TL;DR

The NOAA Storm Prediction Center (SPC) has issued a Marginal Risk for severe thunderstorms across parts of Utah, Idaho, and southwest Montana, as well as the central High Plains from northeast Colorado into western Nebraska. Isolated severe hail and damaging wind gusts are possible this afternoon and evening. The greatest uncertainty is whether enough instability develops across the Intermountain West given morning cloud cover.

Where is the highest risk today?

The Marginal Risk areas span two distinct regions separated by more than 500 miles:

Intermountain West: Southeast Idaho, northern Utah, and southwest Montana face the potential for isolated severe hail and wind gusts this afternoon. A closed mid-level low over northern California will drift eastward today, bringing a 40-50 knot southwesterly jet aloft and increasing large-scale lift across the northern Great Basin.

Central High Plains: Northeast Colorado into western Nebraska will see scattered thunderstorm development by late afternoon as a mid-level vorticity maximum moves northeastward from New Mexico. Isolated severe wind gusts are the primary concern as any storms organize into small clusters through the evening.

Outside these Marginal Risk areas, the SPC outlook notes that ongoing convection across the Midwest and Ohio Valley will continue moving east-southeastward through the morning, though very poor mid-level lapse rates and extensive cloud cover will likely prevent widespread severe weather.

What happened overnight?

The past 24 hours brought 13 wind reports and 4 hail reports across the affected regions, with no tornadoes reported.

Ohio saw the bulk of the activity, with multiple reports of downed trees and structural damage in Darke, Miami, and Richland counties. In Versailles, between five and 10 trees came down in a neighborhood north of Swamp Creek around 2046 UTC. A hardwood tree snapped at its base and fell onto a home's awning, causing minor damage. In Piqua, large hardwood trees were downed near the intersection of Broadway Street and High Street. West of West Alexandria, an older barn lost around 500 square feet of sheet metal roofing.

Missouri recorded three reports near the Lake of the Ozarks, including quarter to half-dollar size hail 2 miles northeast of Linn Creek at 2204 UTC, accompanied by downed tree limbs approximately 2 feet in diameter. A trained spotter reported a condo garage roof removed near Osage Beach at 2220 UTC.

Utah contributed three hail reports, all quarter-sized, in Huntington, East Carbon, and Sunnyside between 2035 and 2204 UTC.

Single wind reports came from Arizona (downed power lines in East Mesa) and Oklahoma (snapped power pole and broken tree limb southwest of Broken Arrow).

When will storms develop today?

Intermountain West timing: Thunderstorm development is expected to increase through this afternoon across southeast Idaho and northern Utah. The SPC notes that the primary uncertainty is how much instability will develop given the prevalence of clouds and ongoing precipitation this morning. If sufficient diurnal heating occurs alongside upper 40s to low 50s surface dewpoints already in place, some guidance suggests 1,000-1,250 J/kg of mixed-layer CAPE could develop by afternoon. Deep-layer shear will be strong enough to support supercells if this instability materializes.

Central High Plains timing: Convective development is expected across northeast Colorado into western Nebraska by late afternoon. Low-level moisture and instability should gradually increase through the day, though mid-level lapse rates will remain modest, likely limiting updraft intensity. Any small clusters that form will spread east-northeastward across the central High Plains through the evening.

Midwest and Ohio Valley: Ongoing elevated convection will move east-southeastward through the rest of the morning in a weakly unstable environment. The SPC notes that various 00Z and 12Z soundings show very poor lapse rates aloft (around 4.7-5.0°C/km in the 700-500 mb layer), which will hinder robust updraft intensities. Extensive cloudiness will slow diurnal heating and dampen low-level lapse rate steepening. While strong gusts may occur with convection, the overall risk for severe or damaging winds appears too limited for higher probabilities.

What does Marginal Risk mean?

Marginal Risk is the lowest category on the SPC's five-tier severe weather scale. It indicates isolated severe storms are possible, but not everyone in the risk area will see severe weather. Storms that do become severe may produce hail one inch in diameter or larger, wind gusts of 58 mph or greater, or in rare cases, a brief tornado.

On Marginal Risk days, the primary challenge is often uncertainty—whether storms will develop at all, and if they do, whether they'll reach severe thresholds. Today's setup exemplifies this: morning cloud cover across the Intermountain West may prevent sufficient heating to generate robust instability, while poor mid-level lapse rates across the Midwest will cap updraft intensity.

Looking ahead to Friday and the weekend

The SPC Day 2 outlook (valid Friday, September 18) maintains a Marginal Risk across parts of Kansas, Nebraska, Missouri, Iowa, and Colorado, as well as portions of Georgia and Ohio. Day 3 (valid Saturday, September 19) shows another Marginal Risk spanning Oklahoma, Texas, Kansas, Missouri, and portions of the Midwest, Southeast, and Great Lakes.

The pattern suggests continued scattered severe weather potential across the central United States through the weekend, though no higher-end threats are currently forecast.

What you should do

If you're in the Marginal Risk areas today:

  • Monitor radar through the afternoon and evening, especially if you're in southeast Idaho, northern Utah, northeast Colorado, or western Nebraska.
  • Have a way to receive warnings. Wireless Emergency Alerts for severe thunderstorm and tornado warnings will sound on your phone even if it's on silent mode. Third-party weather apps are silenced by Do Not Disturb, though Wireless Emergency Alerts for tornado warnings still break through on most phones.
  • Move indoors if you see lightning or hear thunder. All thunderstorms produce lightning, which kills an average of 20 people in the United States each year.
  • Secure loose outdoor items that could become projectiles in strong wind gusts.

For residents of the Midwest and Ohio Valley, ongoing storms this morning may produce gusty winds and brief heavy rain, but the severe weather threat remains low given the atmospheric setup.

VORTEX is a free web app at vortexintel.app that monitors severe weather nationwide. Pro ($4.99/month) places phone calls to your phone when a tornado or flash flood warning is issued for a location you care about — calls can ring through Do Not Disturb once you enable Emergency Bypass for the number, unlike most third-party app notifications.