By Arriana McLymore and Neil J Kanatt

Oct 6 (Reuters) - U.S. holiday online sales are expected to grow at a slower pace this year, according to projections by data firm Adobe Analytics released on Monday, as macroeconomic uncertainty continues to pressure consumer spending.

Adobe expects U.S. online sales to rise 5.3% to $253.4 billion between November 1 and December 31, this year, compared with an 8.7% rise last year.

Cyber Monday, a major online shopping event that takes place on the Monday after Thanksgiving, is expected to be the biggest online shopping day of the season and year, with sales rising 6.3% to $14.2 billion on the day, Adobe said.

The holiday shopping period, a key driver of sales for retailers at the end of the year, will be even more critical this year as shifting trade policies under the Trump administration and persistent inflation take a toll on consumer spending.

"You have consumers dealing with a lot in the broader economy," said Vivek Pandya, director at Adobe Digital Insights. "We anticipate them taking advantage of these major sales moments, and we still see them leaning on the online sector as an area to get better deals."

Adobe Analytics expects shoppers to kick off some of their early holiday purchases during Amazon's AMZN.O October Big Deal Days, October 7 through October 8, and competing sales events. It forecasts $9 billion in spending between the two days, a 6.2% increase compared with last year.

Retailers have issued mixed outlooks ahead of the holiday season. Target TGT.N and Best Buy BBY.N maintained their annual forecasts, while Walmart WMT.N and Macy's M.N raised theirs. Toymaker Mattel MAT.O, however, reduced its expectations.

Adobe's forecast, which relies on direct online transactions based on more than 1 trillion visits to U.S. retail websites, echoes similar expectations of a subdued holiday season, with shoppers prioritizing essentials, hunting for deeper discounts, and cutting back on discretionary purchases.

The forecast sees shopping on mobile devices driving 56.1% of total online spending compared with desktop shopping, and a $2 billion increase in "buy now pay later" spending.

Buy now, pay later services let shoppers expand their purchasing power by paying for merchandise in monthly installments spread out over as many as 36 months; however, the most common payments are four-installment plans.

Adobe said discounts would likely reach up to 28%, similar to last year, as shoppers not just look for the lowest price, but trade up to higher-value items in categories, including sporting goods and electronics, as they seek more value out of their dollar.

(Reporting by Arriana McLymore in New York City and Neil J Kanatt in Bengaluru; Editing by Jacqueline Wong)

((Neil.JKanatt@thomsonreuters.com))