Information on the regional and real-time retrospective analyses available from the U.S. Climate Prediction Center can be found here: http://www.cpc.ncep.noaa.gov/products/precip/realtime/retro.shtml
Original documentation on the U.S. - Mexico data set can be found in the README file at ftpprd.ncep.noaa.gov/pub/precip/wd52ws/us-mex.
The following information is excerpted from the "README" file referred to above:
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The files "us_mex.y????" contain the daily gridded precipitation analysis
over the US and Mexico from Jan. 1, 1948 to December 31, 2001.
(1) Data Source: The US portion of the analysis was based on the CPC Unified
Precipitation Data Set and the Mexico
portion was based on daily station data from about 300 gauge
stations prior to the 90's and ~600 after.
(2) Resolution: 1 degree longitude by 1 degree latitude
(3) Domain: The domain of the analysis is (140W-60W, 10N-60N). The first
grid point is (140W, 10N), the second one is (139W, 10N), ...,
and the last one is (60W, 60N).
(4) Window: From 12Z to 12Z for each day.
(5)Anal. Scheme: Modified Cressman (1959) Scheme (Glahn et al. 1985; Charba
et al. 1992).
(6) Format: The format is direct access 32-bit IEEE floats created on a
big_endian platform (e.g. cray, sun, sgi and hp).
The undefined (missing) value is -9999.
(7) Units: mm/day
Users who use GrADS will find the "control file" very useful because it
contains all information about the data.
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