Wxman, Climatologist Consult; How often can we expect curmulative rainfall of "x"--where is answer? Ed Hopkins Phd Responds

(ed.note: This morning while out for coffee, a citizen made the comment "Well thank goodness we do not have "4" inches of rain very often." That made me wonder how often it did occur and where to look for the answer. One of our local bloggers is a research meteorolgist and he kindly put the question up the chain of command. The only definitive answer probably is the reference book where one can look it up he references--- the "observer" he refers to is not the Evansville Observer but a person who measures. I mentioned a possible 7 over 72 hours for purposes of example. If anyone has the specific rainfall for Evansville since Monday please advise. Here is what he said:

"WXman"


Yes, the rainfall totals across south central Wisconsin during the last
week have been impressive.
Now I can not put my finger on a 7 inch rain in the Evansville area, I
am aware of 24-hr totals in southern Rock county of over 3 inches on
Saturday-Sunday from the feature
http://www.crh.noaa.gov/crnews/display_story.php?wfo=mkx&storyid=9542&source=0
(For last weekend's rain, between 8 to 10 inches were reported in
eastern Iowa County).the

I do not have a report from the Evansville observer for that event, but
that observer did report 3.25 inches for a 24-hour span on Tuesday
morning, which could result in the roughly 7 inches that your friend
mentioned -- for a span of roughly 72-hours.
I consulted the atlas that many engineers and consultants in the
Midwest use for design purposes -- "Rainfall Frequency Atlas of the Midwest"
by F.A. Huff and J. R. Angel (Bull 71) from the Midwestern Climate
Center (available as a 148-pp pdf file at
http://www.sws.uiuc.edu/pubdoc/B/ISWSB-71.pdf ).

Dane and Rock counties are in Division 8 (south central Wisconsin).
So, on page 141, one could estimate for this division that a 3 inch rain
in a 24 hour period would occur once every 2 to 5 years (2.78 inches
once every 2 yrs and 3.53 inches every 5 yrs). A 7-inch rainfall event
in 24 hrs would have a 100-yr recurrence interval, while a 7-inches of
rain spread over 72 hrs would have a recurrence interval somewhere
between 25 and 50 yrs.

I hope that this helps. If you have any questions, please feel free to
contact us.
Ed Hopkins
Edward J. Hopkins, PhD
Assistant State Climatologist