01:03 ‘Total Disbelief’: Families Isolated as Isaias Washes Away Only Road to Their Homes Heavy rain and rising waters washed away a private road in Huntingtown, Maryland, stranding 7 families on the wrong side of the break.

At a Glance Moderate to major flooding is expected along the river basins in the central and southern U.S. this spring.

Nearly two-thirds of the Lower 48 faces an elevated flood risk through May.

Much of the nation is forecast to receive above-average precipitation April through June.

Spring 2019 is expected to bring more historic, widespread flooding to the central United States and above-average precipitation across much of the Lower 48, according to an outlook issued Thursday by NOAA .

A combination of factors, including the melting of a near-record snowpack in early March , frozen ground, heavy rain and ice jams, has already sent some rivers to record levels in the Plains and Midwest , including the upper Mississippi and Missouri River basins in parts of Iowa, Minnesota and Nebraska.

Wetter-than-average conditions, combined with spring snowmelt, will likely prolong and expand the flooding, particularly in the central and southern U.S. This is because excess water will flow downstream through the river basins, worsening the flood threat and causing it to become geographically more widespread.

(MORE: Historic Flooding Due to Rare Confluence of Meteorological Factors)

NOAA's flood outlook says nearly two-thirds of the contiguous U.S. faces an elevated flood risk through May, with the potential for moderate to major flooding in parts of 25 states. Much of the nation will likely receive above-average precipitation, adding insult to injury since soil-moisture values already rank among the top 10 percent for this time of year in the central and eastern states.

The areas under the highest risk of moderate to major flooding, according to NOAA, are the upper, middle and lower Mississippi River basins, including the mainstem Mississippi River, Red River of the North, Great Lakes, eastern Missouri River, lower Ohio River, lower Cumberland River and Tennessee River basins. These are depicted in the red and purple shadings on the map below.

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/noaa-2019-flood-risk-map.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/noaa-2019-flood-risk-map.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/noaa-2019-flood-risk-map.jpg?v=ap&w=980&h=551 800w" > This map depicts the locations with a greater than 50 percent chance of major, moderate or minor flooding from March through May 2019. (NOAA)

Minor flooding is also possible for a large swath of the U.S. east of the Mississippi River, as well as in portions of California, the Pacific Northwest and the Great Basin.

"This is shaping up to be a potentially unprecedented flood season," said Ed Clark, director of NOAA’s National Water Center in Tuscaloosa, Alabama. "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."

NOAA said its flood outlook considers various different factors, such as the current conditions of the snowpack , drought, soil moisture, frost depth, streamflow and precipitation. Locally heavy rain events, particularly thunderstorms, tend to become more common as spring progresses and can lead to flooding in areas not depicted in the outlook.

There's a chance the snowpack in the higher elevations of the West could continue to grow in the weeks ahead, and the flood risk there will depend on future precipitation and temperatures, according to NOAA. Far-above-average rain and snow over the last few months have knocked out the years-long drought in California.

(MORE: California Is Drought-Free for First Time in over 7 Years)

Precipitation Outlook

El Niño's influence this spring is playing a role in the precipitation outlook through June , NOAA forecasters said.

The highest chance of above-average precipitation is from the central Great Basin to the central Plains, as well as along the Gulf and East coasts. Near- or slightly above-average precipitation is expected over much of the Lower 48.

These wetter-than-average conditions will play a role in the increased flood threat discussed earlier.

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/noaa_april_to_june_precip_outlook_2019.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/noaa_april_to_june_precip_outlook_2019.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/noaa_april_to_june_precip_outlook_2019.jpg?v=ap&w=980&h=551 800w" > Areas shaded in dark green have the highest chance of above-average precipitation from April through June 2019, according to NOAA's spring outlook. (Source: NOAA)

The Pacific Northwest is the only area NOAA expects to be drier than average this spring. Precipitation will likely be near average in California and other parts of the West, as well as along the nation's northern tier.

Keep in mind, this outlook is meant to be an overall trend for the three-month period covering April through June.

An individual cold front or an upper-level ridge of high pressure can bring a brief period of enhanced precipitation or a dry spell, respectively, that may not be indicative of the overall trend that is forecast.