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Colorado Flooding: Echoes of Alberta


By Dayna Vettese
Meteorologist
@daynavettese
Tuesday, September 17, 2013, 3:11 PM

A stalled weather system brought a deluge of rain to the Foothills resulting in catastrophic flooding downstream of the heaviest rainfall amounts.

Sound familiar? To many Canadians, this is an echo of June 2013. Travel south from Calgary, about 1500 km to where the Foothills of the Rocky Mountains meet the Great Plains of the United States, and we find another devastating flooding event in Colorado State.

With at least eight people dead, 70 bridges damaged or destroyed and 1500 homes destroyed, the flooding in Colorado is slowly becoming a $1 billion disaster.

THE METEOROLOGY

From September 9 to September 15, it rained and it rained a lot.

An upper level area of low pressure was situated over the Great Basin and was being blocked by high pressure to its north and east. The Jetstream, upper level current that steers pressure systems in the atmosphere, was sitting north across Canada so the weather pattern across the central United States was fairly stagnant. This stalled weather pattern was then able to draw record breaking amounts of moisture from the tropics right into the Great Plains and west toward the Foothills. In fact, the airmass was so sodden that it broke a record for the greatest amount of moisture in the atmosphere for the Front Range area of Colorado. The weather pattern was such that moisture was being drawn toward the Foothills and forced up enhancing the lift necessary to form clouds and rain. This pattern also allowed for bands of heavy rain and thunderstorms to “train” (continue to affect the same areas over and over). All of this rainfall inundated rivers and streams as it drained downstream out of the Foothills creating catastrophic flooding in the Centennial State, Colorado.

THE NUMBERS AND RECORDS

The numbers that were being reported during the flooding event were historical. The National Weather Service referred to the situation as “biblical rainfall amounts”.

From September 9 through September 13, Boulder, Colorado recorded about 375 mm of rain (14.71 inches). Over the week-long period from when the rain began, Boulder recorded almost 440 mm of rain (17.16 inches). On September 12 alone, about 230 mm of rain fell on the Boulder weather station (9.08 inches). The average rainfall for Boulder in September? Just 41 mm (1.63 inches) and an annual average precipitation of 525 mm including rain and melted snow (20.68 inches). Some areas during the event received over 250 mm of rain in under six hours!

As you can imagine, receiving over three quarters of the average annual precipitation in a week resulted in disastrous flooding. The National Weather Service in Boulder released a statement on Monday, September 16 detailing that Boulder has now officially broken their all-time annual precipitation record with a running total this year of 765 mm (30.12 inches). The previous record set in 1995 with 748 mm (29.43 inches). So this historical event broke daily, monthly and yearly records for Boulder, Colorado with record keeping going back to 1893.

When discussing flooding events, we often discuss the frequency of the event. Based on date from the National Weather Service Frequency Data Server, it was determined that this flooding event for certain affected areas could be deemed a 1-in-1000 year event. This does not mean that an event like this will occur every 1000 years but rather than in any given year, there is a 0.1% chance that an event like this would occur. Some other affected areas were 1-in-100 year events and 1-in-500 year events. That being said, it is difficult to discern the frequencies of these events due to the ever changing physical geography of areas: the rerouting of streams, dams, zoning changes to land, urbanization, etc. The best source though still remains river gauges in the area. In fact, the river gauge for Boulder Creek near downtown Boulder crested at about 2.4 metres (7.78 feet) during this event which is the highest water observed in downtown Boulder since the devastating 1894 flood.


SEE ALSO: Calgary floods: It could happen again


DOWNSTREAM DANGERS

As the flood waters slowly recede and the long recovery road begins for parts of Colorado, those downstream along the South Platte River in Nebraska are on high alert. The great majority of the heavy rain that fell, fell over the South Platte River Basin and the river is rising downstream. Exact flood stages are still uncertain but a 3+ metre crest is moving its way downstream.

ECHOES OF ALBERTA

Colorado and Alberta, physically and from a weather perspective, are very similar. They both are home to the Rocky Mountains and their Foothills. The changeable weather patterns of Alberta are the same in Colorado with wild temperature changes and upslope induced thunderstorms.

How does this event in Colorado resemble the flooding in southern Alberta back in June 2013? The weather patterns that lead to the flooding were very similar, just shifted southward for Colorado. Moist air forced upwards over the terrain of the Foothills lead to the increased rainfall in both situations. The rainfall totals were higher in the Colorado event but a lot of that has to do with Colorado’s proximity to the moisture source of the tropics. In both events, areas to the north and west of the flooding had record breaking warmth. During the Alberta flooding event, areas in the Yukon and Northwest Territories experienced record warmth. This past week, north and west of the flooding in Colorado, parts of the Canadian Prairies including Alberta were experienced record-breaking warm temperatures. In both situations, very heavy rainfall amounts were forecasted in advance and as was the same in Alberta and Colorado, the exact impacts of these rainfalls were unknown. We knew there would be flooding in both cases but how exactly the individual rivers and streams react is difficult to know well ahead of time.

Upper level analysis of the weather pattern leading to Alberta flooding.

Upper level analysis of the weather pattern leading to Alberta flooding.

Upper level analysis of the weather pattern leading to Colorado flooding.

Upper level analysis of the weather pattern leading to Colorado flooding.

Colorado and Alberta both have a vast history of flooding events and now have another historical flooding event from 2013 to add to the list. The Alberta flooding event was a 1-in-70 year event but that’s not to say that certain areas of Alberta experienced more historical flooding than a 1-in-70 year, there just isn’t the amount of historical and current weather data available in that area. As Albertans discovered, there are many lessons to be learned from the Colorado flooding event and the hope is that we can apply the knowledge we have gained from the 2013 floods to preventing such disasters in the future.

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