Flooding death toll in Southeast U.S. floods rises to 24; oil slick moving little

The death toll from last weekend's record flooding in Tennessee, Kentucky, and Mississippi has risen to 24, making it the deadliest non-tropical storm or hurricane flood disaster in the U.S. since the October 1998 Central Texas floods that killed 31 when a cold front stalled over Texas. As flood waters recede today, the toll from last weekend's floods is expected to grow higher. Particularly hard-hit was the Nashville, Tennessee area, where ten fatalities were reported. The city had its heaviest 1-day and 2-day rainfall amounts in its history over the weekend. A remarkable 7.25" of rain fell on the city Sunday, breaking the record for most rain in a single day (6.60", set September 13, 1979.) Nashville's third greatest day of rainfall on record occurred Saturday, when 6.32" fell. Nashville also eclipsed its greatest 6-hour and 12-hour rainfall events on record, with 5.57" and 7.20", respectively, falling on Sunday. And, only two days into the month, the weekend rains made it the rainiest May in Nashville's history.

Rainfall records were smashed all across Tennessee, Kentucky, and northern Mississippi over the weekend, with amounts as high as 17.73" recorded at Camden, TN, and 17.02" at Brownsville, TN. According to Chris Burt, the author of the excellent book Extreme Weather, the 13.30" that fell on Camden in 24 hours just missed eclipsing the state's all-time 24-hour precipitation record, the 13.60" inches that fell on Milan on September 13, 1982. Jackson, Tennessee had its rainiest day in its 63-year weather history on Sunday, 7.93". Bowling Green Kentucky had its heaviest 2-day precipitation event on record, 9.67". Records in Bowling Green go back to 1870.


Figure 1. Satellite-estimated precipitable water at 23 UTC (7 pm EDT) Sunday, May 2, 2010. Precipitable water is a measure of how much rain would be produced if all the water vapor and cloud moisture through the depth of the atmosphere were to fall as rain. Values above 50 mm (about 2 inches) are frequently associated with flooding. Sunday's precipitable water image showed a tropical disturbance crossed Mexico into the Gulf of Mexico, dragging a plume of very moist air northwards over the Southeast U.S. Image credit: University of Wisconsin GOES Satellite Blog.


Figure 2. Flood forecast for the Cumberland River in Nashville, Tennessee. Image credit: NOAA.

The record rains were accompanied by a surge of very warm air that set record high temperature marks at 21 major airports across the Eastern U.S. on Saturday. This is not surprising, since more moisture can evaporate into warmer air, making record-setting rainfall events more likely when record high temperatures are present. Accompanying this warm air was moisture from a tropical disturbance that crossed over Mexico from the tropical East Pacific over the weekend (Figure 1.)

The record rains sent the Cumberland River in downtown Nashville surging to 51.86' this morning, 12' over flood height, and the highest level the river has reached since a flood control project was completed in the early 1960s. The previous post-flood control project record level was 47.6', set on March 15, 1975 (the river hit 56.2' in 1929, before the flood control project was built.) The river has now crested (Figure 2) and is expected to recede below flood stage by Wednesday morning. There are no further rains in the forecast this week for Tennessee. At least four rivers in Tennessee reached their greatest flood heights on record this week. Most remarkable was the Duck River at Centreville, which crested at 47', a full 25 feet above flood stage, and ten feet higher than the previous record crest, achieved in 1948 (to check out the flood heights, use our wundermap for Nashville with the "USGS River" layer turned on.)

Funding issues to take 17 Tennessee streamgages offline
According to the USGS web site, seventeen Tennessee streamflow gages with records going back up to 85 years will stop collecting data on July 1 because of budget cuts. With up to eighteen people in Tennessee dying from flooding this weekend, now hardly seems to be the time to be skimping on monitoring river flow levels by taking 17 of Tennessee's 94 streamflow gages out of service. These gages are critical for proper issuance of flood warnings to people in harm's way. Furthermore, Tennessee and most of the northern 2/3 of the U.S. can expect a much higher incidence of record flooding in coming decades. This will be driven by two factors: increased urban development causing faster run-off, and an increase in very heavy precipitation events due to global warming. Both factors have already contributed to significant increases in flooding events in recent decades over much of the U.S. The USGS web site advertises that users who can contribute funding for the non-Federal share of costs to continue operation of these streamgages should contact Shannon Williams of the USGS Tennessee Water Science Center at 615-837-4755 or swilliam@usgs.gov. Tennessee is not the only state with streamgages at risk of closing down; fully 276 gages in 37 states have been shut down or will be shut down later this year. If you have questions about specific streamgages, click on the state of concern on the USGS web page of threatened stream gages.

Oil spill update
The oil slick from the April 20 explosion and blowout of the offshore oil rig Deepwater Horizon has retreated from the coast, thanks to a slackening of the persistent onshore winds that have affected the northern Gulf of Mexico over the past week. According to the latest NWS marine forecast, winds will be light and variable through Wednesday, resulting in little transport of the oil slick. Winds will then resume a weak onshore flow at 5 - 10 knots, Thursday through Friday, then reverse to blow offshore at 5 - 10 knots over the weekend. The net result of this wind pattern will be little transport of the oil slick. The only areas at risk of landfalling oil over the next five days will be the mouth of the Mississippi River in Louisiana, and the Chandeleur Islands. The latest forecast of Gulf currents from the NOAA HYCOM model (see also this alternative view of the HYCOM ocean current forecast) show weak ocean currents affecting the region during the remainder of the week. These currents will not be strong enough to push any oil southwards into the Loop Current over the next five days, so the Keys and South Florida are safe from oil for now. I'll have a post on the long-range prospects for oil to enter the Loop Current later this week, and a discussion of how a hurricane might affect and be affected by the oil spill.


Figure 3. Forecast location at 6pm CDT Tuesday, May 4, 2010, of the oil slick from the Deepwater Horizon blowout. Image credit: NOAA Office of Response and Restoration. See also the trajectory maps available at State of Louisiana web site.

Jeff Masters