The Plains Tornado Outbreak of April 26, 1991

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Weather Map Discussion

The weather conditions that occurred across the Plains states on April 26, 1991, were largely typical of Plains tornado outbreaks. Important features included a dryline, a strong low level jet, a fast upper level jet stream, and a negatively tilted upper level trough.

The Weather Conditions on:
The Morning of April 26, 1991	The Afternoon of April 26, 1991	The Evening of April 26, 1991

The Morning of April 26, 1991

The basic upper level flow pattern indicated a large trough in the western half of the United States with ridging in the eastern half. The western trough had a slight negative tilt to it during the early morning hours. A negatively tilted trough slants to the west with increasing latitude and is often associated with large severe weather outbreaks and winter storms.

Both the polar and subtropical jet streams were seen over the United States that morning. The polar jet was associated with the large scale trough, extending from the Oregon Coast to the desert southwest northeastward into the Central Plains. Polar jet streaks were evident in north central portions of Nevada and in northeast Kansas, with the former measured at 120 knots. A mid level shortwave on the eastern periphery of the large scale trough was associated with the northeast Kansas jet streak. The subtropical jet was aligned from southwest to northeast, stretching from Mexico along the Gulf Coast states. The polar jet was more important, compared to the subtropical jet, to the development of thunderstorms and severe weather in the Plains during the afternoon and evening hours.


300 mb - 7 am CDT	500 mb - 7 am CDT

700 mb - 7 am CDT	850 mb - 7 am CDT

A modest low level jet stretched from eastern Texas to eastern South Dakota, transporting moist air northward. Strong low level warm air advection was present across Oklahoma and north Texas, as well as parts of Nebraska, Iowa, South Dakota, and southern Minnesota.

At the surface, low pressure was evident in southwestern Nebraska, with a dryline trailing southward from the low through western Kansas, the eastern Oklahoma and Texas panhandles, southwestward toward the Permian Basin region of Texas. A warm front extended southeastward through south central Nebraska, eastern Kansas, northeast Oklahoma, and northern Arkansas. Widespread 60 degree dew points were spread across the warm sector, with 70 degree dew points nosing into south central Oklahoma. An outflow boundary from convection early that morning stretched across far north central Oklahoma, but it would become diffuse through the day, not impacting the afternoon thunderstorms.

Data from the morning weather balloon in Norman, Oklahoma, revealed the potential for a very unstable atmosphere, with only a weak inversion keeping convection from developing. Lapse rates above the low level inversion were very steep, contributing to greater than 2500 J/kg of most unstable CAPE. Steep elevated lapse rates were also evident in Topeka, Kansas, but a deeper inversion and dry low and mid levels kept CAPE values lower than locations to the south. Topeka was north of the warm front, resulting in east-southeast surface winds and a corresponding 50 knots of 0-1 km wind shear. Low level wind shear measurements were unavailable for Norman due to weather balloon tracking limitations.


Norman, OK, Weather Balloon Data - 7 am CDT	Topeka, KS, Weather Balloon Data - 7 am CDT

The Afternoon of April 26, 1991

The special midday weather balloon data in Topeka further revealed the ominous conditions in place across Kansas. Steep elevated lapse rates contributed to greater than 1000 J/kg of most unstable CAPE. A shallow surface inversion existed, although a fairly strong cap for a surface based and mixed layer parcel was more the result of dry low level air that remained in the area. The wind field was very strong throughout the troposphere, with almost 40 kts of 0-1 km wind shear despite the shift to a southerly direction in the surface wind.

Topeka, KS, Weather Balloon Data - 1 pm CDT

The Evening of April 26, 1991

The overall upper level flow pattern had changed little since the morning hours, with troughing in the western United States and ridging in the east. Ridging was beginning to move onto the Pacific Coast due to the slight eastward movement of the larger scale trough. The trough had strengthened in the previous 12 hours, becoming more negatively tilted and developing a closed circulation in the mid levels over northeast Wyoming and southeast Montana.

An important jet streak within the polar jet stream had moved from Nevada around the base of the larger trough and into the Oklahoma and Texas panhandles and southwest Kansas. Interestingly, this placed northern Oklahoma and southern Kansas in the right exit region of the upper jet. Typically, this is associated with sinking motion from aloft and therefore, is not associated with thunderstorm and severe weather development. The mid level reflection of the upper level jet was very apparent at 500 mb, with a large swath of 70 kt winds measured from New Mexico to Kansas.

A low level jet remained across eastern Oklahoma and Kansas. Westerly low level winds and a pronounced moisture gradient were evident behind the dryline in the Oklahoma and Texas panhandles, western Oklahoma, western Kansas, and into Nebraska.


300 mb - 7 pm CDT	500 mb - 7 pm CDT

700 mb and 850 mb - 7 pm CDT

Surface low pressure had moved into south central South Dakota and north central Nebraska. The attendant cold front had caught up with the dryline across central Nebraska and into north central Kansas. From that intersection, the dryline extended southward through south central Kansas and near the Interstate 35 corridor in Oklahoma. The cold front stretched southwestward into northwest Oklahoma. A warm front was located in Iowa. Ahead of the dryline, dew point temperatures in the eastern halves of Oklahoma and Kansas had further increased, with 60s and 70s across most of the area.

The evening weather balloon launches at both Norman and Topeka revealed an environment very conducive to the tornadic weather that was already occurring. The Norman profile, in particular, exhibited the look of a prototypical tornadic sounding. Very unstable conditions were evident, with almost 5000 J/kg of most unstable CAPE in Norman and 2000 J/kg in Topeka. Little to no capping inversion existed at either location. The low level CAPE at both locations was also relatively high, with greater than 200 J/kg and about 80 J/kg through the surface to 3 km layer in Norman and Topeka, respectively. Wind shear through the surface to 1 km layer was also more than sufficient for tornado development, with 20 kts in Norman and 40 kts in Topeka.


Norman, OK, Weather Balloon Data - 7 pm CDT	Topeka, KS, Weather Balloon Data - 7 pm CDT

Note: Much of the meteorological information and all of the maps were provided by Bradley S. Churchill and Steven F. Piltz in their November 1991 paper The Northern Oklahoma Tornado Outbreak of April 26, 1991: An Examination of Vertical Wind Profiler, NEXRAD, and Storm-Relative Helicity Data. The weather balloon sounding data were provided by the Storm Prediction Center.

National Weather Service Weather Forecast Office