North Dakota, which on June 17 became just the fourth state to record oil production above 1 million barrels a day (MMbbl/d), could see even stronger growth over the summer as improved weather makes life easier for drilling crew, Bloomberg said June 18.

Output increased to 1,001,149 bbl/d in April, the state’s Department of Mineral Resources reported June 17. Texas, California and Alaska have crossed the million-barrel mark. Only Texas remains above the state, at almost 3 MMbbl/d.

April oilfield work was hampered by heavy rain that shut roads and strong winds that closed down operations. Crews completed 200 wells during the month, and another 600 are already drilled and just waiting on hydraulic fracturing, or fracking. Better weather in the summer months should allow more new wells to start gushing oil.

“As the weather improves, operators should have full utilization of all their rigs, and possibly additional completion crews to whittle down the backlog,” Jonathan Garrett , an upstream analyst at Wood Mackenzie Ltd . in Houston, said in a phone interview June 18. “I wouldn’t be surprised to see quite a bit of production growth over the summer. It should be pretty impressive.”

Oil and gas from the Bakken and other shale formations helped the U.S. produce the equivalent of 87% of its energy needs in 2013, the highest level since 1985, according to data from the Energy Information Administration. The U.S. imported 7.7 MMbbl/d of crude in 2013, the least since 1996.

Most oil produced in North Dakota comes from the Bakken and Three Forks Shales, layers of hydrocarbon-rich rock more than a mile beneath the Earth’s surface. High crude prices and improvements in drilling technology have helped companies like Continental Resources Inc . (NYSE: CLR) and Whiting Petroleum Corp . (NYSE: WLL) tap into the previously inaccessible shale.

Creating wells in the Bakken is a two-step process. Drillers make horizontal bores along the shale, and then completion teams inject a high-pressure mixture of water, chemicals and sand to create micro-fissures in the rock through which gas and oil can seep.

Output from shale wells declines by 60% to 70% in the first year, according to Austin, Texas-based Drillinginfo Inc ., faster than traditional wells. Because of the steep decline rate, companies need to finish new wells constantly. Bad weather can slow the completion process, curbing production growth.

In April, roads were shut for three