FLOOD01363

• County: Statewide
• Title: Colorado High Plains Thunderstorm Systems - A Descriptive Radar-Synoptic Climatology
• Date: 6/1/1974
• Prepared By: CSU
• Floodplain - Doc Type: Educational/Technical/Reference Information

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Table 1 U Characteristics of Si cant Flash F1 Events
Event Date Rain Amount PWI (inch) PF (kin) DeIT(C) De1Z (km)
Big Thompson 7131-8/ln6 10-13' 1.31 2.8 6.5 11.2
Rapid City 6/9m 10-12' 1.38 2.5 8.5 n.5
Maddox Synoptic composite 3.0 4.5 10.5
Mad. Frontal composite 2.2 3.5 9.2
Mad. Meso High composite 3.4 4.5 10.3
Mad. Western composite 1.8 5.0 9.2
Austin ;IX 5/24/81 8'+ 1.65 3.0 4.0 10.6
New Orleans, LA Iln/89 7-13' 1.75 3.7 4.0 10.5
Minneapolis, MN 7/24/87 11' 1.42 4.3 7.8 12.1
Chicago, lL 8/4/89 6'+ 1.19 4.6 7.5 12.4
Chicago, lL 8/13/87 6'+ 1.25 4.0 6.0 10.7
Omaha, NE 818187 4'+ 1.12 3.4 2.8 10.9
Cheyenne:. WY 8/1/85 6'+ 0.92 2.5 8.5 9.5
0pa1, WY 7126190 4-6' 1.22 2.3 6.0 9.6
Albuquerque, NM 7124/89 4'+ 1.17 2.0 4.3 9.0
Atlan GA 6/19/91 3.5'+ 1.95 3.9 3.2 11.5

In many cases when PWI data was compared to a measured rainfall the amount appeared to ..e roughly douhle the
PWl for the peak point hourly ramfoD xeported in the F2P2 District. verifiable until a flood detection nclWOIk of
over 136 ALERT rain gages was insta1led by UDFCD and became operationally accessible to HMS between 1988
and 1990. HMS has verified the 1.51anreiationshipto heavy rainfall since 1990.

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In the four operational F2P2 years from 1990 to 1993 HMS has routinely predicted convective QPF for the F2P2 IUelL
Daily verification of the relationship of the 1.51an warm layer, the PWI and the peak point rainfall in the FDN have
shmw only three cases where the warm layer failed to equal or exceed 1.5 kin. The values on those days were 1.3
kin, 1.3 lanamd 1.4 kin. Verification plots for the past four years and for 25 significant Denver stonn dlites from -
1981 to 1989 will not be presented here due to spa.:. considerations bu1 will be presented at the conference along ,
with QPF verification statistics. ',: " '

'. . " .
This warm layer criteria was tested On flash flooding event dates in Phoenix AZ, Las Vegas NY, Reno NY and on 12
significant flash flood dates in Wyoming> The 1.5 Ian warm layer was appropriate for aU the Nevada and Wyoming
events. However as the altitude dropped in Arizona below 3,500 It the warm layer related to flooding dates increased
to 2.5 kin. The 2.5 Ian factor was tested for the flash flooding cases presented in Table 1 and it appeared to be rather
constant but deserves discussion.

Table 1 presents a SUIJlIIllI1)' of updraft characterisitics as described in the previous section for a number of recent
killer flash flood events and for the Maddox composite flash flood sounding data, The date of the flash flood. the
rainfall associated with it as reported in Storm Data, the PWI. depth of the updraft's warm layer, the DelI and the
Del Z factOrs as calculated in the previous section are presented for each case. . In aU cases the warm layer is vOl)'
deep and meets or exceeds the warm updraft layer values. '

In concl~on it is quite possible that the depth of the warm layer of a predicted updraft can be used to predict the
maximum anticpated rainfall from a thunderstorm. More infonnation will be presented at the conference.