Drought relief and a boost to water supply for the coming year

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By Jeff Lukas, CIRES Western Water Assessment

A month has passed since the devastating floods on the Front Range, and it’s a little easier to discuss the positive effects of the extraordinary rain event for the impacted area, and across Colorado.

The first was the significant drought relief that the rains brought, as shown in the U.S. Drought Monitor. The Drought Monitor synthesizes several different drought indicators into a single scale of drought severity, ranging from D0 (abnormally dry; conditions are generally below the 30th percentile) to D4 (exceptional drought; conditions are generally below the 2nd percentile).

Prior to the rain event, conditions on the Front Range ranged from no drought in western Boulder and Larimer Counties, to D3 (extreme drought) in parts of Pueblo County, with a large swath of D1 (moderate drought) and D2 (severe drought) in between. After the 6” to 18” of rain—roughly 25% to 100% of average annual precipitation falling in one week—all of the Front Range was upgraded by one to three categories, with the drought-free area expanding to cover most of the urban areas and the northern headwaters of the South Platte River. The rest of Colorado also benefited from the lesser—yet still well-above-average—precipitation amounts received there.

In addition, while most of the flood runoff rolled out of the state without being stored, the small to mid-sized Front Range reservoirs with direct stream inflows like Gross Reservoir refilled at a time of year when storage is usually declining fast from an early-summer peak.

On the other side of the Continental Divide, larger reservoirs like Dillon Reservoir also got a big boost. Even enormous Lake Powell far downstream on the Colorado River saw a gain of about 200,000 acre-feet—nearly the entire capacity of Dillon Reservoir—during a period in which it was expected to decline by half that amount. The rains also led to a dramatic reduction in outdoor and agricultural water use, which also benefited the reservoirs.

NCAR/UCAR flood panel

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If you missed the Great Front Range Flood of 2013 research seminar, you can now check it out here. Hydrometeorology experts from NCAR and UCAR combine weather and water models for better prediction of floods. The seminar presented what has been learned so far from observations and modeling, including coupled hydrologic modeling. The presenters also discussed a vision for an end-to-end hydrometeorological prediction system for tackling the challenging problem of flash flood prediction.



Friday afternoon webinar from UCAR

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NCAR’s Research Applications Laboratory and UCAR’s COMET program present a webinar this afternoon (Friday):

The Great Front Range Flood of 2013

Rita Roberts, Jenny Sun, Dave Gochis, Barbara Brown, and Matt Kelsch

The deadly flood that occurred in the Front Range during the week of September 10th, 2013 is believed to be the worst in the state’s history. It claimed several lives and damaged numerous homes and roads. In the context of NCAR’s Short-Term Explicit Prediction (STEP) program, an effort is underway to understand the flood event and to improve our forecasting of future heavy rainfall and flood events. In this seminar, we will present what has been learned so far from observations and modeling, including coupled hydrologic modeling. We will also discuss a vision for an end-to-end hydrometeorological prediction system for tackling the challenging problem of flash flood prediction.

Friday, October 4, 2013


FL 2 – Room 1022

Webcast link:  http://www.fin.ucar.edu/it/mms/fl-live.htm

Flood panel video link

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Did you miss it? If you could not attend the CIRES Western Water Assessment panel discussion on flood, weather, climate today, here it is:  http://cirescolorado.adobeconnect.com/p751cdslj7x/

Our apologies to those who couldn’t get video feed during the panel. We are working to understand the glitch — some people saw it, others did not, creating understandable frustration.

Flood panel & webinar

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Expert panel, Wednesday, 11 am MT:

Extreme weather and connections to climate change: How unusual were September’s floods?

A panel of science experts will convene at the University of Colorado Boulder Wednesday, to discuss weather and climate related to the recent devastating floods.

Panelists will discuss the unusual weather conditions that caused the floods, the historical context and the potential influence of human-caused climate change on this extreme event. CIRES’ Western Water Assessment (WWA) is convening the panel and also will release a brief preliminary assessment of the severe flooding, including information on weather, water, climate and risk.

The panel discussion begins at 11 a.m. in the CIRES auditorium, room 338 in the CIRES building, and will last about 90 minutes. The event is open to the public but space is extremely limited. The event also will be available via webinar, instructions below.

Jeff Lukas, senior research associate with WWA, will lead off the discussion with an overview of the weather and climate context of the flood. Other panelists will include:

  • Kelly Mahoney, a CIRES research scientist who works in NOAA’s Earth System Research Laboratory. Mahoney is an expert on Front Range thunderstorms.
  • Klaus Wolter, a CIRES research scientist who works in NOAA’s Earth System Research Laboratory. Wolter is an expert on extreme weather and seasonal weather patterns in the Front Range. He also is a Jamestown resident.
  • Nolan Doesken, state climatologist for Colorado at Colorado State University. Doesken has an encyclopedic knowledge of Colorado’s climate and weather history.
  • Marty Hoerling, research meteorologist with NOAA’s Earth System Research Laboratory. Hoerling is an expert on attribution of climate conditions to weather events.

Space is limited. Reserve your webinar seat now at: https://www2.gotomeeting.com/register/586012690. After registering with your name and email, you will receive a confirmation email containing information about joining the webinar.

75 years later, weather history repeats itself

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by Jeff Lukas, Western Water Assessment, CIRES

In early September, a favorable atmospheric circulation pattern sent copious subtropical moisture streaming towards the Front Range. As that pattern persisted for three days, areas of heavy rainfall fired up repeatedly over the Front Range foothills, leading to 24-hour amounts over 5 inches in some locations and 3-day totals over 8 inches. The cloudbursts caused flooding in nearly every drainage from Colorado Springs to the Wyoming border, including Bear Creek, Coal Creek, South Boulder Creek, St. Vrain Creek, the Big Thompson River, and the Cache la Poudre River.

Bear Creek burst its banks and badly damaged downtown Morrison. Rampaging South Boulder Creek carried off parts of the Eldorado Springs Resort and undermined streamside houses. In Longmont, the northern residential district was flooded. And in Fort Collins, much of the campus was inundated. At least seven people were killed in the flooding. Never before had so much flood damage occurred over such a broad north-south swath of the Front Range.

While this sounds a lot like what happened 10 days ago, the extraordinary floods described above occurred from September 1st-3rd, 1938, 75 years ago this month. Afterwards, that disaster faded into history, but as the more recent floods subsided, it has reemerged to clearly answer the questions, “Has anything like this ever happened before?” and “Have we ever had severe flooding in September?”

While the total precipitation amounts in the shorter-duration 1938 event were not as high as in 2013’s, the flooding was of a similar magnitude across the affected area, perhaps because there was more convective activity (thunderstorms) in 1938, leading to higher peak rainfall intensities than in 2013.

If we query the NOAA 20th-century reanalysis dataset—which blends historic surface and upper observations in a numerical weather model to create detailed and consistent images of past atmospheric circulation—we can do a quick comparison of the setup for the 1938 event with that for 2013. What we find is that the patterns were remarkably similar: a low pressure area to the west that drew moisture from the south and east, and blocking highs that kept the moisture flow pinned in place for several days.

While research is ongoing at CIRES to investigate how the 2013 flood may have been influenced by human-caused climate change, the 1938 flood tells us that the climate system was capable of producing that type of flood event—in September, no less—under more “natural” atmospheric conditions.

Thanks to Joe Barsugli and Klaus Wolter, CIRES and NOAA ESRL PSD.