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Catastrophic Flood in Boulder, Colorado
An all-time 24-hour record rainfall of 9.08” (as of 6 p.m. 9/12 MT--almost double the previous record) has deluged the city of Boulder, Colorado resulting in widespread flash flooding and the deaths of at least three people so far. 12.27" has accumulated since Monday 5 p.m. (September 9th). Needless to say, these are numbers that surpass most tropical storm events. Other locations in the Boulder and Rocky Mountain Front Range have picked up over 11” of precipitation in just the past 24 hours. The official Colorado state record of 11.08" for a 24-hour period set at Holly on June 17, 1965 might be in jeopardy. UPDATE A site near Eldorado Springs in Jefferson County has reported 14.60" of rainfall as of 9:40 p.m. MT on Thursday evening. It is not clear if this is a storm total or 24-hour total.
Bridge washout and its results at Highway 287 and Dillon Rd. in Lafayette just east of Boulder. Photo by Cliff Grassmick in the Daily Camera Boulder County News.
Radar estimated rainfall for the 18-hour period between 5 p.m. September 11 and around 11 a.m. on September 12th MT across north-central Colorado. You can see how Boulder rests in an area of 8”+. Map from NWS-Denver office web site.
According to the Western Regional Climate Center’s historical data set, Boulder’s former 24-hour precipitation record was 4.79” on July 31, 1919. The September record was 3.05” on September 4, 1909. Records began in October 1893 with some months missing. The CoCoRaHS (Community Collaborative Rain, Hail, and Snow Network) observers have reported the following 24-hour precipitation amounts as of Thursday (September 12th) morning (I'll update this on Friday):
Flooding is also occurring up and down the length of the Front Range from Colorado Springs (where 3.68” of rain has so far fallen) to Longmont (with 3.37” so far). In both cities mandatory evacuations are taking place in several neighborhoods.
The cause of the excessive rainfall is a deep plume of monsoon moisture from the south overriding a cool and moist air mass with orographic lift enhancing the rainfall along the Front Range and its nearby cities.
Show-and-tell graphic of the cause behind all the heavy rainfall in Colorado the past few days. It will continue into the near future. From NWS-Denver office web site.
The full extent of the damage and number of storm-related fatalities has yet to be determined (the rain continues to fall heavily in many locations) but the flooding, at least in Boulder, will be of historic proportions.
Flood History of Boulder
Boulder is considered to be Colorado’s ‘most at risk’ city in terms of potential flood damage. This is because it rests against the mouth of a canyon (the Boulder Canyon) from which a creek (the Boulder Creek) bisects the heart of the town.
Serious floods have affected downtown Boulder in 1894, 1896, 1906, 1909, 1916, 1921, 1938, and 1969 with the worst being those of May 31-June 2, 1894 and May 7, 1969. The flood of 1969 was the result of four days of almost continuous rainfall (11.27” measured in Morrison and 9.34” at the Boulder Hydroelectric Plant three miles up Boulder Canyon from town). There was one death reported and “many thousands of dollars” worth of damage including two bridge wash outs. The flood of 1894 was considerably worse with many causalities although no specific number of fatalities has ever been determined. A brief account of the flood appears on the “Front Range Flood Histories” web site: "In the Left-hand Creek basin to the north of Four Mile Canyon Creek, the mountain towns, mining camps and upstream canyons of the area were perhaps the most severely affected by the 1894 flood. Most were virtually wiped off the map.....Left-hand Canyon areas sustained heavy damages. All bridges were washed out and roads obliterated. Sheriff Dyer stated that Left-hand Creek was over a half mile wide in places and that the farms along that creek were piled knee high with debris and sand....8.54 inches of rain from May 30 through June 1 was reported in Ward.” The estimated water flow on Boulder Creek at 4th Street was 11,000-13,500 cfs (cubic feet per second).
A photo of what was supposed to be Canyon St. in Boulder during the 1894 flood. A caption that goes along with this photo on the Boulder Flood Safety web page says “The eventual repeat flood of this magnitude may well cause hundreds of millions of dollars of damage.” It just may be that that day has arrived. Image from Denver Public Library Archives.
KUDOS: Thanks to Bob Henson of UCAR for updating precipitation totals in Boulder.
