A Regional Outbreak of Severe Weather Possible Today
Afternoon and Evening will see highest threat for strong tornadoes.
A significant severe weather outbreak is unfolding across portions of the Midwest and Upper Midwest today, April 28, 2025, with the potential for strong to intense tornadoes, very large hail, and damaging wind gusts. Below is a detailed discussion of the atmospheric setup, dynamics, areas impacted, and timing based on available forecast information.
Atmospheric Setup
The severe weather threat is driven by a highly dynamic and volatile atmospheric environment characterized by the following key ingredients:
Synoptic Setup:
A deep upper-level trough over the western U.S. is ejecting eastward, interacting with a potent shortwave trough moving across the Plains and Midwest. This trough is sharpening as it progresses, enhancing large-scale ascent and creating favorable conditions for severe thunderstorm development.
A surface low-pressure system is deepening over the Upper Midwest, with an associated warm front extending eastward across Minnesota and Wisconsin and a cold front trailing southward into the Plains. A dryline is positioned across the Mid-Missouri Valley, acting as a focus for storm initiation.
Moisture and Instability:
A broad, moist warm sector is in place, with dewpoints in the mid-60s to low-70s°F across the Midwest and Plains, advected northward by a strong low-level jet (LLJ). This moisture contributes to moderate to high instability, with Mixed-Layer Convective Available Potential Energy (MLCAPE) values ranging from 2000–3500 J/kg, particularly in the warm sector across Iowa, Minnesota, and Wisconsin.
Steep mid-level lapse rates (7–8°C/km) enhance buoyancy, creating an environment conducive to explosive thunderstorm development.
Shear Profiles:
Strong deep-layer shear (40–60 knots) is present due to a robust mid-level jet associated with the approaching trough. This supports organized, long-lived thunderstorms, including supercells.
Low-level shear is particularly impressive, with 0–3 km storm-relative helicity (SRH) values of 300–400 m²/s² across the warm sector. Long, looping hodographs indicate a highly favorable environment for rotating updrafts and tornado production.
Capping and Trigger Mechanisms:
A residual cap (elevated mixed layer) is present early in the day, but increasing atmospheric forcing from the approaching trough and surface heating is expected to erode this cap by mid-afternoon, allowing for rapid storm development.
Differential heating along the warm front and convergence along the dryline and surface low will serve as focal points for convective initiation.
Dynamics Driving the Threat
Several dynamic processes are converging to create a high-end severe weather threat:
Upper-Level Forcing:
The ejecting shortwave trough and associated jet streak provide strong upper-level divergence and large-scale ascent, promoting vigorous thunderstorm development. The jet streak’s exit region enhances vertical motion over the Upper Midwest, maximizing the potential for severe storms.
Low-Level Jet and Moisture Transport:
A 40–50 knot low-level jet is transporting rich Gulf moisture northward, maintaining high dewpoints and fueling instability. This LLJ also enhances low-level shear, increasing the likelihood of tornadic supercells.
Frontal and Mesoscale Boundaries:
The warm front acts as a zone of enhanced low-level vorticity and shear, increasing the tornado threat where storms interact with this boundary. The dryline and cold front provide additional lift, with the dryline serving as a primary trigger for discrete storm initiation.
Outflow boundaries from morning convection may further focus storm development, particularly in the enhanced risk areas.
Supercell and Tornado Potential:
The combination of high instability, strong shear, and favorable hodographs creates an ideal environment for discrete supercells, which are capable of producing strong to intense (EF2–EF5) tornadoes. The greatest tornado threat exists during the late afternoon and evening when surface-based instability peaks.
By late evening, storms may organize into a squall line as the cold front advances, transitioning the primary threat to severe wind gusts (up to 75 mph) and embedded tornadoes.
Hail and Wind Threats:
Steep lapse rates and high CAPE support very large hail (>2 inches in diameter) in supercells.
