Sunshine Aids Colorado Evacuations; 38 Dead in Mexico From Manuel and Ingrid

After nine consecutive days with rain, skies have finally cleared over flood-ravaged Boulder, Colorado. Flooding from the past week's rains have killed at least seven, destroyed over 1,500 homes, damaged 18,000 homes, and caused close to $1 billion in damage, according to insurance broker Aon Benfield. Over 70 bridges have been damaged or destroyed, and hundreds of roads damaged, including major sections of U.S. Highways 34, 36 and 72. Clear skies are forecast for the remainder of the week, which will allow rescue helicopters to safely operate to evacuate the hundreds of people still trapped in mountain towns cut off by the rockslides, collapsed bridges, and destroyed roads. The rains that fell early Monday morning in Boulder officially put the city over its all-time annual precipitation record with three and a half months left in the year. Boulder has been deluged with 30.13" so far this year; the previous record was 29.93", set in 1995.


Figure 1. A raging waterfall destroys a bridge along Highway 34 east of Estes Park, Colorado, on September 15, 2013. (AP Photo/Colorado Heli-Ops, Dennis Pierce)


Figure 2. Damage to Highway 34 along the Big Thompson River, on the road to Estes Park, Colorado. Image credit: Colorado National Guard.


Figure 3. A field of parked cars and trucks sits partially submerged near Greeley, Colorado, Saturday, Sept. 14, 2013, as debris-filled rivers flooded into towns and farms miles from the Rockies (AP Photo/John Wark).

A 1-in-1,000 year flood and rainfall event
The Colorado Emergency Management Agency reported that “Some areas in Larimer County experienced a 100-year flood and other areas experienced a 1,000-year flood. It all depends on where the heaviest rain fell. Areas with more extensive damage experienced the 1,000 year flooding.” The U.S. Geological Survey office in Colorado called the flood of Boulder Creek in the city of Boulder as at least a 1-in-100-year event. In the towns of Lyons and Estes Park, officials separately described the current event in each area as a 1-in-500-year flood. According to Bob Henson of the National Center for Atmospheric Research in Boulder, 17.16" of rain fell in the past week in Boulder. Using about a century of precipitation records, NOAA has constructed a Precipitation Frequency Data Server, which estimates how often we might expect to see extreme rainfall events recur. For Boulder, a 5.87" rain event in one week has an average recurrence interval of once every 1,000 years. The city received almost triple that amount of rain over the past week--a truly extreme and rare weather event.

As extreme as the 2013 Colorado flood was, there are two flood events in Colorado history that compare. One was the June 1965 flood that hit the Colorado Front Range, causing $4 billion in damage (adjusted for inflation.) Colorado's deadliest flood on record was the Big Thompson Flood of 1976, which killed 145 people between Estes Park and Loveland. More than 12" of rain fell in just three hours, causing a flood rated at between a 500-year and 1,000-year event.


Figure 4. The rains that fell in a 24-hour period ending at 12 pm MDT September 13, 2013 over regions near Boulder, Colorado were the type of rains with a 0.2% chance of falling in a particular year, or once every 500 years (purple colors), according to MetStat, Inc. (http://www.metstat.com.) MetStat computed the recurrence interval statistics based on gauge-adjusted radar precipitation and frequency estimates from NOAA Atlas 14 Volume 8, published in 2013 (http://dipper.nws.noaa.gov/hdsc/pfds/.) MetStat does not supply their precipitation recurrence interval forecasts or premium analysis products for free, but anyone can monitor the real-time analysis (observed) at: http://metstat.com/solutions/extreme-precipitation-index-analysis/

Colorado's rains set to cause record flooding in Nebraska
Much of the water from the past week's record rains in Colorado have funneled into the South Platte River, which flows eastwards in Nebraska. A 10 - 11' high flood crest is headed downriver, and is setting all-time flood height records as it heads east. By Wednesday night, the crest will reach western Nebraska at Roscoe, where the flood waters may cover Interstate 80, unless sandbagging efforts to protect the highway are done. Interstate 80 is one of the two most heavily traveled transcontinental highways in the United States.


Figure 5. Observed and predicted flood heights on the South Platte River in Western Nebraska, where an all-time record flood is expected on Wednesday. Records at this gauge go back 30 years. Image credit: NOAA/

Links
Bob Henson of the National Center for Atmospheric Research in Boulder summarizes the great flood.

Wunderblogger Dr. Ricky Rood has a home in Boulder, and discusses his take on the flood.

Colorado’s ‘Biblical’ Flood in Line with Climate Trends by Andrew Freedman of Climate Central.

Wunderground weather historian Christopher C. Burt discusses how this year's flood compares to previous Colorado floods in his latest post.

A map of Boulder flood zones and detailed history of previous floods in the area may be found here.


Video 1. A slow-motion mudslide pours into Boulder Creek, Colorado on September 14, 2013.

Ingrid and Manuel kill 38 in Mexico; Invest 95L headed for Bay of Campeche
Flooding from the combined one-two punch of Hurricane Ingrid on the Atlantic coast and Tropical Storm Manuel on the Pacific coast is being blamed for the deaths of at least 38 people in Mexico, according to AP. The two storms hit Mexico nearly simultaneously on Monday, packing sustained winds of 65 - 70 mph and torrential rains. Hardest hit was the Acapulco region on Mexico's Pacific coast, where the airport is closed, many roads flooded and blocked, and much of the city without water or power.

The waterlogged Gulf Coast of Mexico has yet another tropical rain-making system to worry about this week. An area of disturbed weather (Invest 95L) over the Yucatan Peninsula will emerge into the Southwestern Gulf of Mexico's Bay of Campeche on Wednesday. Belize radar and satellite loops show that 95L already has a pronounced spin and a respectable area of heavy thunderstorms. Wind shear is low and is expected to stay low over the next five days. There is some dry air over the Bay of Campeche, but I doubt this will be an impediment to development, given the low wind shear. NHC gave the disturbance 2-day development odds of 30% and 5-day odds of 50% in their 8 am EDT Tuesday Tropical Weather Outlook. The disturbance is likely to stay trapped in the Bay of Campeche and take a slow west-northwest path towards the same region of coast affected by Hurricane Ingrid. On Saturday, a cold front is expected to push southeastwards over the Gulf of Mexico, and moisture from 95L will likely stream northeastwards along the cold front over much of the U.S. Gulf Coast, bringing heavy rains on Saturday and Sunday. A non-tropical low pressure system could form along this front and move northeastwards into the Florida Gulf Coast, bringing heavy rains to the Southeast U.S. on Sunday and Monday.

Jeff Masters