ST. LOUIS, MISSOURI — A Moderate Risk is in effect in the NWS Weather Prediction Center’s Day 1 Excessive Rainfall Outlook, valid 01Z through 12Z Sunday, September 20, 2026, issued at 0100Z by forecaster Pereira. The outlook traces a long, diagonal corridor of flash flooding concern running from South Dakota and Nebraska through Missouri, Illinois, Kentucky and Ohio, with a separate Marginal Risk zone stretching south through Texas and another reaching up into Virginia and North Carolina.
Missouri and Illinois Sit at the Heart of Tonight’s Moderate Risk
If you’re in St. Louis or elsewhere across eastern Missouri and western Illinois, you’re positioned right inside tonight’s highest-category Moderate Risk zone, carrying at least a 40 percent chance of rainfall exceeding local flash flood guidance within 25 miles of any point. This represents the most significant flooding concern anywhere on the national map tonight, meaning residents throughout this corridor should treat the overnight hours as an active weather situation rather than a routine rain event.
South Dakota and Nebraska Face Slight Risk to the Northwest
To the northwest, a separate Slight Risk zone covers portions of South Dakota and Nebraska, carrying at least a 15 percent chance of rainfall exceeding flash flood guidance. If you’re anywhere across this stretch of the northern Plains, expect a real chance of locally heavy downpours overnight, even though the most intense threat stays concentrated farther southeast toward the Missouri-Illinois core.
Kentucky and Southern Indiana Also Fall Within the Moderate Zone
The Moderate Risk shading extends east from Missouri into Kentucky and southern Indiana, meaning communities including Louisville and surrounding areas fall within the same highest-confidence flash flood zone tonight. Forecasters expect the same repeated rounds of heavy rain responsible for the Missouri threat to continue tracking along this same corridor overnight, keeping the elevated danger in place well into the early morning hours.
Ohio and West Virginia See a Second Slight Risk Pocket to the East
Farther east, a second Slight Risk zone appears over parts of Ohio and West Virginia, including areas near Columbus and Charleston. If you’re in this part of the Ohio Valley, you’re not sitting in tonight’s absolute highest-risk zone the way Missouri and Kentucky are, but the same broader moisture plume responsible for the western threat extends into your region, bringing a real chance of locally heavy rain overnight.
Marginal Risk Extends From Texas Through Oklahoma
A separate, disconnected zone of Marginal Risk, carrying at least a 5 percent chance of isolated flash flooding, covers a broad stretch of Texas, including areas around Dallas and Austin, extending north into Oklahoma. If you’re anywhere across this southern zone, tonight’s flooding concern is considerably lower than what’s forecast for the Missouri-to-Ohio corridor, but isolated heavy downpours remain possible, particularly where storms happen to stall or repeatedly redevelop over the same areas.
Marginal Risk Also Stretches Into Virginia and North Carolina
On the eastern end of the map, another Marginal Risk band stretches from the Ohio Valley through the Appalachians and into Virginia and North Carolina, reaching communities like Roanoke and Charlotte. Residents across this stretch should stay weather aware overnight, even though this zone represents the lowest confidence category on tonight’s outlook compared to the Moderate and Slight Risk areas farther west.
How the WPC’s Four-Tier Excessive Rainfall Risk Scale Works
Tonight’s national outlook uses the Weather Prediction Center’s standard four-category scale, ranging from Marginal, at least a 5 percent chance, and Slight, at least 15 percent, up through Moderate, at least 40 percent, and, at the top, High, at least 70 percent. No areas were placed in the High Risk category tonight, meaning the Missouri, Illinois and Kentucky corridor represents the most significant flash flooding concern anywhere in the country through 12Z Sunday morning.
A Long, Connected Corridor Points to a Single Organized Storm System
The elongated, diagonal shape of tonight’s outlook, stretching continuously from South Dakota through Missouri, Kentucky and Ohio, points to a single organized storm system tracking across the country overnight rather than several unrelated pockets of rain. That kind of connected corridor typically means the same general area of low pressure and associated moisture will continue producing repeated rounds of rain along a fairly predictable path through the night.
What Residents Across the Corridor Should Do Tonight
If you’re anywhere within tonight’s Moderate Risk zone, from St. Louis through Kentucky, avoid driving through any flooded roadway and have a way to receive emergency alerts handy through the overnight hours. If you’re in the surrounding Slight or Marginal Risk areas, including South Dakota, Ohio, Texas or North Carolina, it’s still worth checking updated radar before heading out, since locally heavy downpours remain possible anywhere along this active overnight corridor.
