Heavy rain prompted more flash flood warnings across parts of Texas on Saturday, as another area of heavy thunderstorms swept into the Dallas-Ft. Worth metroplex during the morning, with more rain likely from Austin and San Antonio to Houston later in the day.

These storms will be capable of dumping rainfall at rates of at least 2 to 4 inches per hour, sending creeks and streams back over their banks and re-flooding roadways.

According to the National Weather Service, through Friday, there had been enough rain across Texas to cover the entire state — the nation's second-largest by area, second only to Alaska — in water eight inches deep. In a tweet, the NWS said the rainfall in May amounted to at least 35 trillion gallons statewide.

Even before Saturday's rainfall, the office of the state climatologist reported that the state has already had the wettest month on record.

Rainfall totals during the past two weeks, showing magenta/purple shading where 12-20 inches of rain have fallen in Oklahoma and Texas. Image: NWS/AHPS

Going into Saturday, Dallas already had its wettest May on record, and if it receives a little more than 2 inches of rain before the end of the month — which appears likely — the city will set a record for the wettest month of any month on record.

5:51 AM: WFO Ft. Worth has picked up one half inch of rain in less than 15 minutes. #dfwwx #txwx — NWS Fort Worth (@NWSFortWorth) May 30, 2015

How much is 35 trillion gallons? 35 trillion 1 gal. milk jugs filled with water would stretch from Earth to our Sun 34 times! #txwx — NWS Fort Worth (@NWSFortWorth) May 29, 2015

There has been enough rain across #Texas during May to cover the entire state nearly 8 inches deep. That's over 35 trillion gallons! #txwx — NWS Fort Worth (@NWSFortWorth) May 29, 2015

We've received several questions RE: how we got the 35 trillion gallon number, here's a pic of the math: #txwx pic.twitter.com/TZ3utuKpDL — NWS Fort Worth (@NWSFortWorth) May 29, 2015

Other Texas cities that have already set such all-time records include Wichita Falls, Gainesville and Stephenville.

To the north, Oklahoma City has had nearly 20 inches of rain so far in May, making it the wettest May since records began in 1890, shattering the previous record of 14.52 inches in 2013, and annihilating its record for the wettest month on record, which had been 14.66 inches of rain in 1989.

Norman, Oklahoma and McAlester have both exceeded the 20-inch mark for the month, setting May and all-time monthly records as well. Parts of Arkansas have had copious amounts of rainfall as well.

This was a trail, close to where I live. Its now a river where trees are drowning and people go fishing #dfwflooding pic.twitter.com/NqHIyg7xNw — Iris (@AnsuReg) May 30, 2015

The flooding has hit two of America's largest cities particularly hard, with remarkably similar impacts. Earlier in the week, extreme rainfall and overflowing bayous rendered Houston's network of highways impassible, leading to hundreds of abandoned cars and several deaths from drowning.

Then on Thursday into Friday, it was Dallas' turn to experience flooding on its roadways and alongside rivers and creeks. So far, at least 28 people have been killed in storms that have repeatedly traversed Oklahoma and Texas since Memorial Day weekend, with 12 people still listed as missing as of early Saturday. The heaviest damage was along the Blanco River in Texas Hill Country, where the town of Wimberley was heavily damaged in a flash flood on Sunday.

Flood waters from the Brazos River encroach upon a home in the Horseshoe Bend neighborhood, Friday, May 29, 2015, in Weatherford, Texas. Image: Brandon Wade/Associated Press

Houston may be threatened by more rainfall on Saturday into Sunday as a cold front sags southward across the state. Even without any added rainfall, the city is girding against flooding from the Colorado River as well as Brazos and San Jacinto rivers as floodwaters flow downstream toward the Gulf of Mexico.

All told, it is likely that the southern Plains flooding will become one of the country's billion dollar weather disasters of 2015. President Barack Obama has signed a disaster declaration for Texas after severe flooding this week, ordering federal aid to supplement other recovery efforts in the area affected by severe weather since early May.

Finally, drier days ahead

The flooding comes as a shock to many in the southern Plains who just a few weeks ago were coping with a severe, multiyear drought. Needless to say, that drought is now over.

While Texas droughts have ended in a rapid barrage of downpours before, the amounts of rain falling this May have been unprecedented.

Many climate studies have found that as the world warms in response to rising levels of manmade greenhouse gases, heavy precipitation events are getting more frequent and intense. This is true in the South Central states, but it's most pronounced in the Northeast and Midwest.

The basic explanation for the sudden firehose-like rains in parts of the Plains is a stuck weather pattern that has caused moisture from the Gulf of Mexico to stream northward from Texas into the Plains states, with a steady stream of upper-level low pressure areas and frontal systems moving out from the Southwest and Rockies.

These weather systems have been providing ample lift to force the warm, moist air to rise, cool and condense, thereby forming clouds and precipitation.

The recent weather pattern has some links to thunderstorm activity and winds in the equatorial tropical Pacific, where an El Niño event is gathering strength.

Fortunately for the southern Plains, the weather pattern is forecast to change by early next week, providing at least a week of sunny, dry weather to the waterlogged region. Instead of a dip, or trough, in the jet stream, there will be an area of high pressure across the southern Plains.