Heat Persists in the South Central U.S. and California; Heavy Rain in the Ohio Valley

Sweltering heat remains across the South-Central Plains and Lower/Middle Mississippi Valley, as well as in the Southwest and California this week with limited nighttime relief. Heavy rainfall will lead to localized areas of flash flooding in parts of the Ohio Valley/Central Appalachians. Dry and breezy conditions will enhance fire weather concerns across the northern Rockies and High Plains. Read More >

Overview

Fast Facts:

A brief tornado occurred from a quickly intensifying storm over Waukegan, IL early on Tuesday evening, September 3.
The tornado was rated an EF-1 on the Enhanced Fujita scale with maximum winds of 90 mph.
Damage included a flipped car with one injury, minor damage to commercial buildings, and several trees downed.
The threat for possible severe weather on Tuesday had been noted in NWS products and graphics since Sunday. Coverage of storms on Tuesday evening was very limited, in fact between 5 and 9 p.m. that was the only storm in all of northern Illinois.

Tornadoes
Photos
Radar
Environment
Takeaways

Tornado:

Tornado - NORTH SIDE OF WAUKEGAN
LAKE COUNTY IL

Date	Tuesday, Sept 3, 2019
Time (Local)	6:39 PM - 6:43 PM CDT
EF Rating	1
Est. Peak Winds	90
Path Length	3.1 miles
Max Width	200 yards
Injuries/Deaths	1 minor injury / 0 fatalities
Summary: (Full Summary) Damage from the tornado was minor to commercial buildings and residences, including windows blown out, minor roof damage, fence destruction, and damage to a gas station canopy. There was a vehicle flipped just east of the intersection of York House Road and Lewis Avenue where the one minor injury occurred. Numerous trees felled or experienced heavy branch damage. The tornado was at its widest and strongest in the latter half of its path. The one mile stretch between the last damage point and the lake shore was unable to be surveyed due to a nature area. A waterspout was reported just off shore and so by interpolation we took the tornado path to the lakefront.

Track Map

Track Map EF-Scale

The Enhanced Fujita (EF) Scale classifies tornadoes into the following categories:

EF0 Weak 65-85 mph	EF1 Moderate 86-110 mph	EF2 Significant 111-135 mph	EF3 Severe 136-165 mph	EF4 Extreme 166-200 mph	EF5 Catastrophic 200+ mph

Photos:


Photos of the tornado as it developed near the York House Road & Lewis Avenue intersections near the Waukegan Airport. Photos courtesy of Spencer Dant.

Radar:

NWS Chicago Doppler Radar lowest scan (0.5°) of reflectivity (top) and velocity (bottom). This is one slice -- the lowest and closest to the ground -- that the radar scans. Radar is used in combination with analysis of the environment (i.e. what threats are possible today) as well as trusted reports from storm spotters. Together these make up the basis of NWS warnings.

NWS Chicago Doppler Radar lowest scan (0.5°) of reflectivity (top) near time of tornado beginning. The storm is a supercell with rotation, though the rotation is not particularly pronounced and the storm is somewhat stretched / elongated owing to the high wind shear.

Environment


Satellite water vapor imagery depicting a strong upper system for early September moving across the Great Lakes. The local area is on the far southern edge of the stronger forcing for deep lift. This lift in tandem with enough moisture and instability can assist in deeper thunderstorms. The Waukegan area storm can be seen on this satellite band but almost not until it is out over the lake and deep enough.			Hand surface analysis from 2 pm on September 3, with observations, fronts, and other features plotted.

NWS Storm Prediction Center (SPC) RAP Model mesoanalysis graphics. The upper left is deep layer shear showing >50 kt, more than sufficient for storm organization and supercells, which was the expected storm mode for the day. The upper right depicts effective storm relative helicity, with much of this located in the lowest 1 km. In the lower left is mixed-layer CAPE (instability) which was around 3,000 J/kg. Combined with the shear, this spectrum space is plenty sufficient for supercells with rapidly accelerating updrafts. However, the convective inhibition (CIN) had just eroded or quite possible was not completely eroded despite the model above indicating it had over such a large area. We suspect that was the case because storms were very isolated ahead of the cold front throughout the entire evening. Finally, and maybe one of the most important for this case of tornadogenesis, is the lower right plot of 0-3km CAPE and surface vorticity. The values in excess of 100 J/kg are very high and indicate quickly accelerating updrafts in the low-levels. This

Takeaways

The Lake County tornado reminds us that it is still thunderstorm season and severe weather can happen in September and actually can anytime of the year.
The warning process includes radar analysis, environmental analysis, and what our trained storm spotters are reporting. Despite recent advances in tornado forecasting, there are still some tornadoes that go unwarned. These are primarily the shorter-lived, quickly developing tornadoes, as opposed to the significant, long-track ones.
This quickly-developing tornado formed out of what is known as a "Low precipitation" and "Miniature Supercell." Miniature supercells are still rotating thunderstorms, but are smaller in spatial and vertical extent. This supercell also produced very little rainfall, hence the "Low Precipitation" terminology. The wind fields in these types of supercells are much more difficult for the Doppler Radar to "see" than the larger "Classic" or "High Precipitation" supercells, which makes the tornadic signatures much more difficult to detect. Only about 20-25% of supercells ever produce tornadoes.
We are in the process of researching this event and will continue to in the coming weeks in order to better understand the event evolution.
Forecasts for multiple days and leading up to the event had noted the potential for severe weather, including tornadoes, in northern Illinois and northwest Indiana. It's a good practice to check your forecast daily, especially hazardous weather potential (such as our EHWO and Weather Story graphics), and if there is a threat for severe weather to check the forecast more frequently and be vigilant when skies darken.
While tornadoes themselves are rare, basic thunderstorm safety is important and the first step in severe weather safety too. We like to say when thunder roars, go indoors!

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Heat Persists in the South Central U.S. and California; Heavy Rain in the Ohio Valley

September 3, 2019: EF-1 Tornado in Waukegan, IL