In "A methodology to classify extreme rainfall events in the western mediterranean area" by Casas et al., (2004): http://link.springer.com/article/10.1007%2Fs00704-003-0003-x they show rain rates of up to 0.5 cm/min sustained for several minutes (based on a large network of rain monitors in Barcelona). IF that could somehow be sustained for an hour, that would give a rate of 30 cm/hr (which is what Holt saw).

For comparison, the most extreme rainfall rate measured in one minute in the USA is 3.1 cm (1.2 in; see http://wmo.asu.edu/world-greatest-one-minute-rainfall ). Actual rain doesn't continue at such an extreme rate for much longer, but would equate to 186 cm per hour (6 times what Holt, Missouri saw in 1946). The link that kwinkunks provides http://www.bom.gov.au/water/designRainfalls/rainfallEvents/worldRecRainfall.shtml shows a world rate record of 3.8 cm/min which, if sustained, would equate to 228 cm per hour.