NEW YORK — Retailers should see holiday sales rise a healthy 4% to 4.5% over last year and surpass $1 trillion, according to a forecast by Deloitte, which foresees sales increasing in stores and online.

“The projected uptick in holiday sales ties to four primary factors affecting consumer spending, starting with anticipated strong personal income growth,” commented Daniel Bachman, Deloitte’s senior U.S. economist. “Last year, disposable personal income grew 2% over the year to the holiday period, and we may see that rise to a range of 3.8% to 4.2% this season. Consumer confidence remains elevated, the labor market is strong, and the personal savings rate should remain stable at its current low level.”

Deloitte predicts e-commerce sales of at least $111 billion and as much as $114 billion this holiday season. That would represent a year-over-year increase of between 18% and 21%, Deloitte said in a statement on Wednesday.

Deloitte’s retail and distribution practice expects total holiday sales (seasonally adjusted and excluding motor vehicles and gasoline) of between $1.04 trillion and $1.05 trillion between November and January. By comparison, retail sales in the year-earlier period grew 3.6% to $1 trillion, Deloitte noted, citing U.S. Commerce Department numbers.

While fundamentals remain positive, Bachman cited uncertainties surrounding income growth that could bring the forecast in at the lower end of the Deloitte forecast. These include potential changes in the savings rate that would cause spending to expand more slowly; a U.S. government debt-ceiling crisis; and an unusually active hurricane season, which could either suppress or boost consumer spending, depending on its ultimate severity.

“Sentiment and spending indicators are firing on all cylinders, but the question is: How will retailers respond given the profound disruption across the industry?” remarked Rod Sides, vice chairman of Deloitte LLP and U.S. retail and distribution sector leader. “The good news is retail is thriving, and it is the proliferation of new, niche retailers that is resulting in share constantly changing hands. Consumers have unlimited alternatives and often bounce between brands, touchpoints and influencers, making it more difficult for retailers to attract shoppers without some level of customization. These disruptive factors are likely to combine to create a highly competitive and promotional holiday season. Retailers should modify their assumptions about what drives traffic, engagement and holiday sales growth, and realign around customer experience, creating relevant, emotional and inspirational connections that go beyond just product, price and assortment.”