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After several days of storms and record cold weather forced cancellations of thousands of flights, the major airlines report that their operations are returning to normal. And shares were trading higher Thursday in the wake of strong results for December.

United Continental Holdings Inc. (NYSE: UAL) reported Thursday that its passenger revenue per available seat mile (PRASM) increased by as much as 12.5% in December, compared with the year-ago period. The company also said that it expects fourth-quarter capacity to increase 2.6% and for traffic to rise 2.6% with 82.4% of all seats filled.

United shares rose more than 11% in early trading and set a new 52-week high of $46.19. The 52-week low is $23.62.

Recently merged American Airlines Group Inc. (NYSE: AAL) also rose to a new 52-week high. The company said PRASM for American Airlines increased about 9% in the month of December. Capacity was 14.2 billion available seat miles (ASMs), or 1.1% higher year over year. U.S. Airways’ PRASM for the month of December increased about 12%, and capacity came to 7.4 billion ASMs, or up 3.6%.

Shares of American rose more than 6% in early trading to $29.60. The 52-week low is $12.70.

Southwest Airlines Co. (NYSE: LUV) announced Wednesday that in December ASMs grew 3.4% to 10.8 billion from the December 2012. PRASM for the month was estimated to have increased as much as 15%.

Shares here reached a new high as well. The stock hit $20.75 in morning trading. The 52-week low is $20.36.

Delta Air Lines (NYSE: DAL) also saw its shares rise nearly six percent in early trading Thursday to a new 52-week high of $31.58, compared to a 52-week low of just $13.09.

While JetBlue Airway Corp. (NASDAQ: JBLU) got an early bump as well, hitting a new 52-week high of $9.20, it fell back and was down almost 1% in late morning trading.