Serious Flooding Threat Looms for Tennessee Valley This Week and Beyond

A tenacious weather setup with some atmospheric-river elements will bring several pulses of widespread heavy rain over the next few days from Louisiana to West Virginia. The multi-day episode will hit especially hard from Kentucky across Tennessee to northern Alabama and Mississippi and eastern Arkansas. With soils already saturated in many areas, this pattern will bring a risk of flash floods as each round of rain arrives. Large-scale river flooding will become an increasing concern into next week.

The NOAA/NWS Weather Prediction Center (WPC) is calling for widespread rainfall totals of 5” – 7” across parts of seven southern states over the next week, with pockets of 7” – 10” possible in Tennessee (see Figure 1). The center tagged a region in and near Memphis for a moderate risk of excessive rainfall (enough to cause flash flooding) on Tuesday, with a small area near Huntsville, AL, under a moderate risk of excessive rainfall in the outlook for Wednesday.

Figure 1. 7-day precipitation forecast issued Monday morning for the period starting at 7 am ESfT Monday, February 18, 2019. Image credit: NOAA/NWS/WPC.

A fast, furious jet

The upper-level dynamics on tap to drench the Southeast are in some ways an eastward extension of the pattern that doused California last week. Both events are the result of shots of energy moving through a very persistent and slow-moving upper-level trough (an elongated zone of low pressure) that extended on Monday from central Canada into the southwest U.S. On the east side of this trough, the jet stream has been roaring at extreme speeds—sometimes topping 220 mph at heights of around 35,000 feet.

The 250-hPa jet streak currently over the CONUS contains wind speeds approaching 200 kt (230 mph) and is assoc. w/ standardized anomalies >3σ spanning from Texas to Michigan. I suspect this jet streak is also helping cross-country flights arrive early on the East Coast! pic.twitter.com/z7474e6bXk

— Alicia M Bentley (@AliciaMBentley) February 17, 2019

The core of the strongest winds translated through the Southwest over the weekend, giving eastbound aircraft a major tailwind that shaved up to an hour off the typical length of eastbound coast-to-coast flights and added up to an hour on westbound flights.

This intense jet streak will be pushing across the Ohio Valley and Northeast into the Atlantic over the next several days, but its tail end will remain close to Kentucky and Tennessee through the week, assisting at times with the rain-producing dynamics. Beneath the jet stream, a surface front will be sloshing back and forth across the mid-South. As each upper-level impulse approaches, a rich supply of low-level moisture will be pulled northward from the Gulf of Mexico, intercepting the front and stoking periods of heavy rain.

Where and when

Here’s a rough sense of the expected timing of this prolonged rain and flood threat:

—The first batch of heavy rain will spread across the lower Mississippi Valley on Tuesday, then push across Kentucky, Tennessee, Mississippi, Alabama, and Georgia on Wednesday. From the mid-Atlantic to southern New England, light to moderate snow will shift to ice and eventually to rain as this impulse continues northeast.

—Periods of occasional rain will continue Thursday and Friday as the frontal system slowly lifts north from the Deep South into Tennessee and Kentucky.

—A vigorous spring-like storm system will quickly organize in the Southern Plains on Saturday. Heavy rains as well as severe weather are possible on or around Saturday night as showers and thunderstorms race northeastward from Missouri, Arkansas, and Louisiana into the Ohio and Tennessee valleys. The NOAA/NWS Storm Prediction Center has already issued a Day 6 outlook for potential severe weather on Saturday across this region, noting the possibility of tornado-favorable conditions.

Long-range models are suggesting that at least one more round of heavy rain could affect the South early next week before February draws to a close.

First Day-6 severe weather risk area of 2019 issued by @NWSSPC (for Saturday, 2/23). Stay weather aware this week. pic.twitter.com/Z4v1NIFg6S

— Patrick Marsh (@pmarshwx) February 18, 2019

Where the wettest February on record could happen

Based on predicted rainfall over the next week (as shown in Figure 1 above), central Tennessee in particular has a good chance of a record-wet February, depending on where the heaviest rain bands set up. Here’s how the odds look across a few key locations. It’s striking that just last February (2018) was the wettest on record in some of these areas.

