PHILADELPHIA — A destructive winter storm that has left more than 800,000 customers in the dark barreled into the Northeast today, delaying flights and turning the morning rush into the morning slush as communities braced for the worst.

The storm has been blamed for at least 20 deaths and a glaze of ice and snow caused widespread power failures from the Southern Plains to the East Coast. Authorities said it could be a week before some communities have electricity again.

Tree limbs encased in ice tumbled onto roads and crashed onto power lines in hard-hit Arkansas, Kentucky and Oklahoma on Tuesday and overnight.

Since the storm began building on Monday, the weather had been blamed for at least five deaths in Texas, four in Arkansas, three in Virginia, five in Missouri, two in Oklahoma and one in Indiana. Winter storm warnings were in effect today from Texas to New England.

Power failures continued in eastern Oklahoma and western Arkansas, where travel remained hazardous, officials said. Ice storms overnight in West Virginia knocked out power to thousands more.

More than 300,000 customers in Arkansas lacked power this morning and more than 375,000 in Kentucky. Public Service Commission spokesman Andrew Melnykovych in Kentucky said the figure is second only to the about 600,000 Kentucky customers who lost power when the remnants of Hurricane Ike lashed the state with fierce winds last September.

In Indiana and Ohio, more than 160,000 homes and businesses were in the dark as ice and snow coated the states.