MESOSCALE VORTICITY CENTER AND ITS CONTRIBUTION TO THE GENERATION OF STRONG TO SEVERE CONVECTION: A CASE STUDY
David A. Schmidt
National Weather Service Office
Approximately 2200 UTC on July 7, 1997, thunderstorms rapidly developed over southeast South Dakota and northeast Nebraska. A resultant Mesoscale Convective Complex (MCC) formed and treked east-southeast across the Central Plains. A Mesoscale Vorticity Center (MVC) was produced by the dissipating MCC during the morning hours of July 8th. Once the cirrus anvil debris dissipated somewhat, 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.
2. Synoptic Setting
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 on July 7, 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 July 8th 00 UTC 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 degree Fahrenheit dew points extended northward across eastern Texas into much of Oklahoma and extreme southeast Kansas. Dew points were nearly 60 degrees Fahrenheit at 850 mb across the same region (Figure 4).
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 on July 8, 1997 (Figure 6). According to Johnston (1981), "the dominant characteristic of the MCC is likely to be the large extent of the midtropospheric 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 definitely 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 southeast 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.
3. Development of new convection over southeast Missouri and northern Arkansas
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 8 July, 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, Arkansas to near Fort Smith. This area was on the northern edge of high value theta-e air and therefore the thunderstorm complex built as it moved south through the state. In Missouri, the MVC's once cloud-free center was now filling with towering cumulus and was located between Sedalia and Columbia (Figure 9).
By 2032 UTC on July 8, 1997, a cluster of cumulonimbi was noted in the center of the circulation now located just east of Jefferson City (Figure 10) as the center was propagating further eastward into the unstable airmass across east central Missouri. At 2215 UTC, the center and associated convection was located just south of St. Louis (Figure 11), and by
2233 UTC, the National Weather Service in St. Louis issued a severe thunderstorm warning for southern Jefferson County in east central Missouri. 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 on July 9th, 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 it's 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 is classified as an "active MVC case" (Johnston 1981) because it caused new convection as it propagated east.
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.
Also, be sure to keep up to date with 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 the area.
Dan Ferry, WSFO STL Forecaster, for his copy of the satellite tape containing the MVC
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
Steve Lindenberg, NWSO SGF Forecaster, for his expertise computer advice and help
Ron Przybylinski, WSFO STL SOO, for his expertise and help with research
Saint Louis University for paper copies from their Diagnostic Program
Johnston, E.C., 1981: Mesoscale Vorticity Centers Induced by Mesoscale Convective Complexes. Master's Thesis, University of Wisconsin.
Maddox , R.A., K.W. Howard, D.L. Bartels, D.M. Rodgers, Chapter 17, Mesoscale Meteorology and Forecasting, AMS, 1986
Rasmussen, E., 1979: The Polar low as an extratropical CISK disturbance, Quarterly Journal of the Royal Meteorological Society, 105, 531-549.