Approximately 2200 UTC 7 July 1997, thunderstorms rapidly developed over southeast South Dakota and northeast Nebraska. A resultant Mesoscale Convective Complex (MCC) formed and trekked east-southeast across the Central Plains. A Mesoscale Vorticity Center (MVC) was produced by the dissipating MCC during the morning hours on July 8. Once the cirrus anvil debris dissipated, an MVC became apparent on the visible satellite imagery at 1415 UTC (Figure 1) with a faint circulation center noted near Kansas City, Missouri. The MVC continued to move eastward and was responsible for the generation of strong to severe thunderstorms over east-central Missouri, northern Arkansas, and southern Illinois during the late afternoon hours of July 8, 1997.
This case study will study the "active MVC" (Johnston 1981) and its relation to the development of strong to severe thunderstorms over east central Missouri, northern Arkansas, and southern Illinois.
During this MVC event, the Central Plains was predominantly under the influence of a weak northwest flow aloft. Analysis of the 300 mb chart through the MVC event period showed the Polar Jet in a typical July location with a ridge of high pressure over the western half of the United States and Western Canada, and a shallow trough of low pressure over the Great Lakes region and northeast United States (Figure 2).
Approximately 2200 UTC 7 July 1997, rapid thunderstorm development took place over southeast South Dakota and northeast Nebraska (Figure 3). The thunderstorms developed in response to strong surface convergence ahead of a triple point that was located over the panhandle of Nebraska at the time. Also noted on the 0000 UTC 8 July upper air analysis was a shortwave at 850 mb extending from western Minnesota through eastern Nebraska into western Kansas (Figure 4). This shortwave helped to increase southerly winds as well as low level convergence across Oklahoma, almost all of Kansas, and the southeast half of Nebraska. Strong southerly surface winds across the region helped to transport higher moisture values into the region of concern. At the surface, 70 to 75°F dew points extended northward across eastern Texas into much of Oklahoma and extreme southeast Kansas. Dew points were nearly 60°F at 850 mb across the same region (Figure 4).
Figure 3. GOES-8 visible for 2200 UTC 7 July 1997.
Figure 4. 850 mb analysis for 0000 UTC 8 July 1997
Once the MCC developed in this environment, it continued overnight and propagated southeast but started to dissipate by dawn over eastern Kansas. The dissipation of the MCC was likely caused by the MCC moving east away from the main moisture axis (Figure 5) and loss of the nocturnal low level jet. This scenario fits well with the typical MCC as described by Maddox et al. (1986).
A mesohigh was denoted on the surface chart over Emporia, Kansas by 1100 UTC 8 July 1997 (Figure 6). According to Johnston (1981), "the dominant characteristic of the MCC is likely to be the large extent of the mid-tropospheric upward mass circulation and the attendant large area of precipitation. The warm core nature of this circulation, caused by large amounts of latent heat release, may produce a meso-low aloft above the meso-high associated with the shallow layer of cool air from the thunderstorm downdraft". This seems to be the case with this MVC event. The development of the warm core low (MVC) in the mid-troposphere is accompanied by an increase in positive vorticity (Rasmussen 1979). This is key to future initiation of deep convection. The mesoscale positive vorticity center showed up very well on visible satellite imagery as it progressed across Missouri.
On the surface, the mesohigh continued to move southeastward into northwest Arkansas during the afternoon hours (Figure 7). Meanwhile, over east central and southeast Missouri, northern Arkansas and southern Illinois, destabilization of the atmosphere was taking place with daytime surface heating and dew points remaining in the lower 70s.
Visible satellite imagery clearly showed the MVC and its associated positive vorticity center moving eastward across central Missouri with a well defined cloud-free center depicted near Sedalia, Missouri at 1602 UTC (Figure 8). During the early afternoon hours of July 8, the MVC was propagating into an atmosphere conducive for deep convection over eastern and southeast Missouri. At 1715 UTC , a southward moving "spiral band" (Figure 9) on the southern periphery of the MVC appears to interact with an east-west outflow boundary over northern Arkansas. This interaction caused rapid thunderstorm development extending from near Walnut Ridge to near Fort Smith, Arkansas. This area was also on the northern edge of high value theta-E air, which caused the thunderstorm complex to build as it moved south through the state. In Missouri, the MVC's once cloud-free center, now located between Sedalia and Columbia, was filling with towering cumulus (Figure 9).
Figure 8. GOES-8 visible for 1602 UTC 8 July 1997.
Figure 9. GOES-8 visible for 1715 UTC 9 July 1997.
By 2032 UTC 8 July 1997, a cluster of cumulonimbi was noted in the center of the circulation just east of Jefferson City (Figure 10) as the center was propagating further eastward into the unstable airmass in place across east central Missouri. At 2215 UTC, the center and associated convection was located just south of St. Louis (Figure 11). The National Weather Service in St. Louis issued a severe thunderstorm warning for southern Jefferson County in east central Missouri at 2233 UTC. This severe thunderstorm warning was based on a report of 3/4 inch hail that fell over northeast Washington County, near Richwoods around 2122 UTC. A couple of urban and small stream advisories were also issued for St. Charles and St. Louis Counties from 2200 UTC to 2400 UTC. No other severe weather was reported from this MVC induced thunderstorm complex as it trekked across east central Missouri. By 0015 UTC 9 July, the thunderstorm complex produced by the MVC moved into southern Illinois (Figure 12).
Use of the wind profiler and satellite imagery helped in this event to better understand the circulation pattern of a classic MVC. We were able to track the MVC in its entirety with the use of GOES-8 imagery. We were also fortunate to capture this classic MVC as it passed over the Lathrop, Missouri wind profiler (Figure 13). The profiler allowed us to obtain a vertical cross section of the wind circulation associated with the MVC. The passage of the circulation center was noted around the 650 mb level which is around the level expected for MVC's (Johnston 1981).
Although this MVC event did not seem to have a significant amount of severe weather, it was classified as an "active MVC case" (Johnston 1981) because it caused new convection as it propagated east. One should always be leery of any MVC. This MVC had an easterly track which is favorable for an "active MVC" (Johnston 1981). The reason for the MVC being active is because it propagated into a favorable air mass that was in place over the region. MVC's moving northeast are less likely to be active because they are diverging from moist air that is usually in place to the south. Forecasters should monitor satellite imagery, sounding data, ADAP, surface analysis, and other means in order to assess the potential for severe thunderstorm development should a MVC propagate into their area
The author is grateful to Dan Ferry (WSFO STL), Forecaster, for his satellite archive data containing the MVC, and to David Gaede (NWSO SGF), SOO, for helping me set up the tape and reviewing it on our Scientific Applications Computer (SAC) and for reviewing this article. The author also extends his appreciation to Steve Lindenberg (NWSO SGF), Forecaster, for his computer expertise and advice while extracting the satellite data, and to Ron Przybylinski (WSFO STL), SOO, for his research expertise and help. A final thanks goes to Saint Louis University for the data extracted from their Diagnostics Program.
Johnston, E.C., 1981: Mesoscale Vorticity Centers Induced by Mesoscale Convective Complexes. Master's Thesis, University of Wisconsin, 4-K-5pp.
Maddox , R.A., K.W. Howard, D.L. Bartels, and D.M. Rodgers, 1986: Mesoscale Convective Complexes in the Middle Latitudes. Mesoscale Meteorology and Forecasting, Peter S. Ray, Ed., AMS, 390-413.
Rasmussen, E., 1979: The Polar Low as an Extratropical CISK Disturbance. Quart. J. Roy. Meteor. Soc., 105, 531-549.
*Current affiliation: NWS Office Great Falls, Montana.