"Midwest Snow Event"
06 December 1995

On December 6, 1995, a 14 to 18 mile wide band of heavy snow (greater than or equal to 6 inches) fell across eastern Nebraska and southwest Iowa. The dynamics in this case are subtle, not a classic super-storm system, and therefore the forecast was challenging. The storm came ashore in the Pacific Northwest, and moved across the north-central and central U.S. The system seemed to weaken as it passed over the Rockies, but intensified as it reached central Nebraska. A strong jet streak aloft gave support to the storm, and later, coupled jet streaks seemed to intensify the precipitation over east-central Nebraska and southwest Iowa. Frontogenesis was also a major factor in inducing the mesoscale snow band in the midwest. This case provides an excellent example of heavy snow produced by the union of several subtle features.

The storm came onshore in the Pacific Northwest and moved across the U.S. following the strong jet streak aloft. Here you can see the storm and associated jet streak at 1200Z 5 December and later at 1200Z 6 December.

Interestingly, the 1200Z 6 December OAX (Omaha, NB) sounding showed a very stable profile, and didn't become neutral or conditionally unstable until about 500mb. At this time, heavy snow was falling in the area, so strong lifting mechanisms must have been present to produce this.

Two important features came together to provide this. Frontogenesis was a factor, here you can see 500-700mb Qn vectors showing strong frontogenetical forcing in east central Nebraska and southwest Iowa. The forcing lines up nicely with the precipitation at the time. Also, a second jet streak formed over Kansas between 0600Z and 1200Z 6 December. The coupling of the right entrance and left exit quadrants of these features provided the necessary divergence aloft. Here is a cross section from TUL to FSD showing the vertical velocities at 1200Z 6 December, which coincides well with the band of heavy precipitation over east-central Nebraska.

As the conditions came together in east central Nebraska and southwest Iowa, the storm intensified and a heavy band of precipitation formed. Here are 2km nowrad images showing the quick development of the storm, from 0700Z and 1100Z 6 December.

Case Study 02