Black Sunday

       April 30, 1967

 WEATHER PATTERN:

A dynamic and powerful combination of weather features converged on Minnesota and Iowa during the afternoon and evening hours of April 30, 1967, producing the severe weather and tornadoes across the area.  The storm system wreaked havoc across a large section of the country over a 2 to 3 day period, including blizzard conditions in the Northern Plains and Northern Rockies and severe thunderstorms and tornadoes in Texas, Oklahoma, Kansas, and the Mississippi and Ohio Valley regions.

The Morning of April 30, 1967
  

850 mb Map 700 mb Map 500 mb Map 250 mb Map

850 mb Map

700 mb Map

500 mb Map

250 mb Map

The main branch of the upper level jet stream stretched from the southwestern into west Texas, snaking northeastward into the Central Plains and eastward through the Ohio Valley.  A 135 knot speed maximum in the jet stream had nosed to near the eastern New Mexico and western Texas Panhandle border.  In severe thunderstorm forecasting, the orientation and speed of the jet stream is important in producing large deep layer wind shear values, which maintains and organizes thunderstorm updrafts.

In the mid levels, a closed low pressure area, oriented from northwest to southeast, was positioned from eastern Idaho, across Wyoming, and into north central Colorado.  The diverging winds to the east of the low in the Plains states resulted in a large area of synoptic scale lift, which is a key ingredient in forecasting active weather during all seasons, including thunderstorms, flash floods, and winter storms.

Just above the surface, a southwest to northeast oriented 50 knot speed maximum, known as the low level jet, stretched from central Oklahoma and into northeastern Missouri.  The low level jet was responsible for transporting a large plume of moisture from the Gulf of Mexico into the Upper Midwest.  A large supply of moisture is another critical element in thunderstorm forecasting, as it is necessary to form clouds and also increases the amount of environmental instability, which is necessary for air to accelerate upward to form and sustain a thunderstorm updraft.  The low level jet also accentuates both the deep layer wind shear and the low level wind shear, the latter of which is critical to tornadic thunderstorm organization.

At the surface, a stationary front, associated with a surface low pressure center near the central South Dakota and Nebraska border, stretched through the southern half of Minnesota, just south of the Twin Cities.  Warm and moist air was found to the south of the stationary front, across southern Minnesota and points south.

Midday April 30, 1967

1 pm Surface Weather Map

4 pm Surface Weather Map

1 pm Surface Weather Map

4 pm Surface Weather Map

Maps of the weather conditions above the ground are not available for the middle of the day, as weather balloons were only released twice a day, during the morning and evening.  The surface conditions at this time, however, are available.

Around 1 pm CDT, the surface low pressure center had moved slightly northward to a position just southwest of Pierre, South Dakota.  Several fronts stretched from the low pressure area, including the stationary front, located from north of Sioux Falls, South Dakota to near LaCrosse, Wisconsin.  A warm front also extended from south of Sioux Falls to near Des Moines, Iowa and St. Louis, Missouri.  The air between the two fronts had warmed into the 60s and low 70s, and dew points in the 60s were prevalent.  Winds across far northern Iowa and southern Minnesota were quite strong, with speeds in the 15 to 25 mph range, and from the east-southeast.

By 4 pm CDT, the only element of the surface pattern that had changed substantially was the location of the warm front.  The front had bulged to just south of the Minnesota and Iowa border, extending from Worthington to near Mason City, Iowa.  The stationary front remained in its previous location.

The positions of both the warm front and the stationary front are noted, as the tornadoes developed along and just north of the warm front as it moved northward through northern Iowa and into southern Minnesota, and the stationary front marked the northward extent of the tornadic weather.  Between the two fronts, temperatures were in the low to mid 70s, and dew points remained in the mid and upper 60s.  Surface wind speeds and directions remained comparable to their 1 pm levels.

The Evening of April 30, 1967  

850 mb Map 700 mb Map 500 mb Map 250 mb Map

850 mb Map

700 mb Map

500 mb Map

250 mb Map

                       

6 pm Surface Weather Map 7 pm Weather Balloon Data

6 pm Surface Weather Map

7 pm Weather Balloon Data

 

The weather features above the ground, located relatively far away from Minnesota 12 hours previously, converged on the Upper Midwest during the evening hours.

The upper level jet stream remained in its previous orientation, but the speed maximum had nosed into north central Iowa.  This change increased the deep layer wind shear across southern Minnesota and northern Iowa, increasing the likelihood of supercell thunderstorms in these areas.

The closed mid level low pressure area had ejected into the Northern Plains, placing itself along the South Dakota and Nebraska border, due south of Pierre.  Since the low had maintained a northwest to southeast orientation, divergent winds continued ahead of it across southern Minnesota and Iowa, providing an intense source of lift for thunderstorms in this region.

The low level jet had weakened slightly overall, typical afternoon and early evening behavior for this feature, but remained strong with 40 to 50 knots from Little Rock, Arkansas to Saint Cloud.  The associated plume of Gulf moisture also accepted this orientation, as did the tongue of warm temperatures.  The strongest thunderstorms developed on the western edge of the speed, temperature, and moisture maximum, something that is not uncommon during severe thunderstorm outbreaks.

The data from the weather balloon launched from St. Cloud at 6 pm, modified near the surface using the weather conditions observed at the same time in southern Minnesota indicate a very unstable environment containing ample low level wind shear, conducive to violent tornado development.

Note:  All weather maps, except the sounding profile, are courtesy of Jonathan Finch, Forecaster at the National Weather Service Forecast Office in Dodge City, Kansas.  The sounding profile is courtesy of Dan Miller, Science and Operations Officer at the National Weather Service Forecast Office in Duluth, Minnesota.


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