FLOOD11686

• County: Statewide
• Basin: Statewide
• Title: Flood Task Force Summary - 06/08/05
• Date: 6/8/2005
• Prepared For: FTF
• Floodplain - Doc Type: Flood Task Force Meeting Materials

there precipitation totals come from April, May, and
June. So it is opposite from mountains to plains. These
individual stations east of the mountains share the dry
winter phenomena.
The Heavy Rain Probabilities graphic shows that the
chances are best for heavy rains in May with a drop
off to the lowest point at the Fourth of July. He
warned that there are no absolutes with climate but
that this is the general trend in Colorado. Interpreting
this graph he explained that during the middle and late
May there is good chances for heavy rains. During the
July 4th period there is a remarkable lull. One
exception was the July 3-4 Frijole Creek storm event that was 16 inches of rain in not much time.
Then you see an interesting return to last ten days of July that is the onset of the monsoon and
last hurrahs of the winter. There is a second lull in end of august and then the third peak.
September through October is southwest dominated for precipitation and April and May is plains
dominated for precipitation.
Once its warmer the air holds more water vapor.
Max I hour precipitation versus elevation now has
a data set for a 45- year period. Here we are
looking at max one-hour rainfall. For a big
summer thunderstorm, the higher you go in
elevation the lower your max rainfall is expected
to be, but there are some outliers even at high
altitudes.
Nolan showed the NOAA precipitation frequency
atlas that has the 100-year 24 hour precipitation
values or what is called the "IOO-year storm" for
24 hour durations. This was last updated from 1973 using data from late 1960s precipitation and
precipitation since 1970 has not been incorporated. The largest values are in southeast Colorado,
6.5 inches. Whereas the Front Range is 5 inches from Golden to Loveland and is slightly
decreasing to the east.
Larry Lang pointed out that Utah and others had updated their NOAA Atlas with new maps for
their states and they had more definition to their
lines but the average numbers didn't go up or down
that much.
Nolan asked about how many places experience
100-year storm on average? The answer is about
150 places in Colorado have 100-year storms in
different locations per year.
Nolan showed the drought monitor map from May
31 that had only small portions of northeast and
northern east plains in drought. There is an
emerging drought in Oklahoma and southwest
Kansas. hrtp"'o"'"ght.unl....ulOm

Nolan stated that the history of the month of May for the last 6-8 years is a drier trend.

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HEAVY RAIN PROBABIUTlES
> 2" IN 24 HOURS SOMEWHERE IN COLORADO

DURING CONSECUTIVE 10-DA Y PERIOD

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MAR APR MAY JUN JUl AlIG SEP OCT NOV

Max. 1-hour Precipitation VS. Elevation
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BASED ON 19411 -1993 HPD DATA FOR COlORAOO

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Maximum 1-hour Precipitation

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U.S. Drought Monitor

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May 31, 2005
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