Damaging winds are likely in both discrete storms and eventual mesoscale convective systems (MCS), with upscale growth expected late Monday night.
Areas Impacted
The severe weather threat spans a broad region, with the highest risk concentrated in the Upper Midwest. Specific areas and risk levels include:
Moderate Risk (Level 4/5):
Southeastern Minnesota, Northeastern Iowa, Southwestern Wisconsin: This region, including cities like St. Paul, Des Moines, and Cedar Rapids, faces the greatest threat for strong to intense tornadoes, very large hail, and damaging winds. The population at risk is approximately 4.87 million.
Key infrastructure, including Interstates 35, 80, and 90, and airports like Minneapolis–Saint Paul (MSP) and Des Moines (DSM), may face significant disruptions.
Enhanced Risk (Level 3/5):
A broader area encompassing parts of Kansas, Missouri, Nebraska, Iowa, Minnesota, and Wisconsin, including cities like Kansas City, Omaha, Minneapolis, Madison, and Overland Park. This zone affects an estimated 11.54 million people and includes critical transportation corridors (Interstates 29, 35, 70). All severe hazards are possible, with a focus on large hail and damaging winds.
Slight Risk (Level 2/5):
Extends from the Southern Plains (eastern Kansas, Oklahoma, northern Texas) to the Great Lakes (Illinois, Michigan). Isolated severe storms are possible, with risks of hail, wind, and a few tornadoes.
Marginal Risk (Level 1/5):
Surrounds the primary risk areas, including parts of the Dakotas, Montana, and the Mid-South. Isolated hail and wind threats are possible, particularly in southeast Montana and western South Dakota.
Timing
The severe weather threat will evolve throughout the day and night, with the following timeline:
Morning (6 AM–12 PM CDT):
Elevated thunderstorms are ongoing at 12Z (7 AM CDT) from North Dakota across the Upper Mississippi Valley, producing isolated large hail. These storms will move east-northeast, with limited surface-based severe potential due to the cap.
The warm front begins to strengthen, and differential heating sets the stage for later storm development.
Early Afternoon (12 PM–3 PM CDT):
Additional elevated storms redevelop northeast of the surface low, slowing the warm front’s northward progression but enhancing instability in the warm sector. Surface-based storm initiation is expected along the dryline and near the surface low in the Mid-Missouri Valley and southwest/west-central Minnesota by mid-afternoon.
The cap begins to erode as surface temperatures climb into the 70s°F and forcing increases.
Late Afternoon to Evening (3 PM–9 PM CDT):
This is the most dangerous period, with discrete supercells forming in the warm sector across the moderate risk area. These storms are capable of producing strong to intense tornadoes, very large hail, and damaging winds. The tornado threat peaks during this window due to maximum surface-based instability and optimal shear.
Storms may begin to cluster in some areas, increasing the wind threat.
Late Evening to Overnight (9 PM CDT–3 AM CDT, April 29):
Storms are expected to organize into a squall line as the cold front advances southeastward, overtaking discrete convection. Severe wind gusts (up to 75 mph) and embedded tornadoes remain possible, particularly in the enhanced risk area.
The severe threat gradually shifts eastward into Wisconsin, northern Illinois, and parts of the Great Lakes, with a diminishing but still notable risk.
Additional Notes
Uncertainty: While the environment is highly favorable, the extent of storm coverage depends on sufficient convective initiation. Some forecasts note uncertainty about whether storms will remain isolated or become widespread, which could modulate the outbreak’s severity.
Flash Flooding: In addition to the severe weather threat, heavy rainfall (1–2 inches, locally higher) may cause flash flooding, particularly in the Southern Plains and Missouri Valley, where a Moderate Risk of Excessive Rainfall is in effect for April 29.
Safety Recommendations: Residents in the affected areas should have multiple ways to receive warnings (e.g., NOAA weather radio, smartphone alerts), know their sheltering plan, and be prepared to take immediate action, especially in the moderate risk area.