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A Quickie Case Study of an MCS’s Demise

A Quickie Case Study of an MCS’s Demise. Gail Hartfield WFO Raleigh, NC.

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A Quickie Case Study of an MCS’s Demise

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  1. A Quickie Case Study of an MCS’s Demise Gail Hartfield WFO Raleigh, NC

  2. Check out this satellite imagery of a recent MCS. It had produced widespread severe weather over Ohio and Indiana. Those of us on the evening shift had to determine if this system would survive and where it would go. The mean wind (850-500 mb) was northwesterly, so there was some concern that the MCS (or resultant mesoscale vorticity center) might move into North Carolina.

  3. The regional radar loop showed a southwesterly movement toward north-central North Carolina– where we had no PoPs. The Storm Prediction Center even put up a watch box just to our north. We were starting to get nervous!

  4. The Eta model (12H forecast, valid 00Z/8th) gave us some clues as to where the system would go. Since MCSs typically track with the thermal wind (shown by thickness contours, thick blue dashed lines), we thought that it might take a right turn (to the south) and head west of the CWA– rather than follow the mean “steering” wind (850-500 mb wind, black barbs).

  5. Also, the MCS was heading into an area of diffluent thicknesses. Since MCSs tend to backbuild into such an area, we were growing more confident that this system would largely avoid our CWA.

  6. Regarding the MCS’s overall health, it was moving out of the favorable convergence zone in the nose of the low level jet (850 mb isotachs in yellow, wind shown by white barbs)– an indication that it was not probably going to live much longer in its current state.

  7. Also, the 250 mb raob plot showed that persistent upper divergence—a common characteristic of long-lived MCSs—didn’t exist over the RNK and RAH CWAs. So, we concluded that the MCS would continue to die, and that its mesoscale vorticity center (MVC) would trend to the right (more southerly movement), avoiding the RAH CWA.

  8. So what happened? Well, we were mostly correct—the MCS did dissipate, as expected. However, the bulk of the remnant cloud shield followed the mean wind and headed toward WFO Wakefield’s CWA (southeast VA/northeast NC).

  9. Why did the MVC end up moving with the mean wind after all? Check out the graphic on the right. MCS movement, Vmbe in the graphic below, is determined by subtracting the low level jet vector (which is a surrogate for the propagation vector, Vprop -- equal in magnitude but opposite in direction) from the cloud layer wind, Vcl. In this case, once the MCS moved away from the low level jet, Vprop became negligible, and VmbeVcl. In other words, the system began moving more with the mean wind. Graphic above from Corfidi, S.F; Predicting the movement of mesoscale convective complexes. Wea. and Fcstg, Sept. 1995.

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