A Perfect Environment For Thunderstorms
By noon on Sunday, several long-lived thunderstorms were active in central North Dakota and a cluster of storms was crossing from South Dakota into Minnesota (figure 1). In response to the rapid intensification of these storms and the increasing atmospheric instability downstream in Minnesota and Wisconsin, the SPC upgraded the potential for severe weather to moderate for portions of North Dakota and Minnesota with the Day One Outlook issued at 12:29 pm (EST). The combination of MLCAPE values approaching 6000 J/KG, strong 0-6 km wind shear, and diminishing CIN raised concerns regarding the potential for supercell thunderstorms and the possible development of a squall line. The accompanying graphic outlining the area at risk for severe wind gusts covered a broad area extending from North Dakota into Michigan. All of Lake Michigan was included in the revised area of elevated risk.
The national radar composite at 5:08 pm (2108Z) (figure 1) shows the cluster of storms had nearly crossed Minnesota and were approaching Wisconsin's western border. The surface analysis valid at 5:00 PM (2100Z) (figure 2) indicated the presence of a warm front stretching from the area of low pressure along the South and North Dakota border east to the southwestern tip of Lake Superior. The placement of the warm front marked the boundary between warm and moist air in Wisconsin and slightly cooler and moist air to the west of the boundary. The conceptual model of a warm front suggests that the ongoing storms to the west of this boundary (in Minnesota and the Dakotas) were examples of elevated convection, a situation where surface air does not flow directly into the storm's updraft. While elevated storms have a reputation for producing large hail, the potential for tornadoes and damaging surface winds is much less than with surface-based convection.
The placement of a warm front across Wisconsin's northwest corner suggested that if thunderstorms were to develop over the state they would likely be surface-based and thereby possessing a greater potential to produce tornadoes and strong winds. The curve in the eastern end of the warm front oriented the boundary perpendicular to the approaching storms, thereby maximizing low level convergence.
The formation of a warm front along the Minnesota - Wisconsin border was likely the result of a disparity in cloud cover over the two states between 1:00 pm (1700Z) and 5:00 pm (2100Z). A review of the visible satellite images for this period (click here) shows that while the skies over Wisconsin were not cloud-free, the cloud cover over Minnesota was far denser. The reduced cloud cover, in combination with a persistent southerly breeze, prompted surface temperatures to surge across most of Wisconsion.
The environment in Wisconsin was primed for the intensification and development of thunderstorms. The analysis of MLCAPE valid at 4:00 pm (2000Z) (figure 3) shows that Convective Inhibition (CIN) across the entire state of Wisconsin was very low while MLCAPE values were at least 5000 J/kg. A swath of central Wisconsin had extraordinary MLCAPE values of 6000 J/kg. The low level of CIN was predominantly the result of daytime heating. A comparison of the observed soundings at Green Bay shows that lowest portion of the atmosphere warmed significantly between 8:00 am (1200Z) and 8:00 pm (0000Z - 7/18) on July 17th. The 8:00 am (1200Z) sounding indicated that the surface temperature needed to overcome the robust level of surface-based CIN (-604 J/kg) was 87F. The surface temperature and dew point at the time of this calculation were 72F and 67.5F, respectively. At 8:00 pm (0000Z-7/18), the surface temperature had risen to 92.1F and surface-based CIN had been eroded to a mere -2 J/kg. The dew point had risen to 77.7F resulting in a very oppressive environment. In contrast, the temperature 1,500 meters above the surface increased only 4.5 F from 66.2 F to 72.7 F.
The analysis of 0 to 6 km wind shear at 5:00 pm (2100Z) (figure 4) indicated that while values in southern Wisconsin were relatively low, the northern part of the state was under the influence of 30 to 40 knots of west to northwest deep-layer shear. Shear values in this range were more than adequate to support the formation and maintainance of severe thunderstorms.
The synoptic, or large scale, pattern was also supportive of the development of severe weather. The analysis of 500 mb (18,000 feet) heights and vorticity valid at 5:00 pm (2100Z) (figure 5) displays a subtle shortwave trough near the southwestern tip of Lake Superior. Shortwave troughs produce mid-level divergence slightly downstream of the trough axis. This area of mid-level divergence supported upward motion in the air column above northern Wisconsin.
The corresponding 300 mb analysis (30,000 feet) (figure 6) indicates an area of upper-level divergence (outlined in magenta contours) near the Minnesota-Wisconsin border and directly above the 500 mb shortwave trough. This area of upper-level divergence was associated with a jet streak, a pocket of much faster winds travelling in the jet stream.
Surface dynamics, extreme values of CAPE, moderate deep-layer wind shear, and a mid- and upper-atmosphere supportive of upward motion set the stage for the explosive development of thunderstorms.
