Solar and wind power are being blamed as largely responsible for the potential danger. From Naveen Athrappully at The Epoch Times via zerohedge.com:
Parts of America could face difficulties in meeting electricity demand during the summer season, with renewable energy sources like wind and solar power posing a potential risk to reliable power supply, according to a report by the North American Electric Reliability Corporation (NERC).

The NERC report classifies several parts of the country as facing an “elevated” risk of summer electricity reliability for the upcoming June-September period.
Elevated risk means there is “potential for insufficient operating reserves” when the region faces above-normal demand conditions. Such regions include parts of Louisiana, Texas, New Mexico, Arizona, California, Illinois, and Iowa. The determination of elevated risk is based on various factors, including potential low wind or solar energy condions that could lead to a lower electricity supply.
The North American power bulk power system (BPS) is made up of six regional entities—Midwest Reliability Organization (MRO), Northeast Power Coordinating Council (NPCC), ReliabilityFirst (RF), SERC Reliability Corporation (SERC), Texas Reliability Entity (Texas RE), Western Electricity Coordinating Council (WECC)—with elevated risk upcoming in certain regions.
Midcontinent Independent System Operator (MISO), which manages the electricity capacity market, operates in 15 U.S. states, including Texas, Illinois, Montana, Arkansas, and Kentucky. MISO is expected to have “sufficient resources” to meet normal summer peak demand, the NERC report said.