Ohio storm of July 4, 1969...What happened ?

I have some questions about the July 4, 1969 Ohio superstorm, possibly the most severe convective event in the history of Ohio. Descriptions of the storm describe a wide swath of devastating surface winds coming ashore from Lake Erie from the N along a broad front (tens of miles). This description makes me think that the storm had a LEWP structure, or less likely a very wide single bow at this stage in its development. If this was the case, one could surmise that this MCC was building forward (in the down-shear direction).

The puzzle to me is that this same storm is credited with producing a staggering 24 hour 15 inch rainfall total in the area of heaviest rain. This is truly staggering when it is considered that the ratio of the 24 hour storm rainfall to the 100 year/24 hour estimate for this storm was greater then that of the notorious Aurora Illinois storm of July 19,1996!!!! :shock: The Aurora storm is considered to be the greatest documented 24 hour rainstorm for the Upper Midwest.

My impression is that MCC’s are more likely to produce enormous rainfalls when they resemble the classic Maddox type of MCC, where regeneration is in the up-shear direction (back-building) causing the MCC to be very slow moving. In forward building MCC’s or bow echo storms, the heavy rainfall usually does not stay over a given area for long.

How does one explain the July 4, 1969 Ohio event? :? Was there more then one MCC? Or did the system somehow switch from dominantly forward-building to back-building modes of regeneration? Is that even possible? I have not had much luck finding literature regarding this truly extraordinary event.
:study:
 
Originally posted by Glenn Rivers
....In forward building MCC’s or bow echo storms....

Technically... bow echoes are most associated with MCSs, not MCCs. Remember, Mesoscale Convective Complexes (MCC) are a type of Mesoscale Convective System (MCS). For an MCS to be classified as an MCC, it must meet the following criteria, per Maddox (1980):

1. Temperature -- Continuous cold cloud temperature (per IR) of <-32C at least 100,000 km^2
2. Temperature --Central cold core temperature of <-52C at least 50,000 km^2
3. Longevity -- The above two requirements must be met for at least 6 hours
4. Shape -- Eccentricity (minor / major axes) must be at least 0.7 at the time of maximum extent

MCCs tend to be slow-moving, as you noted, while forward-propagating MCSs / bow echoes tend to be fast moving... MCCs, partly because of their slow movement, tend to be prodigous rain-makers, so hefty rainfall totals are very believable. MCCs, however, tend NOT to be the best damaging wind-producers...
 
More about July 4, 1969

Thank you Jeff Snyder for the ready at hand tips on definition of MCC’s. I really can’t prove that the July 4,1969 Ohio superstorm met the criterion of an MCC but I think that it probably did. After looking at the site bookmarked for me by Jeremy Lemanski there is no doubt in my mind that in the first couple hours of this event, the storm took on the form of a derecho exhibiting a LEWP.

The amazing thing is that newspaper accounts (thanks Tim Vasquez) indicate that severe thunderstorms with flooding rain continued for the entire night, for 8-12 hours after the leading edge of the derecho. I don’t think that all of this could have been due to the trailing stratiform precip region behind the initial derecho, especially considering the staggering rainfall totals.

I am left wondering if this storm was actually two or more events, with the derecho being the first event. Is it possible that after the derecho charged SE, departing the area, that new convection fired along the upwind edge of the cold pool left behind, leading to a second large MCS that unlike the first one remained in a back-building mode? Does this seem possible?

Does anyone know of an event were an area was struck first by a fast moving bowing MCS, only to be almost immediately blindsided by a second, back-building type MCS, which then trained multiple cells of flooding rain across an area already left reeling from a full blown derecho? That would be some trip!!!!!
 
Here is a look at the storm total rainfall for the event, taken from "Thunder In the Heartland" by Thomas W. Schmidlin & Jeanne Appelhans Schmidlin...

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The area of heaviest rain is located along the southern edge of the wind damage area. The 500mb data for 0Z July 5, 1969 shows a ~50 knot west-northwesterly flow across the region with a ~40 knot west-southwesterly low level jet, most notable at Dayton (DAY). Considering this information, the forward speed of the derecho and the approximate time the flooding started, I'd guess additional storms developed from upward motion caused by the low level jet interacting with the wake cold pool. I experienced one of these systems in July of 1998, so I've done research on the subject. Radar photos on microfilm from NCDC would be helpful, but I havne't had success in geting any thus far (something to consider).
 
Thank you

Originally posted by Nick Smith
Here is a look at the storm total rainfall for the event, taken from \"Thunder In the Heartland\" by Thomas W. Schmidlin & Jeanne Appelhans Schmidlin...

The area of heaviest rain is located along the southern edge of the wind damage area. The 500mb data for 0Z July 5, 1969 shows a ~50 knot west-northwesterly flow across the region with a ~40 knot west-southwesterly low level jet, most notable at Dayton (DAY). Considering this information, the forward speed of the derecho and the approximate time the flooding started, I'd guess additional storms developed from upward motion caused by the low level jet interacting with the wake cold pool. I experienced one of these systems in July of 1998, so I've done research on the subject. Radar photos on microfilm from NCDC would be helpful, but I havne't had success in geting any thus far (something to consider).

Thank you, Nick Smith, this information and your description of what you think happened is very helpful.
 
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