Nashville, TN:  8.32” through Sun. 2/17; 6”-8” predicted through Sun. 2/24; record wettest Feb. is 12.37” in 1880 (period of record starts in 1874)
Memphis, TN:  3.74” through Sun. 2/17, 6”-8” predicted through Sun. 2/24; record wettest Feb. 13.43” in 2018 (period of record starts in 1875)
Bowling Green, KY:  6.62” through Sun. 2/17; 4”-6” predicted through Sun. 2/24; record wettest Feb. is 11.27” in 1989 (period of record starts in 1893)
Louisville, KY:  5.06” through Sun. 2/17; 3”-5” predicted through Sun. 2/24; record wettest Feb. is 10.54” in 2018 (period of record starts in 1873)
Paducah, KY:  6.49” through Sun. 2/17; 3”-5” predicted through Sun. 2/24; record wettest Feb. is 13.33” in 1989 (period of record starts in 1937)
Huntsville, AL:  3.13” through Sun. 2/17, 5”-7” predicted through Sun. 2/24; record wettest Feb. is 12.52” in 1939 (period of record starts in 1894)

Potential flood and flash floods are a concern between Tuesday and Wednesday. The #Mississippi River and the #Tennessee river are both in the moderate excessive rainfall category. Both rivers have areas that are expected to go to major flood stage. #turnarounddontdrown pic.twitter.com/LgJLWnOZ9X

— NWS Memphis (@NWSMemphis) February 18, 2019

River flooding: a real concern into March

Significant river flooding is already in progress over parts of the lower Ohio Valley, and the risk of river flooding will be on the rise later this week into next week, especially over parts of western Kentucky and Tennessee and northern Alabama. For example, the Tennessee River at Savannah, TN, was projected on Monday to reach 390 feet by Sunday, with the water still rising at that point. Such a long-range projection should not be taken as a literal forecast, since the outcome will hinge on exactly where and how much it rains this week, but a 390-foot crest would be among the ten highest in more than 100 years of recordkeeping at Savannah.

Further downstream, we can expect water levels to rise along the lower Mississippi River well into early March, as weeks of heavy rain work their way downstream.

Guntersville Dam in North Alabama is one of the nine dams on the Tennessee River that is generating at full capacity and spilling excess water. Guntersville is discharging 1.1 million gallons per second as we prepare for heavy rainfall this week. #Guntersville #ALWX pic.twitter.com/o1wQQ8uDkw

— Tennessee Valley Authority (@TVAnews) February 17, 2019

Is this an atmospheric river?

The extreme rains and snows that pummeled parts of California late last week—including the third heaviest calendar-day rainfall on record at Palm Springs, 3.69” on Thurs., Feb. 14—were closely linked to an atmospheric river (AR), an elongated moisture channel extending from the tropical Pacific into the West Coast. As we discussed in a post on February 5, there's a new scale for assessing the strength of ARs, which takes into account the strength and duration of atmospheric moisture transport at any given spot. This prototype scale hasn’t yet been adopted by the National Weather Service. The Center for Western Weather and Water Extremes (CW3E), whose researchers collaborated with NWS forecasters on the scale, is issuing updates on West Coast AR events using the new scale.

Atmospheric rivers occur in many parts of the world. One common trajectory runs from Central America across the Caribbean and western Gulf of Mexico into the U.S. Plains and Midwest. Paul Dirmeyer and James Kinter (University of Maryland) dubbed this flow the Maya Express, a nod to the Pineapple Express that rolls from the vicinity of Hawaii into the West Coast. (See the related Forbes column by Marshall Shepard.) Similar flows can extend into parts of the U.S. east of the Mississippi.

Although CW3E has not weighed in on the Southeast’s rain event, it appears (based on output from the 06Z Monday run of the GFS model showing integrated water vapor) that the early-week surge of moisture could rank as high as an AR-Cat 3 over parts of Mississippi, Alabama, and Tennessee. The late-week system could rank as high as an AR-Cat 4 in the vicinity of western Kentucky and Tennessee, according to this GFS run. Note that AR-Cat ratings are not being used officially or operationally—these estimates are presented here only for discussion value—and the ratings apply only to specific locations rather than to an entire event across a region.

Figure 2. A strong moisture channel from the western Gulf of Mexico into the Mississippi Valley will bring high values of integrated water vapor transport (IVT), a measure of the amount of water vapor flowing through an atmospheric river per second at a given point. Shown here is IVT as predicted by the 06Z Monday run of the GFS model for 06Z Wednesday (1 am EST). Arrows show the orientation and strength of the IVT. Image credit: CW3E.

One crucial difference between ARs in the West and East is topography. ARs slamming into the West Coast often run headlong into California’s north-south mountain ranges, and that leads to terrain-induced precipitation on top of the dynamically produced rain and snow. The geography is much more muted in the Southeast outside of the Appalachians, so it’s more important that there be “atmospheric topography” to help squeeze out the moisture, such as a frontal system near the surface. Heavy thunderstorms can also boost precipitation totals during an AR event over the Southeast, a factor that’s typically less important on the West Coast.

Jim LaDue, a meteorologist with the NWS Warning Decision Training Branch, discusses this week’s event together with relevant AR research in an illuminating post from Sunday on his personal website, Should we be talking about the next atmospheric rivers in the southeast US?.