Boulder's 1-in-100 Year Flood Diminishing; Ingrid a Dangerous Flood Threat for Mexico

Colorado's epic deluge is finally winding down, as a trough of low pressure moves across the state and pushes out the moist, tropical airmass that has brought record-breaking rainfall amounts and flooding. Devastating flash floods swept though numerous canyons along the Front Range of Colorado's Rocky Mountains Wednesday night and Thursday morning, washing out roads, collapsing houses, and killing at least three people. The flood that swept down Boulder Creek into Boulder, Colorado was a 1-in-100 year event, said the U.S. Geological Survey. A flash flood watch continues through noon Friday in Boulder. According to the National Weather Service, Boulder's total 3-day rainfall as of Thursday night was 12.30". Based on data from the NWS Precipitation Frequency Data Server, this was a greater than 1-in-1000 year rainfall event. The city's previous record rainfall for any month, going back to 1897, was 9.59", set in May 1995. Some other rainfall totals through Thursday night include 14.60" at Eldorado Springs, 11.88" at Aurora, and 9.08" at Colorado Springs. These are the sort of rains one expects on the coast in a tropical storm, not in the interior of North America! The rains were due to a strong, slow-moving upper level low pressure system to the west of Colorado that got trapped to the south of an unusually strong ridge of high pressure over Western Canada. This is the same sort of odd atmospheric flow pattern that led to the most expensive flood disaster in Canadian history, the $5.3 billion Calgary flood of mid-June this summer. The upper-level low responsible for this week's Colorado flood drove a southeasterly flow of extremely moist tropical air from Mexico that pushed up against the mountains and was lifted over a stationary front draped over the mountains. As the air flowed uphill and over the front, it expanded and cooled, forcing the moisture in it to fall as rain. Balloon soundings from Denver this morning continued to show levels of September moisture among the highest on record for the station, as measured by the total Precipitable Water (PW), which is how much water would fall at the ground if the entire amount of water vapor through the depth of the atmosphere was condensed. Four of the top eight all-time September highs for Precipitable Water since records began in 1948 have been recorded over the past two days:

1.33" 12Z September 12, 2013
1.31" 00Z September 12, 2013
1.24" 12Z September 13, 2013
1.23" 12Z September 10, 1980
1.22" 00Z September 2, 1997
1.21" 00Z September 7, 2002
1.20" 00Z September 13, 2013

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.


Figure 1. A torrent of water rushes alongside a swamped house following flash flooding near Left Hand Canyon, south of Lyons, Colo., Sept 12, 2013. (AP Photo/Brennan Linsley)



Figure 2. Observed rainfall for the Colorado's Front Range from the September 11 - 13 rain event. Rainfall amounts greater than 10" (pink colors) were indicated near Boulder. Image credit: NWS Denver.

Tropical Storm Ingrid a Dangerous Rainfall Threat for Mexico
Tropical Storm Ingrid, the ninth named storm of the 2013 Atlantic hurricane season, has formed in the Southwestern Gulf of Mexico's Bay of Campeche. Ingrid is the most dangerous Atlantic tropical cyclone of 2013 thus far, due to its rainfall potential. Ingrid is embedded in an exceptionally moist environment, and is already bringing heavy rains to the Mexican coast in Veracruz state, as seen on Mexican radar. Satellite loops show that Ingrid is not well-organized, and has only a modest area of heavy thunderstorms. However, the Friday morning hurricane hunter mission found 45 mph winds, prompting NHC to upgrade Ingrid. Moderate wind shear of 15 - 20 knots is interfering with development, but ocean temperatures are a very warm 29 - 29.5°C (84 - 85°F).


Figure 3. Percent chance of receiving more than 16" of rain during a five day period, from the Friday 2 am EDT run of the experimental GFDL ensemble model for Tropical Storm Ingrid. More than 16" of rain are predicted for the Oaxaca and Tampico areas of Mexico. Image credit: NOAA/GFDL.

Forecast for Ingrid
The soils along the Mexican Gulf Coast are already saturated from the rains of Tropical Depression Eight and Tropical Storm Fernand, and it won't take much additional rain to generate dangerous flash floods and mudslides. An added danger is the presence of tropical disturbance 90E in the Eastern Pacific Ocean, on the other side of Mexico. If Ingrid intensifies sufficiently, it could draw in the moisture from 90E across Southern Mexico, resulting in torrential rains on both the Pacific and Atlantic sides of Southern Mexico. Ninety-E represents a threat to develop into a tropical depression in its own right; in their 8 am EDT Friday Tropical Weather Outlook, NHC gave 2-day odds of development of 70% for the disturbance, and predicted a north to northwest motion of the storm towards the coast. This morning's 2 am EDT run of the experimental GFDL ensemble model predicted that a some areas of Mexico are at high risk of 16+ inches of rain due to the combined effects of Ingrid and 90E. The greatest danger is on the Pacific side in Oaxaca state, where the combined effects of the circulations of Ingrid and 90E will pull a flow of very moist air upwards over the mountains, creating torrential rains. All of the models predict a west-northwest to northwest track for Ingrid into Mexico, but heavy rains of 2 - 4" may also affect extreme South Texas by early next week.

Today is the 25th anniversary of the intensification of Hurricane Gilbert into the strongest hurricane ever observed in the Atlantic (at the time.) I was on the hurricane hunter flight into Gilbert that day, and will be posted an account of the mission later today.

Jeff Masters