July 12 (Reuters) - Online spending soared $24.1 billion across U.S. retailers during the stretch from July 8 to 11 – dubbed "Black Friday in Summer", outpacing Adobe Analytics' prior forecast, as eager shoppers rushed to snap up deep discounts on back-to-school essentials.

Retailers recorded online sales growth of 30.3% during events that included Amazon Prime Day, Adobe said on Saturday, compared with its projections of 28.4%.

Online retail sales in the U.S. rose 11% to $14.2 billion a year ago.

The Prime Day event has also been cemented as a "back-to-school" shopping moment, as consumers jumped on early deals to stock up on essentials and get ahead of the back-to-school rush, Adobe said.

Several major retailers including e-commerce giant Amazon.com AMZN.O, Walmart WMT.N, Target TGT.N as well as Best Buy BBY.N have been launching deal events with strong discounts, encouraging shoppers to trade up on expensive items they might normally pass on.

Amazon.com extended its sales window to 96 hours, up from the typical 48, featuring aggressive promotions on categories ranging from apparel to electronics.

A wave of enticing deals comes at a time when heightened trade tensions amid the Trump administration's volatile tariff policies have rattled consumers and businesses, and as the new August 1 deadline for countries to renegotiate trade agreements with the United States looms.

According to the report, shopping on mobile was the dominant transaction channel during the Prime Day event, driving 53.2% of online sales, above Adobe's forecast of 52.5%.

According to Adobe, overall discounts across U.S. retailers were between 11% and 24%, compared with the prior forecast range of 10% to 24%.

Apparel had the biggest deals at 24%, compared with 20% last year, while electronics were at 23%, similar to last year.

(Reporting by Anuja Bharat Mistry and Shivani Tanna in Bengaluru; Editing by Shailesh Kuber and Diane Craft)

((AnujaBharat.Mistry@thomsonreuters.com;))