FLOOD05189

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: Survey of Thunderstorm Rainfall Characteristics - 1979 to 1988
• Date: 9/1/1989
• Prepared For: CWCB
• Prepared By: Henz Kelly Associates
• Floodplain - Doc Type: Educational/Technical/Reference Information

I

I

I

I

I

I
I

I

I

I

I

I

I

I

I

I

I

I

I

the ColDrado PrDnt Range.

It should be clear from this survey that a heavy thunderstDrm
rainfall threat exists at elevatiDns abDve 7,500 feet. Storm point
rainfall totals have routinely exceeded 2.00 inches and storm
intensities support hourly rainfall rate of 3-4 inches per hDur.
Given the results of the survey three foothills thunderstorm
rainfall profiles are constructed as potentially representative of
storm rainfall until a more rigorous study can be completed. The
next section of the survey addresses these storm profiles.

5.0 PrDpDsed Heavy ThunderstDrm Rainfall Profiles

The thunderstorm rainfall survey discussed in the previous
sections was used tD prepare a ssmple family of heavy
thunderstDrms fDr the central CDlorsdo Pront Range. The primary
inputs to the storm family were the average survey storm
rainfsll characteristics described in SectiDn 4.0.

The 5 minute rainfsll for the storms has been calculated using
the HKA Excessive Convective Rainfall(ECR) model (Henz and
Kelly,1988) and the average heavy thunderstDrm rainfall
tempersture and mDisture structure repDrted in Henz snd Kelly,
1985. The similarity of the average structure to August 1, 1985
actual Denver sounding is shDwn in Pigure 6. The average
precipitable surface tD 500mb water is 0.90 inches while the August
I, 1985 sDunding's is 0.92 inches. Indian Hills and a significant
pDrtion of the Jefferson County fODthills were afffected by a
severe foothills thunderstorm Dn this date and the Storm Data
report included 2.13"/25 minutes with 6-8 inches of hail.
Appendix A describes the stmDspheric variables presented in Table
2 .

The 5 minute and total rainfall were calculated for a family of
three foothills storms:

a. Storm A: A classic "front-end" dumper storm with heaviest
rainfall production in first 30 minutes and a
duratiDn of 90 minutes

b. Storm B: A multi-cell heavy stDrm which tracks two
thunderstorm cells in a train effect over the
basin in just less than a 2 hour period

c. Storm C: A classic S-curve storm with a duration Df 90
minutes and heaviest rainfall during the middle Df
the storm's life-cycle