Stage Is Set for Potentially Devastating U.S. Spring Floods

A rare set of preconditions has left some of the nation’s biggest river basins vulnerable to what could be widespread and historic flooding, NOAA warned in its spring flood outlook issued on Thursday. Upcoming weather will play a huge role in the outcome, which is far from a done deal, but the agency didn’t pull any punches on what could happen—especially now that rainfall is projected to be above normal yet again this spring over much of the nation (see below).

“The stage is set for record flooding now through May,” said Mary Erickson, deputy director of the National Weather Service, at a news conference. The disastrous floods that hammered Nebraska and Iowa this month—inflicting what may be billions in damage and taking three lives—may only be a “preview” of what’s to come, said Erickson: “In fact, we expect the flooding will become worse and more widespread.”

Ed Clark, director of NOAA’s National Water Center, painted a similarly grim picture in a news release: “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.…This is shaping up to be a potentially unprecedented flood season."

Figure 1. NOAA’s Spring Flood Outlook issued on March 21, 2019. The outlook depicts areas where the odds favor minor, moderate, or major flooding for the period from March through May. This period includes the time frame in mid-March when severe flooding occurred along and near the lower Missouri River. Image credit: NOAA.

Where the risk is greatest, and why

As shown in Figure 1 above, vast parts of the country are at risk of at least minor flooding this spring. Such floods could be excerbated regionally by major storm systems and locally by heavy showers and thunderstorms.

Based on current conditions, the largest-scale and most serious flood threat will affect three of the nation’s great river basins.

•  Additional flooding is quite possible along and near the Missouri River from eastern South Dakota to northeast Kansas and northwest Missouri. "Several of the prior major floods along the Missouri River have occurred during the months of April through June," said soil physicist Tyson Ochsner (Oklahoma State University). "The current high soil moisture levels in Nebraska and northern Kansas likely increase the probability of additional flooding this spring."

The risk may be somewhat less than depicted in Figure 1 over some parts of eastern Nebraska, where large amounts of snow and ice have already been dislodged or melted and flowed downstream during this month’s catastrophic floods.

•  Saturated soils over much of the Upper Mississippi River basin will hike the regional flooding risk, especially over southeast Minnesota, western Wisconsin, and eastern Iowa. As these waters flow downstream, flood risk will be heightened along and near the Mid- and Lower Mississippi River, especially if heavy spring rains develop in the region. 

A great view from "above" this afternoon! Notice all that remaining deep snow just to our north. That snow is the reason we continue to have concern about additional flooding over the next 1-2 weeks. pic.twitter.com/DPsCKDN1vx

— NWS La Crosse (@NWSLaCrosse) March 21, 2019

•  An unusually large snowpack over the northern Great Plains and Upper Midwest could lead to major flooding across the Red River Valley of the North in eastern North Dakota and western Minnesota. The threat will be greatest if temperatures suddenly spike far above freezing during April, especially if the warmth is accompanied by heavy rain. Unusually cold conditions in late winter and early spring have frozen the soil to unusual depths, and that will limit the soil’s ability to absorb rain and snowmelt until thawing is complete.

Another important variable is how quickly warming and heavy rains translate north. The Red River of the North is the nation’s only major river that flows northward into Canada, which means meltwaters tend to pile up as they flow toward progressively colder conditions.

As of March 19, the Upper Midwest region was covered by an average of 11” of snow, holding an average 3” of water, according to snow analyses from the NWS National Operational Hydrologic Remote Sensing Center. These amounts easily top anything observed for the date in data going back to 2004, with one exception. There was an average of 12.2” of snow on March 19, 2013—but that snow held only 2.2” of water, on average. All other years had less than 2” of water held in snowpack.

Figure 2. The equivalent amount of rainfall that the snow over the Upper Midwest held on March 21, 2019. Large portions of the Mississippi River watershed in Minnesota, North Dakota, South Dakota, and Wisconsin had the equivalent of more than 2” of rainfall stored in the snowpack (purple colors), and a significant portion had more than 5.9” of liquid water equivalent (pink colors). Image credit: NOAA.

Grounds for concern: Soils are phenomenally wet

Literally underpinning the flood threat this spring is unusually high soil moisture across most of the U.S. east of the Rockies—the result of heavy summer and autumn rains followed by the wettest winter on record for the contiguous U.S.

