The Southern Minnesota Tornadoes of
March 29th, 1998
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Unusually warm and moist surface conditions combined with a very dynamic upper level storm system to produce severe weather and tornadoes over the Upper Midwest on March 29, 1998. The same storm system was also responsible for wintry weather over a portion of the Northern Rockies and the Northern Plains.
The Morning of March 29, 1998
|850 mb Map at 6 am CST||700 mb Map at 6 am CST||500 mb Map at 6 am CST|
|250 mb Map at 6 am CST||Surface Map at 6 am CST||Surface Map at 9 am CST|
|KMPX RAOB at 6 am CST|
The basic upper level weather pattern was characterized by an amplified long wave trough in the western third of the Continental United States and weak ridging east of the Rockies. The trough was positively tilted, meaning that the trough slanted to the east with increasing latitude, a condition not typically associated with major tornado or severe weather outbreaks.
Two branches of the upper level jet stream were apparent, with the main branch stretching from west to east across the far southern United States. The speed maximum of 140 knots was concentrated in Arizona, New Mexico, and west Texas. The secondary branch curled northeastward on the east side of the upper level trough, with a 100 knot speed maximum positioned from north central Kansas into eastern Iowa. 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. The jet stream can also provide a lifting mechanism for thunderstorm development. The secondary branch of the upper jet stream was most important to the thunderstorms over southern Minnesota.
The presence of the upper level trough was also felt in the mid levels of the atmosphere. The southwest to northeast orientation of the mid level speed maximum on the east side of the mid level trough, measured at approximately 100 knots, was feeding warmer air into the Upper Midwest. Warm air advection is another source of synoptic scale lift that can initiate thunderstorm development. The mid level speed maximum also greatly enhanced the deep layer wind shear over the region.
Just above the surface, a southwest to northeast oriented 40 to 50 knot speed maximum, known as the low level jet, stretched from the Texas Panhandle, across central Kansas, and into western Iowa. 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.
A surface low was centered over northeastern Wyoming, with the attendant warm front stretching across Nebraska and into southeastern Iowa. Elevated thunderstorms, some of them severe, were ongoing during the morning hours to the north of the warm front in response to the low level jet and the mid level warm air advection. Temperatures in the 40s and easterly winds at 10 to 15 knots were present north of the warm front, while upper 50s and 60s and south to southeast winds at 10 to 20 knots were located south of the boundary. An inverted surface trough extended from northeastern Nebraska northward to near the Minnesota border with the Dakotas.
By 9:00 am CST, the surface low had moved to near the Wyoming and southwestern Nebraska border, and the warm front had lifted into northeastern Nebraska and northern Iowa. Temperatures in the warm sector, south of the front, had warmed into the upper 60s with continued south to southeast winds at 15 to 20 knots. Dewpoint temperatures in the lower 60s had crept into far southern Iowa. North of the warm front, temperatures had changed little from 6:00 am, remaining in the 40s and lower 50s, with easterly winds at 5 to 10 knots. Rain continued from the early morning elevated thunderstorms over parts of east central Minnesota and west central Wisconsin.
Data from the morning weather balloon released from the Twin Cities forecast office indicated a relative lack of environmental instability but considerable wind shear. The observed low level wind shear of 18 knots is more than supportive of tornadoes, while the 50 knots of deep layer wind shear is conducive to the development of supercells.
Midday March 29, 1998
Maps of the weather conditions above the ground are not available for the middle of the day, as weather balloons are only regularly released twice a day, during the morning and evening. A special weather balloon was released at the Twin Cities office around noon on the 29 th, however, these data, alone, are not enough to make an accurate diagnosis of the exact upper level conditions over the Upper Midwest around midday. The surface conditions at this time are available.
At noon CST, the surface low pressure center had moved eastward into north central Nebraska. The warm front had lifted a good distance northward from its morning position, now stretching from the surface low to just north of the Interstate 90 corridor over southern Minnesota and eastward toward Madison and Milwaukee. A dryline extended southward from the surface low, through central Nebraska and into central Kansas. To the north of the warm front, temperatures and dew points were in the upper 40s and lower 50s, with east to northeast winds at 10 to 15 knots. Temperatures had warmed well into the 70s to the south of the front, and southerly winds at 10 to 20 knots had transported upper 50s and lower 60s dew points to locales south of the warm front and east of the dryline.
