Updated on June 6th, 2019
The upper Midwest and Mississippi River valley have experienced crippling rainfall this Spring. The Washington Post recently documented these "biblical" Spring rains and how the Corn Belt is nearing a breaking point. The period May 2018 to April 2019 was the wettest 12-month period on record in the United States, and there is no immediate end in sight. Some scholars have asked whether "The Brown Ocean Effect," which has been associated with wet soils maintaining or enhancing the intensity of tropical cyclones, is contributing to the current rainfall. However, my hunch is that a combination of deep tropical moisture and something called "precipitation recycling" are the likely culprits.
The National Weather Service discussion issued by the Weather Prediction Center provides the meteorological context for the events depicted in the weather map above:
Deep, tropical moisture ahead of an upper level system will continue to support widespread moderate to heavy rains as it moves east across the southern Plains into the lower Mississippi valley Thursday and Friday. Heaviest rains on Thursday are expected to focus from the central Gulf Coast northward into a frontal band settling south into the lower Ohio and Tennessee valleys and the Arkansas Basin. These rains may pose flash flooding concerns, with the Weather Prediction Center highlighting this area with a Slight Risk for excessive rainfall on Thursday.
By Friday and Saturday, the flash flooding threat moves into the southern Appalachians and southern Mid-Atlantic states.
Many questions have emerged on social media and weather discussion boards about the role of pre-existing surface moisture and current weather events. Meteorologist and Weather Underground scientist Bob Henson tweeted:
Hey, #brownocean experts...do you think today's heavy rains in Oklahoma (triggered by a slow-moving upper low) might be getting a boost from all the extra surface water/wet soils from recent flooding?
As one of the scientists that published the original papers on the Brown Ocean Effect, I am always keeping an eye on these types of scenarios. My scientific hunch is that something called "precipitation recycling" is a better description of what may be contributing to this event since there is no maintenance or intensification of a tropical cyclone. University of Oklahoma meteorology professor Jeff Basara responded to Henson's Tweet:
There are banded inflow structures as tropical-like precipitation develops with an apparent LCL that appears to be only several hundred meters above the surface. Surface conditions are certainly inot inhibiting the environment and likely enhancing it.
To translate Basara's meteorological terminology, the LCL stands for Lifted Condensation Level. He is basically pointing out that moisture air is not having to travel very far from the surface before condensing into cloud water.
At this point, I want to introduce the concept of precipitation recycling. Moisture is an essential ingredient for precipitation systems. Moisture can be transported from other regions or supplied locally by evapotranspiration. Precipitation that comes from surface moisture in the local region has been referred to as "recycled precipitation." A 2006 study in the Journal of Climate is a good primer on this topic. They found, at daily scales in some regions, that local storage terms are not trivial and can be as high as 50% of the total moisture moved in from elsewhere. A more recent study examined the contributions of precipitation recycling to the Great Plains region.
Undoubtedly the above-normal precipitation this spring helped with moisture availability. A deeply moist sounding with light flow through the troposphere, plus a nearby lifting mechanism, create an environment conducive to efficient rainfall.
The map below shows that much of the Great Plains and Midwest experienced above normal (125 to 400% of normal) rainfall throughout the month of May. While moisture feeding these rainfall systems is likely from terrestrial and oceanic sources, I certainly wouldn't rule out some recycling as well.
