Storms pummeled the Mississippi River Valley Sunday following a weekend of deadly tornadoes in Oklahoma. Violent twisters also struck throughout Nebraska and Iowa amid more widespread severe weather across the Central states Saturday.

Threat level: The states hit by severe weather Sunday included northeastern Texas and parts of Louisiana. This follows the tornadoes that left towns in ruins in Oklahoma.

• Multiple rounds of severe weather occurred through the weekend, immediately after an outbreak of tornadoes struck Nebraska and Iowa.
• The communities of Sulphur, Holdenville, and Marietta, Okla., were hard hit. Sulphur appears to have been struck by multiple tornadoes in rapid succession overnight as storms intensified well after dark.
• According to local media in Oklahoma. four fatalities have been reported from the tornadoes that struck on Saturday into early Sunday. Nighttime tornadoes are known to be associated with greater fatalities, as people have fewer ways to see threatening weather and receive warnings that wake them up.
• Norman, Okla., home to the Storm Prediction Center and the University of Oklahoma, was also hit by a tornado, though damage was not extensive.

Zoom in: NWS storm survey teams have confirmed that the Sulphur tornado, and another tornado that hit Marietta, were at least EF-3 intensity, judging from the damage they did.

• Experts will continue to examine the damage in case they need to upgrade the ratings.

The intrigue: The tornado warning for the Sulphur storm came after another tornado had already hit or come close to the community, prompting this unusual language in the NWS' next warning, "First responders need to prepare for additional tornado impacts immediately!!!"

Catch up quick: The storms on Saturday stretched nearly continuously for about 1,000 miles from north Texas to Michigan, with the most dangerous storms hitting Texas, Oklahoma and Kansas, where ingredients were most conducive to forming tornadoes and causing flash flooding.

• Numerous events were canceled during the day and into the evening in Oklahoma City, as forecasters warned of a potentially historic outbreak, but it wasn't until overnight that the severe thunderstorms moved in and took advantage of the favorable environment.
• Overnight, repeated warnings were issued for some communities as multiple large and extremely dangerous tornadoes moved through parts of the state.

The big picture: During the period from April 26-28, at least 72 confirmed tornadoes touched down in the U.S., from Nebraska to Texas.

What they're saying: "We urge those who can to remain near a shelter today as the environment resembles that of previous historic tornadic outbreaks. Please heed warnings!" warned NWS forecasters at the Oklahoma City office in a forecast discussion Saturday afternoon.

What's next: Storms continued on Sunday across southeastern Texas and parts of Louisiana. The weather is far more quiet on Monday, but more severe weather is expected this week as an active pattern continues across the Central states.

Between the lines: The storms on Saturday and into Sunday were the result of a separate weather disturbance from the one that spawned tornadoes in Nebraska and Iowa Friday.

• It is extremely rare, if not unheard of, for two tornado outbreaks to hit on back-to-back days caused by two distinct storm systems.
• The storms on Saturday and Sunday were the result of an upper level low pressure area with associated spin that provided the trigger for thunderstorms to form in a warm, humid air mass across the Plains and Mississippi River Valley.

Context: Climate change is affecting the atmospheric conditions that give rise to severe thunderstorms, in particular by increasing instability.

• Some studies show a warming climate can make tornado outbreaks more prolific, but potentially less frequent.
• Climate science shows robust links between heavy rainfall events and a warming climate, since warm air holds more water vapor.

Editor's note: This story has been updated with the latest conditions.