Nuevo Leon/Coahuila Rainfall Reviewed

Left: Two-week analyzed rainfall
Analysis: West Gulf River Forecast Center

Here is an excellent blogpost related to that seriously-undercovered story: the floods in northeastern Mexico.

When Alex was coming in, I was floored by the forecast precipitation being generated by the NOGAPS model over at FNMOC WXMAP. Amazing amounts! Heavy rain was forecast to fall in places that were either among Mexico's most arid areas, or adjacent to them.

Desert areas generally cope poorly with high rainfall. High caliche content in the soils make the rain run off instead of soak in. It is exceedingly sad that the forecasts were not inaccurate. And these areas still remain vulnerable to heavy rains. Indeed, even light rains can cause chaos right now, given how soaked the ground already is.

Here is a sampling from the blogpost:

Hurricane Alex may have come and gone, but it brought copious rains to Mexico and has temporarily eliminated the capping inversion that normally suppresses summertime convection in the area. As a result, eye-popping rainfall has come to the northeastern Mexico states of Nuevo Leon and Coahuila.

[This] analysis ... was produced by the National Weather Service's West Gulf River Forecast Center, and is based on a combination of radar, satellite, and rain gauge data. The pinks indicate rainfall totals over the past two weeks exceeding 10 inches. Purple is 15 inches, and white is 20 inches. As you can see, almost the entire eastern slope of the Sierra Madre Oriental in Coahuila is estimated to have received over 20 inches of rain in the past two weeks. Most of that fell during the past week.

Near the bottom of the image, a second area of heavy rainfall is centered around the major Mexican city of Monterrey. Monterrey is located near the base of the Sierra Madre Oriental. According to rain gauge data from CONAGUA, Monterrey itself received 11 inches (282 mm) of rain on July 1 and a three-day total of 18 inches (456 mm). Just upstream, Santa Catarina received 14 inches (355 mm) in one day and 24 inches (610 mm) in three days. This rain was directly associated with Alex itself.

Not surprisingly, all this rain has caused problems. At last count, 15 deaths were attributed to the flooding, according to the Mexican newspaper La Prensa. The urban flooding was considerable.

The normal annual rainfall for the state of Nuevo Leon is about 24 inches (602 mm). The five stations reporting from Nuevo Leon received an average of 13 inches (329 mm) in the past week. Farther north, the normal annual rainfall for the state of Coahuila is about 13 inches (327 mm). The eleven stations reporting from Coahuila received an average of 7 inches (184 mm) in the past week.

None of the Coahuila stations were in the bulls-eye 20 inch region. Two were on the eastern margin: Santa Cecilia and Nueva Rosita, both of which have received at least 16 inches (400 mm) of rain since June 26, more than their normal annual average. Nueva Rosita reported 8 inches (202 mm) on July 5 alone. These observations give me some confidence that the 20+ inches analyzed over a broad area just upstream is realistic.

The northern end of the heavy rain area is in the drainage basin for Amistad Reservoir, and reservoir operators have increased their releases of water to maintain flood control capacity. Meanwhile, other tributaries flowing into the Rio Grande below Amistad have contributed to flood conditions in areas such as Laredo, between Amistad and Falcon reservoirs, causing some border bridges to close.