(Reuters) - Retailers are expected to show a sizable increase in sales for the month of May, but erratic trends of consumer spending could grow more pronounced over a seasonally weaker period for shopping.

Retail chains from Target Corp (TGT.N) to Costco Wholesale Corp (COST.O) to teen retailer Abercrombie & Fitch Co (ANF.N) are scheduled to report sales at stores open at least a year -- an industry gauge also known as same-store sales -- on Wednesday and Thursday.

Analysts expect May same-store sales to be up 3.8 percent, compared with a decline of 4.8 percent last year, according to Thomson Reuters data. Standard & Poor's sees a 3.4 percent increase, or 2.5 percent excluding gasoline sales.

COOLER WEATHER HURT

Driving the expected sales gain in May will be discount retailers like Target and Costco, which are expected to report an increase of 6.3 percent, according to Thomson Reuters. Gains for department stores and apparel chains are forecast at 1.5 percent and 1.8 percent, respectively.

However, teen retailers, including American Eagle Outfitters Inc (AEO.N) and Hot Topic Inc (HOTT.O), are expected to post a decline in same-store sales of 0.2 percent, Thomson Reuters said.

Major factors that influenced shopping in May include a Memorial Day holiday that falls on the last weekend of the month, pushing more sales into June and suggesting a low- to mid-single-digit drag on May, analysts said.

In addition, cooler weather everywhere but the Northeast, as well as heavier rain in the Northwest, hurt sales of summer apparel, analysts said.

"There's a lot of bodies in the Northeast. Is that enough to carry the rest of the nation?" said Scott Bernhardt, chief operating officer for Planalytics, which provides weather data for businesses. He said Texas also saw pretty good weather.

Regardless of what happens in May, June looks like a hit in the making with the holiday shopping and with "glorious" weather expected everywhere but the Southeast, Bernhardt said.