Soils are currently holding anywhere from 200 to 600+ mm of water (7.9” to 23.6+”) over many parts of the central and eastern U.S., based on NOAA data. The higher end of this range would correspond to nearly saturated conditions throughout the topmost 2 meters (6.6 feet) of soil.

There are more than 1800 measuring sites within the North American Soil Moisture Database (NASMD). These in-place observations become even more useful when combined with satellite-based data and with soil moisture models that integrate the available observations. (Among the newest innovations in soil moisture sensing are devices that count fast-moving neutrons from cosmic rays, which lose energy as they penetrate the soil and encounter moisture; see this great explainer from UK-COSMOS.)

Figure 3 below shows percentile output from a single-layer NOAA model that does not take into account soil characteristics or soil depth. it suggests that soil moisture is in the top 1% of the range of conditions expected in late March over broad areas.

Figure 3. Soil moisture on March 20 was in the top 1% of all expected values for the time of year over large parts of the Great Plains, Midwest, Ohio and Tennessee Valleys, and mid-Atlantic, according to CPC’s one-layer soil moisture model. Image credit: NOAA/NWS/CPC.

A more detailed ensemble product called NLDAS-2, which blends the output from four models—each with multiple soil layers—shows more regional variation (see Figure 4 below). However, it agrees that soil moisture is remarkably high for late March from the Central Plains into the western Great Lakes. Note that soil moisture sensors are not very accurate at detecting the amount of ice within soil, so these maps are less reliable where soil is still frozen, such as North Dakota.

In some areas, "the soil temperature is just getting above the freezing point," said Witek Krajewski, who directs the Iowa Flood Center and its Iowa Flood Information System. "That means the soil will be releasing all that water that’s been sitting there since last fall."

Figure 4. Analysis for March 17, 2019, from the National Land Data Assimilation System (NLDAS), blending four land surface models, shows soil moisture in the 90th to 99th percentile for this time of year from much of Nebraska and Kansas into southern Wisconsin and northern Illinois. Image credit: NLDAS Drought Monitor.

Another sign of our sodden times: For four straight weeks, through March 19, the weekly U.S. Drought Monitor has shown the Midwest completely free of any areas of drought or abnormal dryness. Only two other weeks have managed this feat since the monitor was established in 2000, and those were non-consecutive weeks.

Short-range and seasonal weather outlooks

Up to an inch of moisture could fall from the central Plains into the Upper Midwest this weekend with a moderately strong storm system, which could ramp up snowmelt in some areas. Otherwise, the next week should bring mostly dry and cool-to-mild conditions. A late-week warmup could push readings above 60°F as far north as southern Minnesota, hastening snowmelt.

Recent runs of the GFS and European models agree that a more potent storm may take shape across the central and northern Plains about a week from now, with the potential for locally heavy snow from the central and northern Plains into Minnesota and/or significant rain from Iowa to Wisconsin and Michigan (see Figure 6 at bottom).

Figure 5. U.S. precipitation outlook for spring 2019. Areas in green have enhanced odds of a wetter-than-average spring. Image credit: NOAA/climate.gov.

Part of the heightened concern about spring flood risk stems from NOAA’s latest seasonal precipitation forecast (Figure 5). Most of the contiguous U.S. now has an elevated chance of above-normal precipitation for April – June, according to NOAA. In fact, the widespread higher-than-average odds for wetness are projected to continue through the summer.

Guidance from the North American Multi-Model Ensemble, which incorporates several seasonal-to-interannual forecast models, is strongly behind the idea of a wet central U.S. this spring and summer. The outlook is also consistent with the weak El Niño event now in place, which NOAA predicts is likely to continue through at least summer 2019.

Figure 6. Predicted precipitation for the 5-day period ending at 0Z Monday, April 1, 2019, from the 0Z Friday, March 22, 2019 run of the European model. The model predicted that rains of 1 – 2” would affect portions of Wisconsin and Michigan’s Upper Peninsula where a very heavy snowpack lies. Rains of this magnitude are likely to cause flooding problems if they fall on a heavy snowpack. Up to 3" of rain is projected over parts of Nebraska and Iowa. Image credit: WSI.

Jeff Masters contributed to this post.