NOAA issued its annual Spring Outlook (April – June 2019) last Thursday, March 21, 2019 which includes outlooks for temperature, precipitation, and flood risk. As those living in the upper Mississippi and Missouri River basins including Nebraska, Minnesota and Iowa are still recovering from recent monumental and devastating flooding, forecasters have declared that above-average spring rain and snow will likely worsen flood conditions through May.
The early flooding seen this year was caused by rapid snow melt combined with heavy spring rain and late season snowfall in areas where soil moisture is high. Soil moisture plays an important role in both temperature and precipitation forecasting. When the ground is drier, storms do not produce as much widespread flooding as they would if the soils were wet because most of the water will infiltrate the soil (although drought conditions where the soil is exceptionally dry can also create extreme runoff during heavy rain because it doesn’t absorb as well either). Higher soil moisture content leads to increasing runoff and greater potential for flooding. Rainfall or snow melt on frozen ground generally poses an even greater concern as pores are blocked with ice. In addition, ice jams are also further exacerbating the flooding in some areas across the United States. These occur when pieces of floating ice carried with a stream’s current accumulate at any obstruction to the stream flow. As the ice builds up and causes these obstructions, water upstream that is held back may cause flooding or flash flooding. If the obstruction breaks suddenly, then flash flooding can occur downstream as well.
Offices across the National Weather Service have been working with local communities, providing decision-support services and special briefings to emergency managers and other leaders in local, state and federal government to ensure the highest level of readiness before the flooding began.
Spring Flood Risk
Looking ahead, NOAA’s Flood Risk Outlook for spring indicates that additional spring rain and melting snow will prolong and expand flooding, especially in the central and southern US. As this excess water flows downstream through the river basins, the flood threat will become worse and geographically more widespread. Nearly two-thirds of the continental US faces an elevated risk of flooding through May, with the potential for major or moderate flooding in 25 states. The flood risk is increased by the fact that the majority of the country is favored to experience above-average precipitation this spring.
Dr. Neil Jacobs, NOAA’s acting administrator, said “This outlook will help emergency managers and community decision-makers all along the nation’s major waterways prepare people and businesses for the flood threat. In addition to the safety aspects, our rivers are critical to the economic vitality of the nation, supporting commerce, recreation and transportation. NOAA forecasts and outlooks help people navigate extreme seasonal weather and water events to keep the country safe and moving forward.”
Unfortunately, the areas who have seen the record-setting precipitation this year (up to 200% above normal in the upper Mississippi and Red River of the North basins!) are at among those who have the most elevated flood risk. The areas of greatest risk for moderate to major flooding include the upper, middle, and lower Mississippi River basins including the main stem Mississippi River, Red River of the North, the Great Lakes, eastern Missouri River, lower Ohio, lower Cumberland, and Tennessee River basins. Additionally, much of the U.S. east of the Mississippi River and portions of California and Nevada are at risk for minor flooding. The info-graphic below explains how the National Weather Service defines the difference between minor, moderate, and major flooding.
“The extensive flooding we’ve seen in the past two weeks will continue through May and become more dire and may be exacerbated in the coming weeks as the water flows downstream,” said Ed Clark, director of NOAA’s National Water Center in Tuscaloosa, Alabama. “This is shaping up to be a potentially unprecedented flood season, with more than 200 million people at risk for flooding in their communities.”
Note that the flood risk outlook is based on a number of factors, including current conditions of snowpack, drought, soil moisture, frost depth, streamflow and precipitation. Local heavy rainfall, especially associated with thunderstorms, can occur throughout the spring and lead to flooding even in areas where overall risk is considered low. In the western U.S., snowpacks at higher elevations may continue to build over the next month, and the flood risk will depend on future precipitation and temperatures.
Spring Outlook for Temperature, Precipitation, and Drought
Above-average precipitation is favored from the Central Great Basin to the East Coast and in Alaska, compounding the flood risk for many states, especially in the Central and Northern Rockies and in the Southeast. Hawaii, the Alaskan panhandle, and the Pacific Northwest are favored to have drier than average conditions this coming spring.
For those of us in the New York’s Capital Region who have been waiting for summer-like conditions since it ended last year, there is some good news! Warmer-than-average temperatures are forecast to extend from the Pacific Northwest to the Central Rockies, and from southern Texas, northward through the Great Lakes and eastward to encompass the entire East Coast. The greatest chance for above-average temperatures exist in Alaska, the Northeast and mid-Atlantic. The interior of the U.S. from the Dakotas southward to northern Texas are favored to have below-average temperatures this spring.
Above-average rain and snow in California this winter pulled the entire state out of its seven-year drought. Scattered areas of the Southwest, Southeast and Pacific Northwest are abnormally dry, but the worst drought conditions in the U.S. are in northern New Mexico. Springtime rain and melting of deep snowpack are favored to slightly improve the drought there. Drought will persist through spring in southern Alaska and Oregon, and may develop in Hawaii.
NOAA produces seasonal outlooks to help communities prepare for weather and environmental conditions that are likely during the coming months, but keep in mind that these outlooks are meant to be an overall trend for the next three months. Heavy rainfall at any time can lead to flooding, even in areas where overall risk is considered low. The latest information for a specific area, including official watches and warnings are available at weather.gov. Empowering people with the information they need to prepare and take action is key to NOAA’s effort to build a Weather-Ready Nation, and Shade Tree Meteorology is proud to be a Weather-Ready Nation Ambassador.