Christopher C. Burt
Weather Historian
An all-time 24-hour record rainfall of 9.08” (as of 6 p.m. 9/12 MT--almost double the previous record) has deluged the city of Boulder, Colorado resulting in widespread flash flooding and the deaths of at least three people so far. 12.27" has accumulated since Monday 5 p.m. (September 9th). Needless to say, these are numbers that surpass most tropical storm events. Other locations in the Boulder and Rocky Mountain Front Range have picked up over 11” of precipitation in just the past 24 hours. The official Colorado state record of 11.08" for a 24-hour period set at Holly on June 17, 1965 might be in jeopardy. UPDATE A site near Eldorado Springs in Jefferson County has reported 14.60" of rainfall as of 9:40 p.m. MT on Thursday evening. It is not clear if this is a storm total or 24-hour total.
Bridge washout and its results at Highway 287 and Dillon Rd. in Lafayette just east of Boulder. Photo by Cliff Grassmick in the Daily Camera Boulder County News.
Radar estimated rainfall for the 18-hour period between 5 p.m. September 11 and around 11 a.m. on September 12th MT across north-central Colorado. You can see how Boulder rests in an area of 8”+. Map from NWS-Denver office web site.
According to the Western Regional Climate Center’s historical data set, Boulder’s former 24-hour precipitation record was 4.79” on July 31, 1919. The September record was 3.05” on September 4, 1909. Records began in October 1893 with some months missing. The CoCoRaHS (Community Collaborative Rain, Hail, and Snow Network) observers have reported the following 24-hour precipitation amounts as of Thursday (September 12th) morning (I'll update this on Friday):
Flooding is also occurring up and down the length of the Front Range from Colorado Springs (where 3.68” of rain has so far fallen) to Longmont (with 3.37” so far). In both cities mandatory evacuations are taking place in several neighborhoods.
The cause of the excessive rainfall is a deep plume of monsoon moisture from the south overriding a cool and moist air mass with orographic lift enhancing the rainfall along the Front Range and its nearby cities.
Show-and-tell graphic of the cause behind all the heavy rainfall in Colorado the past few days. It will continue into the near future. From NWS-Denver office web site.
The full extent of the damage and number of storm-related fatalities has yet to be determined (the rain continues to fall heavily in many locations) but the flooding, at least in Boulder, will be of historic proportions.
Flood History of Boulder
Boulder is considered to be Colorado’s ‘most at risk’ city in terms of potential flood damage. This is because it rests against the mouth of a canyon (the Boulder Canyon) from which a creek (the Boulder Creek) bisects the heart of the town.
Serious floods have affected downtown Boulder in 1894, 1896, 1906, 1909, 1916, 1921, 1938, and 1969 with the worst being those of May 31-June 2, 1894 and May 7, 1969. The flood of 1969 was the result of four days of almost continuous rainfall (11.27” measured in Morrison and 9.34” at the Boulder Hydroelectric Plant three miles up Boulder Canyon from town). There was one death reported and “many thousands of dollars” worth of damage including two bridge wash outs. The flood of 1894 was considerably worse with many causalities although no specific number of fatalities has ever been determined. A brief account of the flood appears on the “Front Range Flood Histories” web site: "In the Left-hand Creek basin to the north of Four Mile Canyon Creek, the mountain towns, mining camps and upstream canyons of the area were perhaps the most severely affected by the 1894 flood. Most were virtually wiped off the map.....Left-hand Canyon areas sustained heavy damages. All bridges were washed out and roads obliterated. Sheriff Dyer stated that Left-hand Creek was over a half mile wide in places and that the farms along that creek were piled knee high with debris and sand....8.54 inches of rain from May 30 through June 1 was reported in Ward.” The estimated water flow on Boulder Creek at 4th Street was 11,000-13,500 cfs (cubic feet per second).
A photo of what was supposed to be Canyon St. in Boulder during the 1894 flood. A caption that goes along with this photo on the Boulder Flood Safety web page says “The eventual repeat flood of this magnitude may well cause hundreds of millions of dollars of damage.” It just may be that that day has arrived. Image from Denver Public Library Archives.
KUDOS: Thanks to Bob Henson of UCAR for updating precipitation totals in Boulder.
Christopher C. Burt
Weather Historian
Extreme Weather Flood Precipitation Records
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