WSP09246

• File Number: 5000.475
• Description: Flood Protection Section - Wetzel Creek Flood - 1986
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 8/2/1986
• Author: John Henz-Ron Kelly
• Title: Wetzel Creek Colorado Flash Flood - August 2 1986 - A Reconstitution of Thunderstorm Rainfall - 1545-1845 MDT
• Water Supply Pro - Doc Type: Report/Study

. ,

000821

INTRODUCTION

During the afternoon and early evening hours of August
2, 1986 a damaging flash flood roared down Wetzel Creek in
extreme eastern Adams County. The flash flood drowned
several head of cattle and did extensive rangeland damage to
property and fences. The flash flood was accompanied
by copious amounts of hail ranging in size from pea-size to
nearly egg-size. The hail and flooding was produced by
a thunderstorm complex which crossed the basin of Wetzel
Creek in several surges and is described along with the
resulting rainfall in this report.

This report describes the meteorological setting, the
thunderstorm complex which produced the flash flood, the re-
constituted thunderstorm rainfall. A brief discussion of
the methodology and data sources used to derive the
re-constituted rainfall is presented.

METEOROLOGICAL SETTING

The basic weather pattern that influenced northeastern
Colorado on August 2, 1986 is fairly normal for early
August. Monsoonal moisture from the desert Southwest
continued to flow into the state at mountain top level and
above. A weak and shallow pool of cool air formed a
northwest to southeast boundary from the foothills near Fort
Collins into southeastern Colorado near Lamar. Moist air
from the Gulf of Mexico and the southern plains flowed
northward along this boundary into northeastern Colorado.
These moisture sources provided the fuel and water supply
for the heavy rain and hail that fell on this day from heavy
thunderstorms. Above mountain top level a disturbance
crossed the region from west to east helping to trigger and
intensify storm development.

As the air grew more unstable locally heavy
thunderstorms developed in three areas of northeastern
Colorado: the Larimer County foothills, Palmer Lake Divide
and along the low level boundary of cool air from Lamar to
Fort Collins. Storm formation began abou" mid-afternoon,
peaked from 1600MDT un"il 2~00MDT and subsided before
midnight. The Wezel Creek storm was one of atleast ten
impressive thunderstorm systems that crossed northeastern
Colorado during the peak period. One of the storms which
formed in Larimer County about 1930HDT produced over $40
million of hail damage in a swath from Fort Collins through
Longmont and southwestern Weld County into central Arapahoe
County. This storm moved from northwest to southeast at a
fairly steady rate of about 25 mph. It produced a report
from the National weather Service in Denver of 2.25 inches of
rain in 30 minutes at Platteville in Weld County.eSee Figure
1). This storm was classified as arigh~-propagating super-
cell thunderstorm with a life cycle of over three hours. The
rainfall rate of this storm was used to help in the re-
constitution of the Wetzel Creek storm.