Epic Rains, Disastrous Floods Plague Texas, Oklahoma

Water cascaded through the streets, creeks, and bayous of downtown Austin and Houston on Monday as an upper-level storm inched its way across the southern Great Plains. Slow-moving thunderstorms dumped 6” to 8” across the western Houston metro area between 8:00 and 11:00 p.m., and heavy rains continued well past midnight across much of the south and west metro area, bringing some totals as high as 10+”. Though the Houston flooding came well short of that in 2001’s catastrophic Tropical Storm Allison, countless roads and interstate highways were submerged, and hundreds of homes reportedly took water. This was the latest salvo in a remarkable three-day stretch of torrential rain and destructive flooding across much of Oklahoma and Texas and parts of neighboring states. As of Tuesday morning, the floods had taken at least 8 lives, with at least 12 people missing, and damaged or destroyed many hundreds of buildings.


Figure 1. Damage along River Road in Wimberley, Texas, from a devastating flash flood on Saturday night, May 23. Photo credit: Jerry Lara/The San Antonio Express-News, via AP.

Some of the worst damage occurred in and near the much-touristed town of Wimberley, located along the Blanco River in the Texas Hill Country west of Austin. After a mesoscale convective system (MCS) dropped upward of 7” upstream of Wimberley on Saturday afternoon, the river surged to a new record high in spectacular fashion later that night (see Figure 2 below). The Hill Country has a tragic history of flash flooding, and prompt evacuations no doubt saved many lives. This weekend also saw several “flash flood emergency” declarations by the National Weather Service. This wording is reserved for the highest-end events, much like the “tornado emergency” tag issued when substantial numbers of people are in particular danger. From Saturday through Monday, flash flood emergencies were declared in parts of the Tulsa, Austin, and Houston areas, as well as five counties in west-central Oklahoma. It was the first time that Tulsa and Houston had ever been placed under such alerts.



Figure 2. The level of the Blanco River at Wimberly soared from a more 5 feet on Saturday afternoon, May 23, to a record 40.21 feet at 1:01 am CDT Sunday, more than 7 feet above the prior record. The river gauge stopped reporting after this point. Image credit: NWS Advanced Hydrologic Prediction Service.


Figure 3. Infrared satellite imagery shows the extremely cold cloud tops (gray) associated with the torrential rains in the Houston area at 9:15 pm CDT on Monday night, May 25. Cloud-top temperatures at the height of the storms were colder than -100°F. Note the two eerie faces visible across southeast Texas! Larger version here [with time code but without state boundaries]. Image credit: NASA Earth Science Office, courtesy Stu Ostro.


The scope and intensity of this weekend’s rains resulted from a rich feed of low-level moisture pumping into the region from the Gulf of Mexico, combined with the excruciatingly slow movement of the large-scale storm system. Upper-level winds were largely aligned with the low-level frontal zone, an ideal setup for “training echoes” (successive downpours over the same area). In addition, rains were concentrated by several mesoscale convective vortices, small-scale centers of low-pressure that developed along the frontal bands, in some cases resembling mini-tropical cyclones.


Figure 4. Damaged homes stand next to others that were razed when a powerful tornado tore through a seven-block area in Ciudad Acuna, Mexico, on Monday morning, May 25. Image credit: AP.

A deadly tornado strikes northern Mexico at daybreak
Although flooding was the main threat across Texas and Oklahoma, nearly 100 tornado reports were logged nationwide by the NOAA Storm Prediction Center over the three-day holiday weekend. The deadliest twister so far this year in North America struck on Monday morning in Cuidad Acuña, Mexico, a city of about 200,000 located on the Rio Grande across from Del Rio, Texas. With reported wind estimates in the EF4 range, the tornado killed at least 13 people and injured at least 150, with more than 1,000 homes damaged. [Update: Maria de Jesus Guadalulpe Gallo Banda, from Mexico's National Water Commission, has estimated that peak gusts were 220 kilometers per hour (137 mph), which would correspond to an EF3 rating, or an F2 rating in the original Fujita scale, which is still used in Mexico.] The powerful twister emerged around 7:00 am CDT from an isolated supercell thunderstorm that likely benefited from very strong dynamics associated with an upper-level jet streak plowing across Mexico (see Figure 4). Monday’s disaster brings to mind a similar event that occurred 60 miles to the southeast of Cuidad Acuña on April 24, 2007, when a tornado produced F4 damage in Piedras Negras, Mexico, killing 3 people, and EF3 damage across the Rio Grande in Eagle Pass, Texas, where 7 people died. In general, North American tornadoes this strong are very unusual at such a low latitude. No F5/EF5 events have ever been confirmed this far south in the U.S. or Mexico, and TornadoHistoryProject.com shows that only 10 of 555 U.S. tornadoes rated F4 or EF4 between 1950 and 2014 occurred near or south of 30°N. Six of these were in Texas, two in Louisiana, and two in Florida. Half of them occurred during El Niño conditions (about twice the number you would expect based on El Niño frequency alone), and half occurred between 1:00 am and 11:00 am local time. The strong subtropical jet streams associated with El Niño typically bring very cold upper-level air, which enhances instability, and the powerful jet-level winds help ventilate the rising air within thunderstorms. Moreover, at these low latitudes, surface temperature and moisture values can be high enough to produce very unstable air even during the late night and early morning.


