AN MCS MERGER - JUNE 3rd, 2008
by Philip Lutzak – June 2008
This is a short analysis of two MCSs (Mesoscale Convective Systems) that merged over the mid-western U.S. on June 3rd, 2008. Figures 1 and 2 below show these two systems as the faster, western MCS caught up to and merged with the more slowly moving eastern MCS. The western MCS originated over Iowa, Minnesota and Wisconsin, and was a DCS, or Derecho Mesoscale System, in which the gust front (main convergence zone with strongest thunderstorms) runs at the leading edge (or downwind side) of the system. This type of MCS is known as a forward-propagating system. The eastern MCS was an SCS, or Severe Mesoscale System, in which the gust front is quasi-stationary and remains near the rear of the system. In these SCS type of Mesoscale Convective Systems the strongest thunderstorm cells form on the back end (or upwind side) of the system, so they are known as backward-propagating systems. These features are clearly visible in the radar images in Figures 2a thru 2d. After the DCS caught up to the SCS and they began to merge, the resultant MCS was an SCS type, with the gust front clearly visible at the rear of the merged system in Figure 2d.
The surface weather maps in Figure 3 below show the synoptic weather for June 3rd at 7AM and 1PM CDT. Note how remarkably little the surface conditions changed over this time period, illustrating a classic stagnant weather pattern so common in the warm season. And as can often happen in the summer season, the MCSs that developed overnight moved along and parallel to the stationary fronts as the upper winds drove them eastward.
Note in Figure 4a below that winds at 300mb at 20Z or 3PM CDT were blowing at 60-70 knots or better over northwestern Illinois where the DCS was located, but were blowing at 40-50 knots over southern Indiana where the more slowly moving SCS was located. The most salient point in this chart is the significant area of maximum divergence from south central Indiana to the Ohio-Kentucky border around 20Z as the front edge of the DCS merged with the back end of the SCS. In the 500mb chart in Figure 4b, we can also see much faster wind speeds over central Indiana where the DCS was located (60-70 knots), and much slower winds (30-40 knots) over southern Ohio/Kentucky at the bottom of the NW to SE tilted 500mb trough where the SCS was located. So we can see why the DCS caught the SCS, and why the resultant system slowed down considerably.
From the previous satellite and radar images in Figures 1 and 2 we can see that the states of Illinois and Indiana took quite a beating from these systems.
Since DCSs tend to produce more severe weather and SCSs more flooding, it's not surprising that Illinois took the brunt of the severe weather and Indiana/Ohio experienced the flooding rains. Figure 5a, below left, shows the severe weather associated with both systems. It's important to note that almost all of the the severe weather in Indiana and Ohio did NOT occur with these systems, but rather during the evening before between 00 and 04Z in a different event. The only severe weather in Indiana/Ohio that afternoon were a few high wind events caused by the still potent gust front of the DCS as it caught up to the SCS. Figure 5b, below right, shows the flooding rains that occurred over central Indiana as the two systems merged and the resultant SCS slowed down.