The Twin Cities weather balloon data indicated an increasingly unstable atmosphere over southern and central Minnesota, with strengthening low and deep layer wind shear, measured at 37 and 76 knots, respectively. These data solidified the growing concern that supercells containing tornadoes remained possible across the area.
By 3:00 pm CST, a mere half hour before the first tornado touchdown, the surface low had moved slightly eastward into northeastern Nebraska, with the warm front extending northeastward to near Sioux Falls and Mankato. The tornadoes developed just north of the warm frontal boundary. An outflow boundary, a boundary between rain cooled air from prior thunderstorms and the ambient warm and moist air, was evident over south central Minnesota along the warm front. The synoptic warm front continued from near Rochester, through southeast Minnesota and across southern Wisconsin. The dryline had moved eastward with the surface low, now located in eastern Nebraska. Temperatures remained in the 50s to the north of the front, under north-northeasterly winds at 5 to 15 knots, while upper 70s prevailed in the warm sector with southerly winds at 15 to 25 knots. The wind observation at Worthington, located just south of the warm front and just south of tornadic supercell should be noted, as the wind direction was more easterly than at other locations in the warm sector. This is perhaps due to the proximity of the aforementioned outflow boundary. Nevertheless, the more easterly direction served to enhance the low level wind shear in this location and perhaps, increased the tornadic potential along that region of the warm front. Another notable feature in the surface observations is the presence of haze along and just north of the warm front, something that helped obscure the tornadoes, as many witnesses attested.
The Evening of March 29, 1998
|850 mb Map at 6 pm CST||700 mb Map at 6 pm CST||500 mb Map at 6 pm CST|
|250 mb Map at 6 pm CST||Surface Map at 6 pm CST||KMPX RAOB at 6 pm CST|
The strength and positions of the above ground weather features did not change considerably from the previous observations, taken 12 hours prior, but there was enough alteration to result in the significant severe weather seen during the afternoon and early evening hours.
The basic weather pattern remained the same, featuring the amplified positively tilted upper level trough in the west, with weak ridging in the east. The trough had moved slightly eastward during the day. The upper level jet stream remained split with the main branch stretching from northern Mexico across the southeastern United States. The secondary branch, now with a maximum speed of 110 knots, extended along the east side of the trough, with its nose located over western Iowa and southwestern Minnesota. This orientation placed southern and central Minnesota in the left exit region of the jet stream, which is an area favored for enhanced synoptic scale lift and subsequent thunderstorm development. Mid level winds also remained quite strong on the east side of the mid level trough, whose center was now located over Utah. Eighty knots were measured at about 18,000 feet above the ground at Omaha, near the nose of the mid level speed maximum.
The low level jet had weakened slightly overall, typical afternoon and early evening behavior for this feature, but remained at 30 to 35 knots from the southwest. One significant change from the observations 12 hours ago was the strength of the above surface fronts. The mid level warm front had lifted well north of the affected area, into northern Minnesota, but the mid level cold front had tightened considerably over eastern North Dakota and central Nebraska, a process termed frontogenesis. Frontogenesis is a mesoscale lifting mechanism and likely assisted in the development of the parent supercell thunderstorm over eastern South Dakota.
The surface low had moved into southwestern Minnesota, with the warm front extending eastward through the Twin Cities and across central Wisconsin. The outflow boundary had spread into the southern Saint Croix and Mississippi River Valleys in east central and southeast Minnesota, and west central Wisconsin.
Data from the Twin Cities evening weather balloon showed a very unstable atmosphere, featuring 2000 J/kg of Most Unstable CAPE, with ample low level and deep layer shear, measured at 44 and 87 knots, respectively.
Note: All weather maps, except the sounding profiles, are courtesy of Phil Schumacher, Science and Operations Officer at the National Weather Service Forecast Office in Sioux Falls, South Dakota. The sounding profiles are courtesy of Jeff Peters, Storm Prediction Center.