A Quickie Case Study of an A Quickie Case Study of an MCS’s DemiseMCS’s Demise

Gail HartfieldGail Hartfield

WFO Raleigh, NCWFO Raleigh, NC

Check out this satellite imagery of a recent MCS. It had produced Check out this satellite imagery of a recent MCS. It had produced widespread severe weather over Ohio and Indiana. Those of us on the 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 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 would go. The mean wind (850-500 mb) was northwesterly, so there was some concern that the MCS (or resultant mesoscale vorticity center) some concern that the MCS (or resultant mesoscale vorticity center)

might move into North Carolina.might move into North Carolina.

The regional radar loop showed a southwesterly movement The regional radar loop showed a southwesterly movement toward north-central North Carolina– where we had no PoPs. toward north-central North Carolina– where we had no PoPs. The Storm Prediction Center even put up a watch box just to The Storm Prediction Center even put up a watch box just to

our north. We were starting to get nervous!our north. We were starting to get nervous!

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

(850-500 mb wind, black barbs).(850-500 mb wind, black barbs).

Also, the MCS was heading into an area of diffluent thicknesses. Since Also, the MCS was heading into an area of diffluent thicknesses. Since MCSs tend to backbuild into such an area, we were growing more MCSs tend to backbuild into such an area, we were growing more

confident that this system would largely avoid our CWA.confident that this system would largely avoid our CWA.

Regarding the MCS’s overall health, it was moving out of the favorable 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 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 yellow, wind shown by white barbs)– an indication that it was not probably going to live much longer in its current state.going to live much longer in its current state.

Also, the 250 mb raob plot showed that persistent upper Also, the 250 mb raob plot showed that persistent upper divergence—a common characteristic of long-lived MCSs—divergence—a common characteristic of long-lived MCSs—

didn’t exist over the RNK and RAH CWAs. didn’t exist over the RNK and RAH CWAs.

So, we concluded that the MCS would continue to die, and So, we concluded that the MCS would continue to die, and that its mesoscale vorticity center (MVC) would trend to the that its mesoscale vorticity center (MVC) would trend to the right (more southerly movement), avoiding the RAH CWA.right (more southerly movement), avoiding the RAH CWA.

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

(southeast VA/northeast NC).(southeast VA/northeast NC).

Why did the MVC end up moving with the mean wind after all? Check out Why did the MVC end up moving with the mean wind after all? Check out the graphic on the right. MCS movement, the graphic on the right. MCS movement, VmbeVmbe in the graphic below, is in the graphic below, is

determined by subtracting the low level jet vector (which is a surrogate for determined by subtracting the low level jet vector (which is a surrogate for the propagation vector, the propagation vector, VpropVprop -- equal in magnitude but opposite in -- equal in magnitude but opposite in direction) from the cloud layer wind, direction) from the cloud layer wind, VclVcl. In this case, once the MCS . In this case, once the MCS moved away from the low level jet, moved away from the low level jet, VpropVprop became negligible, and became negligible, and

VmbeVmbe VclVcl. In other words, the system began moving more with the . In other words, the system began moving more with the mean wind.mean wind.

Graphic above from Corfidi, S.F; Predicting the movement of mesoscale convectivecomplexes. Wea. and Fcstg, Sept. 1995.