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The number of locations along rivers in the United States expected to experience some level of flooding by the end of the week is expected to swell to almost 400 after another round of torrential rain only worsened flooding brought on by a soggy May, spring and excessive snow melt.

Parts of Kansas, Oklahoma and Missouri picked up 4 to 9 inches of rain from this latest drenching. Water rose quickly, trapping people in homes and vehicles, forcing dramatic rescues and evacuations and even flooding a stretch of Interstate 40 west of Oklahoma City.

(NEWS: Flooding Prompts Water Rescues, Evacuations)

Record flooding was measured on Bird Creek in Avant, Oklahoma, and the Chikaskia River near Blackwell, Oklahoma, crested near its 2008 record for the second time in two weeks.

The Arkansas River in Tulsa was expected to reach its highest level since the October 1986 flood, though several feet below that crest. Downstream, the town of Webbers Falls, Oklahoma, was urged to evacuate. Several rural roads in northeast Oklahoma and southeast Kansas were closed due to flooding.

As of May 21, the National Weather Service River Forecast Center predicted just under 400 locations with gauges along rivers in the U.S. would experience some level of flooding by later in the week, covering an impressive area from the northern Plains and upper Midwest to the Gulf Coast.

This massive number, which includes areas already reporting flooding right now, comprises just under 18% of all U.S. Geological Survey river gauges for which forecasts are currently issued by the NWS.

Other streamflow data from the USGS indicated dozens more gauges were reporting streamflow in the 95th percentile or higher for late May, not only in the Plains and Midwest, but also in parts of the Northeast and West.

The distribution of river gauges in the map above nicely defines the current Plains flooding, and two other flood zones:

East Texas to Louisiana and Mississippi: Parts of these areas picked up 10 to 14 inches of rain in the first full week of May. Heavy rain also soaked parts of Oklahoma, Kansas and Arkansas, triggering significant flooding there and keeping ground saturated ahead of the current Plains event.

Mississippi River Valley: Heavy winter rain in the South, and a combination of snowmelt and heavy rain in the spring, has kept a large stretch of the Mississippi Valley in flood for months, breaking flood longevity records from Davenport, Iowa, to Vicksburg, Mississippi, and Baton Rouge, Louisiana.

The 12-month stretch from May 2018 through April 2019 was the wettest 12-month period in U.S. history dating to 1895, Weather Underground meteorologist Bob Henson laid out in a Category 6 blog on May 8.

The first 20 days of May were the wettest since 1989 in Dallas and since 1943 in Tulsa, Oklahoma, according to the Southeast Regional Climate Center.

There seems to be only a little relief ahead.

Rounds of locally heavy rain are possible again in the Plains from Oklahoma and the Texas panhandle to Nebraska and Iowa through Memorial Day weekend, according to NOAA's rainfall forecast.

Given saturated ground and some rivers still in flood stage, these locally heavy rain bursts could trigger more flash flooding of smaller creeks and streams and may eventually prolong or worsen flooding of some mainstem rivers.

Beyond that, the end of May and beginning of June may continue to be wet in that same Plains zone, though drier weather is expected in the lower Mississippi Valley.