich, Mendota, Amboy, Sheridan, Somonauk, Earlville, Leland, and Millington. Alongside the Flash Flood Warning, NWS Chicago also issued a companion Flood Advisory for the same three-county area, valid through 10:45 AM CDT, with 2 to 4 inches of rainfall confirmed under that broader advisory footprint. Together, the dual alerts signal a significant, rapid-onset flood event across one of north-central Illinois’s most flood-sensitive corridors.

What the National Weather Service Confirmed

The Flash Flood Warning issued by NWS Chicago at 7:51 AM CDT on September 17, 2026 was supported by a combination of Doppler radar analysis and readings from automated rain gauges across the three-county warned area. According to NWS Chicago, key confirmed facts include:

• 3 to 5 inches of rain had already fallen across southern De Kalb, northern La Salle, and southeastern Lee counties as of 7:51 AM CDT
• An additional 1 inch of rainfall remained possible at the time the warning was issued
• Flash flooding was already occurring across the warned area — confirmed by Doppler radar and automated gauges, not projected
• Rapid-onset flooding was identified in creeks, streams, drainage ditches, streets, underpasses, and low-lying areas throughout the region

The companion Flood Advisory, also issued by NWS Chicago and valid through 10:45 AM CDT, covered the same De Kalb, La Salle, and Lee county footprint and confirmed 2 to 4 inches of rainfall had accumulated under that advisory. The simultaneous issuance of both a Flash Flood Warning and a Flood Advisory underscores the scale and speed of this morning’s precipitation event.

Property owners across the affected region should monitor real-time updates directly from the National Weather Service at weather.gov as the morning progresses and conditions evolve.

What Homeowners in the Affected Towns Should Check Now

Once it is safe to move around your property — and only after any active warnings in your immediate area have expired — residents of Sandwich, Mendota, Amboy, Sheridan, Somonauk, Earlville, Leland, Millington, and surrounding communities should conduct a careful exterior and interior inspection for storm- and flood-related damage. Rapid-onset flooding of the kind confirmed by NWS Chicago this morning can affect structures in ways that are not immediately visible, so a systematic walk-through matters.

Roof and Gutters

• Gutters and downspouts: Inspect for clogs, separations, or sections pulled away from the fascia under the weight of heavy runoff. Overloaded gutters can redirect water directly into soffits and wall cavities rather than away from the foundation.
• Roof surface: From the ground using binoculars if possible, look for displaced or missing shingles, debris accumulation in roof valleys, or standing water on flat or low-slope sections. Sustained heavy rainfall at 3 to 5 inches can expose pre-existing vulnerabilities that were not yet causing problems.
• Roof penetrations: Vents, skylights, chimneys, and pipe boots are among the most common points of water entry. After checking the exterior, step into your attic and look for dripping, staining, or moisture on the underside of the roof deck.

Siding and Exterior Walls

• Siding panels: Look for warped, cracked, or separated sections — especially at exterior corners and around window and door frames, where water tends to work its way behind the cladding during wind-driven rain.
• Foundation grading: Check whether soil has washed or eroded away from the foundation perimeter. When the grade slopes toward the structure rather than away from it, water pools against the foundation rather than draining off.
• Window and door frames: Look for moisture staining, soft or spongy wood, or visible swelling, particularly on exposures that faced the storm most directly.

Trees, Landscaping, and Drainage

• Trees and large shrubs: Look for split limbs, cracked trunks, or trees showing signs of root uplift. Do not approach a leaning tree before a professional arborist has assessed it.
• Yard drainage: Standing water pooling near foundations, overflowing window wells, or saturated ground that is not draining can drive moisture into basements and crawl spaces within hours of the rain stopping.
• Storm drains and swales: If it is safe to do so, clearing debris from street-side storm drains near your property can help reduce localized ponding as runoff continues.

Interior — Water Intrusion

• Basement and crawl space: Check for standing water, seepage along wall-floor joints, or moisture spreading across the floor slab. Homeowners in Earlville, Somonauk, and Leland — communities with older housing stock and terrain that can channel runoff — should pay particular attention to below-grade spaces after an event of this magnitude.
• Attic and ceilings: Water stains, dripping, or sagging drywall are obvious signs, but ceiling materials that appear dry on the surface can hold significant absorbed moisture. Probe any soft spots gently and document what you find.
• Mechanical equipment: If your furnace, water heater, or electrical panels are located in a basement or ground-level utility room, confirm they have not been reached by standing water before restoring power to those circuits.

If You Find Damage

Discovering storm or flood damage is stressful, but the steps you take in the first 24 to 48 hours can significantly affect both your property’s condition and your ability to move through an insurance claim effectively.

Document everything before you touch it. Walk through all affected areas with your phone and photograph or video every instance of damage from multiple angles — roof surfaces, interior water intrusion, water-damaged contents, and any visible structural concerns. Date-stamped photos and video are among the most useful things you can bring to your insurance carrier. Do not discard damaged materials until your adjuster has seen them.

Take reasonable steps to limit further damage. Most homeowner insurance policies include a duty-to-mitigate provision, which means you are expected to take reasonable action to prevent additional loss from occurring. This can include placing a tarp over an exposed roof section, removing standing water with a wet-dry vacuum, or relocating water-damaged belongings to a dry area. Keep receipts for any emergency materials you purchase — those costs are often reimbursable.

Report your claim promptly. Contact your insurance carrier as soon as possible to open a claim and receive a claim number. Your adjuster will schedule an on-site inspection. You are entitled to have a licensed contractor present during that inspection to help ensure that damage is identified thoroughly and documented accurately.

Choose a licensed restoration contractor. A qualified contractor can document damage in detail, prepare a line-item repair estimate, and meet with your adjuster on-site. Allied Emergency Services is an Illinois-licensed restoration contractor — we document damage, provide comprehensive repair estimates, and perform covered restoration work. We are not a public adjuster and do not negotiate, settle, or adjust insurance claims on your behalf. If you have questions about your specific policy rights or claim settlement, consult a licensed public adjuster or an attorney. Learn more about our storm damage restoration services.

When vetting any contractor after a storm, ask to see their state license, verify active insurance, and be cautious of door-to-door solicitors who cannot provide a verifiable local track record. Allied Emergency Services has been serving the greater Midwest since 2015 — more than a decade of licensed storm restoration work across Illinois and the surrounding region.

For general emergency preparedness planning ahead of future events, ready.gov offers practical guidance on building a home emergency kit and developing a family preparedness plan.

This article is provided as general information only and does not constitute legal or insurance advice; consult your policy, your carrier, and qualified professionals for guidance specific to your situation.

Contact Allied Emergency Services — Available 24/7

If you are a property owner in Sandwich, Mendota, Amboy, Sheridan, Somonauk, Earlville, Leland, Millington, or anywhere across southern De Kalb, northern La Salle, or southeastern Lee County and you have found storm or flood damage this morning, Allied Emergency Services is ready to respond.

We are available 24 hours a day, 7 days a week — because storms do not follow business hours, and neither do we. Our team will assess the damage, document it thoroughly, and get your restoration on track as quickly as possible.

• Call us now: (800) 792-0212
• Text ESTIMATE to: (844) 907-2546

See the full range of our storm damage restoration services and learn how Allied can help you move from damage to restored — the right way, with a licensed team that has been doing this work across the Midwest for more than a decade.