That's a good question because the NOAA/CIRES Twentieth Century Global Reanalysis Version 2c (20thCR V2c) only ingests three things: surface pressure, sea ice coverage and sea surface temperature. The rest of the analysis is generated by the physical models of NOAA's Global Forecast System (GFS).
The short answer is that 20thCR V2c extends all the way back to 1851, which means you can compare storm intensity between storms, including events that occurred before the modern satellite era.
E.g. how does the AR event of 2004-2005 compare with one responsible for the Los Angeles flood of 1938, that felled thousands of buildings? Let's take a look.
|Global animation of PWV from 20thCR V2c for 1938-02-27-00 UTC through 1938-03-03-18 UTC.|
|Regional animation of PWV from 20thCR V2c for 1938-02-27-00 UTC through 1938-03-03-18 UTC.|
Between February 27th and March 3rd, 1938 Los Angeles was inundated with two storm systems delivering record breaking rainfalls. By March 3rd, the San Gabriel Mountains received 32 inches of rain, more than their average yearly total, and Los Angeles received over 10 inches of rainfall over the 5-day storm. 115 people lost their lives, thousands more were evacuated, over 6000 homes were damaged or destroyed, and 108,000 acres - one third of Los Angeles - was flooded.Click to see more pictures of the storm damage.
The CSUN Digital Library offers more images, including this aerial photograph of Victory Boulevard in 1938.
|Aerial view of 1938 flood damage in San Fernando Valley courtesy of CSUN Digital Library and used here with permission.|
Of course, precipitable water vapor aloft is not a perfect predictor of localized rainfall. That depends on terrain, lifting, and wind direction. More accurate estimates can be obtained by ingesting reanalysis fields into numerical weather prediction models such as WRF.