Retail sales are expected to grow modestly this holiday season.

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The National Retail Federation (NRF) expects sales in the months of November and December to increase 3.9 percent to $602.1 billion, over 2012's 3.5 percent holiday season sales growth.  The forecast is higher than the 10-year average holiday sales growth of 3.3 percent.

"For retailers it's a lot more than just two months out of the year, it's the biggest time of the year," said Kathy Grannis, a spokesperson for the NRF.

While the forecast is for actual growth, there are some questions that remain.  "There are a few variables and a few factors that could derail the holiday season for retailers," said Grannis.

Grannis said a lot remains unknown about the federal stalemate, hiring and the economy.

"Our forecast is a realistic look at where we are right now in this economy - balancing continued uncertainly in Washington and an economy that has been teetering on incremental growth for years," said Matthew Shay, president and CEO of NRF.

Overall, retail sales drives at least 70 percent of the United State economy.