Flooding from record rains kills 11 in Tennessee; oil spill update

Nashville, Tennessee remains virtually paralyzed this morning thanks to that city's heaviest recorded 1-day and 2-day rainfall in its history. A remarkable 7.25" of rain fell on the city Sunday, breaking the record for most rain in a single day (previously 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, remarkably, only 2 days into the month, May 2010 is already the wettest May on record for Nashville.

Rainfall records were smashed all across Tennessee and Kentucky, 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. The University of Wisconsin GOES Satellite Blog has some excellent imagery showing the remarkable plume of tropical moisture that crossed over Central America from the Eastern Pacific and fed the record rains.


Figure 1. Radar estimated rainfall for May 1 - 2, 2010 from the Nashville, Tennessee radar. A large region of the Tennessee and Kentucky received over ten inches of rain, with many areas receiving more than fifteen inches.


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

The record rains triggered massive flooding that has killed at least eleven people in Tennessee, with two missing. The Cumberland River in downtown Nashville has surged to a height of 51', which is 11' over flood height, and the highest level the river has reached since a flood control project was completed in the early 1960s. The river is expected to crest this afternoon at 11.5' above flood stage, bringing flood waters to a large portion of the downtown area. The mayor has ordered all schools, parks, and libraries closed today, and commuter bus and train services have also been suspended. Five people died in Nashville due to the flooding. The Harpeth River at Bellevue, Tennessee to its greatest flood height since record keeping began in 1921. The river had a depth of 2 feet early Saturday morning before the rains began, and was running at a depth of 29' early this morning, breaking the record of 24.34' set in 1948. (To check out the flood heights, use our wundermap for Nashville with the "USGS River" layer turned on.) The Duck River at Hurricane Mills reached 28.7' yesterday morning before its streamgage stopped operating, its 2nd greatest flood height since record keeping began in 1926 (record: 30.7' in 1948.)

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; 19 more records were set on Sunday. 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.


Figure 3. A portable classroom building from a nearby high school floats past submerged cars on I-24 near Nashville, TN on May 1, 2010. One person died in the flooding in this region of I-24. Roughly 200 - 250 vehicles got submerged on this section of I-24, according to wunderphotographer laughingjester, who was a tow truck operator called in to clear out the stranded vehicles.

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 thirteen 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. According the landmark 2009 U.S. Climate Impact Report from the U.S. Global Change Research Program, "the amount of rain falling in the heaviest downpours has increased approximately 20 percent on average in the past century, and this trend is very likely to continue, with the largest increases in the wettest places." 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 (Figure 4.)


Figure 4. Streamgages that have been discontinued or are being considered for discontinuation or for conversion from continuous record discharge to stage-only stations. Funds for these 276 threatened streamgages are from the U.S. Geological Survey and other Federal, State, Tribal, and local agencies. For those streamgages that have already been discontinued, extensive efforts were made to find another funding source; however, when no funding was made available the streamgages had to be discontinued. For those streamgages at risk for discontinuation, the current funding source has indicated that it can no longer fund the streamgage. Efforts are currently underway to identify another funding source for the operation of these streamgages; however, if no funding is identified, then these streamgages will have to be discontinued also. 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. Onshore winds of 10 - 15 knots will continue today, according to the latest NWS marine forecast, then shift to sideshore (out of the west) on Tuesday, as a cold front passes. Winds will then resume a weak onshore flow at 5 - 10 knots, Wednesday through Friday. These winds should cause only a slow transport of the oil slick towards the coast. I don't expect the spill will affect any land areas for the remainder of the week, with the possible exception of 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.


Figure 5. Previous location and forecast location for tomorrow of the Deepwater Horizon oil spill. Image credit: NOAA Office of Response and Restoration. See also the trajectory maps available at State of Louisiana web site.

Jeff Masters