PHOENIX, ARIZONA — A Moderate Risk is in effect in the National Weather Service’s Day 1 Excessive Rainfall Outlook, issued by the NWS Weather Prediction Center, covering two separate zones across the Desert Southwest today. The official outlook, valid 16Z Wednesday, September 16, 2026, through 12Z Thursday, September 17, 2026, and issued at 1556Z by forecaster Campbell, also outlines a much broader Slight and Marginal Risk corridor stretching from the Southwest through the Central Plains and into a second, distinct zone covering Iowa, Illinois, Indiana and Ohio.
NWS Issues Day 1 Excessive Rainfall Outlook With Moderate Risk Across the Southwest
The Weather Prediction Center’s Day 1 Excessive Rainfall Outlook represents the National Weather Service’s official national assessment of where rainfall is most likely to exceed local flash flood guidance over the next roughly 20-hour period. Today’s outlook places the highest category reached anywhere on the map, a Moderate Risk, over two separate zones within the Desert Southwest, reflecting forecasters’ high confidence that rainfall rates in those specific areas will be intense enough to overwhelm drainage and trigger flooding.
Dual Moderate Risk Zones Target Arizona and New Mexico Through Thursday Morning
The two Moderate Risk zones on today’s map form distinct red-shaded ovals, one centered over southeastern Arizona and a second smaller zone over southern New Mexico near the Texas border. Both zones remain valid through the full outlook period ending 12Z Thursday morning, meaning the threat for numerous flash floods in these areas persists not just through today’s daytime hours but into the overnight period as well, as repeated rounds of heavy rain continue targeting the same terrain.
Broad Slight and Marginal Risk Corridor Stretches From the Southwest Into the Central Plains
Surrounding both Moderate Risk ovals, a wide Slight Risk zone, shaded in yellow, covers much of Arizona, New Mexico and west Texas, before an even broader Marginal Risk, shaded green, extends the footprint of today’s outlook considerably farther. That Marginal Risk band stretches northeast out of the Southwest, arcing up through the Central Plains, including parts of Colorado, Kansas and Nebraska, before curving east across Iowa and continuing toward the Ohio Valley, connecting the Southwest’s flash flood threat to a second corridor of concern farther northeast.
Second Slight Risk Zone Emerges Across Iowa, Illinois, Indiana and Ohio
Embedded within that broader Marginal Risk corridor, a second, distinct Slight Risk zone appears across the Upper Midwest and Ohio Valley, forming its own yellow-shaded pocket stretching from Iowa through Illinois and Indiana and into Ohio. This second Slight Risk area is separate from the Southwest’s dual Moderate Risk zones but represents a real, elevated flash flooding concern in its own right, with forecasters flagging at least a 15 percent chance of rainfall exceeding flash flood guidance within 25 miles of any point inside that corridor today into tonight.
How the WPC’s Four-Tier Excessive Rainfall Risk Scale Works
Today’s national outlook uses the Weather Prediction Center’s standard four-category scale for measuring flash flood potential, ranging from Marginal, defined as 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 anywhere on today’s national map, meaning the Moderate Risk zones over Arizona and New Mexico represent the most significant flash flooding concern in the country for today’s outlook period.
Southwest Monsoon Moisture and a Midwest Storm System Drive Today’s Two Separate Threats
The two highlighted corridors on today’s map stem from two different atmospheric setups working simultaneously across the country. Across the Southwest, deep tropical moisture streaming north out of the Gulf of California continues fueling the dual Moderate Risk zones over Arizona and New Mexico, while a separate storm system tracking across the Midwest is responsible for the second Slight Risk corridor stretching from Iowa through Illinois, Indiana and Ohio. Forecasters note that while these two threats are geographically distinct, both are contributing to what has become a nationally significant flash flooding day.
What Residents in Both Corridors Should Expect Today Into Thursday Morning
With the outlook valid through 12Z Thursday morning, residents across both highlighted regions should expect the flash flooding threat to persist well into the overnight hours rather than easing after sunset. In the Southwest, communities across Arizona and New Mexico face the greatest risk given their Moderate Risk designation, while across the Midwest, residents from Iowa through Illinois, Indiana and Ohio should stay alert for locally heavy downpours capable of producing rapid, dangerous runoff. Forecasters continue to urge anyone in either corridor to avoid driving through flooded roadways as this two-region excessive rainfall event unfolds through the day and into Thursday morning.