Figure 5. WunderMap analysis of flow at the 200-hPa level (about 39,000 feet) at 1200 GMT on Monday, May 25. A powerful subtropical jet can be seen flowing eastward across Mexico, then northward through the Mississippi Valley. The tornadic supercell at Cuidad Acuna, Mexico, occurred about an hour before the time of this analysis, near the left front corner of a powerful “jet max” (the greenish strip across Mexico). Severe storms tend to occur most often at the left front and right rear of jet maxima, where upward motion is favored. Also evident is the mammoth ridge extending from western Canada into the Arctic Ocean, where record-high temperatures have been observed in recent days. The northwestern ridge and western U.S. trough have persisted through much of May.

Precipitation persistence: the story of 2015
One of the most intriguing questions in climate change research is whether blocking-type patterns might be fostered by rising global temperatures and the resulting effects on jet-stream behavior. Over the last month, the same pattern favoring heavy rain across Texas and Oklahoma has kept rain away from the Northeast. It was just three months ago that an unprecedented month-long stretch of heavy snowfall brought Boston and much of New England to its frosty knees. A couple of individual snowstorms within that stretch were among Boston’s heaviest, but it was the relentlessness of the cold, snowy conditions that truly stood out and caused such misery. Likewise, the unrelenting rainfall across the southern Plains this month has caused pile-on effects, as downpours flow off saturated soil and farmers struggle to get spring crops planted. During the 30 days ending on May 25, Norman, OK, received an astounding 24.10”. Oklahoma is now assured of its wettest month on record, according to the Oklahoma Climatological Survey.


Figure 6. 30-day precipitation from 1200 GMT April 25 through May 25, 2015. Some parts of southern Oklahoma and far north Texas have received more than 20” of rain, while central New England has recorded less than half an inch. Image credit: NWS Advanced Hydrologic Prediction Service.

Here are some individual locations that have already set records for their wettest May as of midnight Monday night, with nearly a week left to go in the month. (Thanks to Nick Wiltgen at the Weather Channel for compiling these statistics.)

Oklahoma City, OK: 18.85” (previous May record 14.52” in 2013; previous all-time record 14.66” in June 1989)
Fort Smith, AR: 18.07” (previous May record 13.67” in 1943; previous all-time record 15.02” in June 1945)
Austin, TX (Camp Mabry): 16.72” (previous May record 14.10” in 1895; all-time record 20.78” in Sept. 1921)
Wichita Falls, TX: 14.15” (previous all-time record 13.22” in May 1982)

Meanwhile, back in the Northeast, the tables have turned in a way that might have seemed hard to fathom three months ago. The most recent U.S. Drought Monitor shows moderate drought (D1) covering all of Massachusetts, Connecticut, and Rhode Island, as well as parts of New York, Maine, and Pennsylvania. Even the enormous snowfalls of February didn’t contain especially large amounts of moisture compared to what can fall in a midwinter New England rainstorm, and the tap hasn’t been flowing much at all lately. In the 30 days ending on May 26, less than an inch of rain fell across most of New Jersey, southeast New York, and southern New England, with some locations getting little more than sprinkles. Further south, only a trace has fallen this month in Charlotte, North Carolina. Reservoirs and water tables in New England are still in relatively good shape, but streamflows are quite low. Potential records for driest or second-driest May at locations with more than a century of data include:

Concord, NH: 0.07” (record low 0.50”, 2008; records begin in 1903)
Providence, RI: 0.51” (record low 0.57”, 1939; records begin in 1904)
Hartford, CT: 0.60” (record low 0.73”, 1959; records begin in 1905)
Boston, MA: 0.31” (record low 0.25”, 1944; runner-up 0.32”, 1903; records begin in 1872)
Albany, NY: 0.31” (record low 0.15”, 1903; runner-up 0.73”, 1920; records begin in 1874)
New York, NY (Central Park): 0.32” (record low 0.30”, 1903; runner-up 0.34”, 1887; records begin in 1871).

A change in the prevailing upper-level features toward more zonal (west-to-east flow) across North America, as outlined by Steve Gregory in his latest blog, may help put some of these dry-May records out of reach. A midweek front will bring scattered showers and thunderstorms, with a better chance of widespread wetting rains over the weekend into early next week as a front stalls across the mid-Atlantic.

Watch for a report from Jeff Masters on NOAA’s Atlantic hurricane outlook for 2015, scheduled for release on Wednesday. We will also be covering the devastating pre-monsoonal heat wave that has taken hundreds of lives across India.

Bob Henson


Figure 7. Departures from average outgoing longwave radiation (OLR) for the period from April 23 to May 22, 2015. OLR is maximized when skies are clear and minimized when outgoing radiation is blocked by clouds and precipitation. A belt of lower-than-average OLR values extends from the eastern tropical Pacific (where El Niño has warmed the sea surface) to the central and western United States, illustrating the likely connection between El Niño and this month’s heavy rainfall. See the AGU blog by Dan Satterfield for more on this apparent link. Image credit: NOAA/ESRL/PSD, courtesy Eric Blake.


Video 1. Barton Springs Rapids, on Barton Creek within the city limits of Austin, lived up to its name on Sunday, May 24, prior to the area’s second round of major flooding on Monday. If embedded video does not play, it can be viewed here. Image credit: Austin Parks and Recreation Department.


Figure 8. An ironic scene from the town of Cache, in southwest OK, on Sunday, May 24. Image credit: wunderphotographer lmalcolm.