|Above: Flint resident Brayden Bend, 5, sits on top of his mother's friend's vehicle on Robert T. Longway Boulevard at North Center Road on Tuesday, Feb. 20, 2018, in Flint, Michigan. (Bronte Wittpenn/The Flint Journal-MLive.com via AP).|
An unusually severe winter flood event is underway across the center of the U.S., from Texas to Michigan, thanks to heavy rains that fell during the week, fed by record to near-record atmospheric moisture for this time of year. The most significant flooding thus far has been in Southwest Michigan, Northwest Indiana, and Northeast Illinois, where the heavy rains fell on a snowpack that completely melted, releasing meltwater equivalent to another ½" – 1” of rain. In East Lansing, Michigan, floodwaters rose on the campus of Michigan State University from the Red Cedar River, causing some classes to be moved on Friday to non-flooded areas. Three drowning deaths have been blamed on the flood thus far, one death each in Michigan, Oklahoma, and Illinois.
This was one of the greatest winter rainfall events on record in Southern Michigan (rivaling Feb 1997). Forecast river flooding may require actions to mitigate property loss. Every river point has a different history/vulnerability/forecast. https://t.co/JEjTkJxE7u #wmiwx #miwx pic.twitter.com/bRDr6A2XrD— NWS Grand Rapids (@NWSGrandRapids) February 21, 2018
|Figure 1. Predicted precipitation for the 5-day period ending at 12Z (7 am EST) Wednesday, February 28, 2018. Monticello, AR has already had record February rainfall of 13.35” (previous record: 12.85” in 2001), and will add to that total. Memphis, TN has had 8.73” of precipitation from Feb 1 – 22; the forecast above calls for an additional 3” in February, which would beat their February precipitation record of 11.14", set in 1948. Image credit: National Weather Service.|
Moderate to major flooding expected on the Ohio River
A relatively stationary front over the Ohio Valley will continue to be the focus of periods of heavy rain until late Sunday morning, when the front will finally push to the east. With soils saturated, these additional rains of 2 – 4” will exacerbate the moderate to major flooding already occurring along the Ohio River and its tributaries. Dry conditions on Monday and Tuesday will bring a welcome change to the wet pattern, before another significant rain-making storm moves in on Wednesday. Moderate to major flooding is predicted for the coming week along several hundred miles of the Ohio River, from Pennsylvania to the river’s confluence with the Mississippi River at Cairo, Illinois. The flood waters from the Ohio River will dump into the Lower Mississippi River by late next week; the forecast is for minor to moderate flooding on the Mississippi River by late next week.
|Figure 2. By Monday, the Ohio River along the West Virginia/Ohio border at Racine is predicted to reach major flood stage; the NWS advises that at the predicted water levels, the town of Racine, Ohio (population 650) will flood. Image credit: NWS.|
Record atmospheric moisture fed the flood
Accompanying the exceptional February warmth this week were record levels of February moisture, as a flow of unusually moist air rode northwards from the Gulf of Mexico, where water temperatures were about 1.0°C (1.8°F) above average. Meteorologists use a term called "precipitable water" to discuss how much water vapor is in the atmosphere. Precipitable water is defined as how much rain would fall on the ground if one took a vertical slice of the atmosphere above a given location and condensed all the water vapor into rain. Precipitable water levels tend to be higher when the temperature is warmer, since warm air holds more water vapor. This week, an extraordinarily large number of upper air balloon soundings set all-time records for February moisture. There are 73 radiosonde stations in the contiguous U.S. that take routine measurements twice per day, and six of them set all-time February precipitable water records this week; four of these stations broke their previous February record multiple times. That is a very rare occurrence, as radiosonde data goes back 70 years. Here are the new February precipitable water records set this week:
Detroit, MI, 3 new records: 1.38” at 0Z 21 Feb, 1.36” at 12Z 20 Feb, and 1.29” at 0Z 20 Feb; Old record: 1.15", 2/23/1985
Lincoln, IL, 2 new records: 1.47" at 12Z 2/20 and 1.36” at 0Z 2/20; Old record: 1.34", 2/25/2001
Colombia, MO, 2 new records: 1.44" at 0Z 2/20 and 1.39” at 12Z 2/19; Old record: 1.33", 2/21/1997
Shreveport, LA, 3 new records: 1.75" at 0Z 2/21, 1.69” at 12Z 2/21, and 1.75” at 0Z 2/23; Old record: 1.66", 2/21/1974
Alpena, MI, 1 new record: 1.11” at 0Z 2/21; Old record: 1.09”, 2/12/1999
Davenport, IA, 1 new record: 1.18” at 12Z 2/21; Old record: 1.09”, 2/25/2001
Green Bay, WI (4th), Slidell, LA (6th) Nashville, TN (4th), and Jackson, MS (5th) all recorded top-ten February precipitable water values during one of their balloon soundings this week. Radiosonde data goes back to 1948.
|Figure 3. Change in extreme precipitation by U.S. region. Left: change in the amount of precipitation falling in daily events that exceed the 99th percentile of all non-zero precipitation days, as calculated over 1958–2016. Right: the number of 2-day events with a precipitation total exceeding the largest 2-day amount that is expected to occur, on average, only once every 5 years, as calculated over 1958–2016. The numerical value is the percent change over the entire period 1958–2016. Image credit: U.S. National Climate Assessment and NOAA NCEI.|
Global warming plays an important role in causing heavier downpours
An increase in heavy downpours (exceeding the 99th percentile) has been documented across every region of the contiguous U.S. since 1958 (Figure 3), and this increase has been partially attributed to the increase in atmospheric moisture due to human-caused global warming. Every degree Centigrade that the air warms up increases the amount of water vapor the air can hold by 7%, due to increased evaporation from the oceans. Thus, record-breaking atmospheric moisture and the resultant major flooding--like the situation observed over the central U.S. this week--are something we will see a lot more of as the climate continues to warm in coming decades. The average moisture content of the atmosphere has already increased by about 4% since the 1970s, as expected from theory (the Clausius–Clapeyron equation). This increase has been attributed, in part, to human-caused global warming. There is research showing that in the summer, we can expect to see an increase in hourly precipitation extremes (greater than the 99th percentile) of up to 14% per degree Centigrade of warming; at colder temperatures, the increase is about 7% per degree Centigrade of warming, as one would expect from the Clausius–Clapeyron equation. The authors argue that the "super Clausius-Clapeyron scaling" of 14% more hourly rainfall per degree Centigrade of warming in extreme events in summer happens because of the dynamics of convective (thunderstorm) clouds--the extra moisture causes more rainfall formation and more latent heat release when the water vapor condenses into liquid, which forces stronger updrafts, invigorating the thunderstorm, and potentially leading to a stronger rate of condensation and even more rainfall formation.