Sunday's Forecast
Sunday, July 18, 2011
By the time the NWS Convective Outlooks (figure 1) and forecast (figure 2) were issued on early Sunday morning, the ingredients for the development of severe weather were either already in place or were forecast to develop across the upper Midwest over the next several hours. (Special Note: If you are unfamiliar with the ingredients for thunderstorm development, or need a refresher, please review this primer.) At 8:00 am, dew point temperatures were already in the mid- to upper-70s F. An analysis at 850 mb (5,000 feet) also showed significantly high dew point values aloft, confirming the presence of a thick warm and moist layer of air. This low-level moisture had been transported to the region from the Gulf of Mexico by steady winds at 850 mb over the previous few days.
Although Convective Available Potential Energy (CAPE) is a measure of buoyancy and the potential for strong updrafts, it is commonly used by meteorologists as a proxy for atmospheric instability. There are different methods of calculating CAPE based upon the altitude from which the analysis begins. Mixed-Layer CAPE relies upon the average temperature and moisture parameters in the lowest 100 mb to determine a starting point and is considered more realistic than using a surface-based parcel. The Mixed-Layer CAPE (MLCAPE) analysis valid at 8:00 am (1200Z) on Sunday morning (figure 3) indicated values (red contours) from 1500 J/kg to more than 3000 J/kg across the upper Midwest. CAPE values in excess of 2500 J/kg suggest a very unstable atmosphere.
The light and dark blue shading on the MLCAPE analysis (figure 3) represents Convective Inhibition (CIN) in negative J/kg, with the shading representing -25 J/kg and >= - 100 J/kg, respectively. CIN identifies an area where the energy and instability represented by the values of CAPE is inhibited by low-level stability. The presence of CIN on an analysis of CAPE indicates a warm layer of air above the surface which acts to suppress the upward motion associated with a thunderstorm updraft. This warm layer aloft is clearly evident on the atmospheric profiles from Green Bay, WI and Chanhassen, MN, valid at 8:00 am (1200Z) on July 17th.
In summary, the chart of MLCAPE provides both a quantitative analysis of the amount of energy available for a thunderstorm updraft, and the strength of the stable layer aloft which the fledgling updraft must first overcome. The analysis above suggests that the atmosphere over the region was very unstable, but this instability was offset by reasonably strong inhibition, particularly over most of the Dakotas, Minnesota, and northern Wisconsin. The computer forecast valid at 2:00 pm (1800Z) and 8:00 pm (0000Z) on Sunday indicated that MLCAPE would remain very high across the region.
Vertical wind shear, the change in speed and direction of the wind as altitude increases, plays a critical role in the initiation and development of severe thunderstorms. When wind shear is weak, the rain-cooled downdraft quickly undercuts the updraft's access to warm, moist air and quickly leads to dissipation of the thunderstorm. Wind shear acts to separate a thunderstorm's updraft from the rain-cooled downdraft, thereby prolonging the life of the storm. The potential for a thunderstorm to be categorized as severe is related to its life span, with long-lived storms being much more likely to be associated with tornadoes, large hail and damaging wind gusts.
The analysis of 0 to 6 km wind shear valid at 8:00 am (1200Z) on Sunday, July 17 (figure 4), indicates that wind shear values above North Dakota and northern Minnesota were quite strong -- in the range of 40 to 60 knots. In contrast, the wind shear values over northern Wisconsin were quite modest, and fell below the chart's threshold. Severe thunderstorms are more likely to occur when the environment is characterized by at least moderate values of CAPE (1000 to 2500 J/kg) and 0 to 6 km wind shear values of at least 20 to 45 knots.
The forecast of 0 to 6 km wind shear valid at 2:00 pm (1800Z) (figure 5) and 8:00 pm (0000Z) (figure 6) suggested that wind shear values would increase dramatically over the upper Midwest as the day progressed. By 8:00 pm (0000Z), wind shear values approaching 80 knots were expected over North Dakota while 50 to 60 knots were forecast for northwestern Wisconsin. If thunderstorms were able to develop, more than adequate wind shear was available to promote long-lived storms.
Provided that the low-level stability could be overcome, it appeared that the atmosphere was primed for the development of thunderstorms -- perhaps even severe thunderstorms. The final ingredient was a source of lift to promote upward motion from buoyant air parcels. In this instance, a possible trigger was an approaching cold front. Based upon the forecast valid at 8:00 pm (00Z) on Sunday night issued by the HPC (figure 7), the cold front was expected to pressing towards the northern border of Minnesota.