Heavy rainfall and flooding battered restaurant sales in August, and that trend looks like it will continue well into September.

Hurricane Harvey ripped through Houston and other parts of southeast Texas last week, weighing heavily on same-store sales of restaurants and dragging down overall same-store sales for the industry.

"August is really a tale of two different periods," said Victor Fernandez, executive director of insights and knowledge for TDn2K, a segment of Black Box Intelligence.

He said that during the first three weeks of the month, same-store sales were down about 1.4 percent, a narrower drop than the 2.8 percent in July. However, following Harvey, same-store sales at the end of August were down 2 percent, with traffic lagging 3.9 percent, according to Black Box Intelligence.

"Results from the final week of August were very soft, with two major events impacting results," Fernandez said. "The first, and most extensive, was the heartbreaking tragedy of Hurricane Harvey. Same-store sales in Texas declined 15 percent that last week, almost 13 percentage points below the average of the first three weeks of the month."

Fernandez said the second event was the Mayweather-McGregor boxing match, which may have drawn customers to casual dining bar and grill chains but likely negatively impacted full-service restaurants that do not showcase sports in restaurant.

Houston has some 13,000 restaurants, and more than 7,000 of those locations are chain restaurants. Lynne Collier, an analyst at Canaccord, said last month that Houston, which is one of the nation's largest restaurant markets, represents about 2 percent of the restaurant footprint in the country.

On Friday, Collier said she expects that same-store sales for the casual dining industry, in particular, fell 2.5 percent in August.

With Hurricane Irma aiming at Florida and part of the lower East Coast, Collier said she also expects September same-store sales in this sector to be down 2 to 4 percent.

In a note to investors, Collier updated the list of restaurants she covers that she sees as the most at risk for sales losses due to destruction by Irma: Bojangles, Fiesta Restaurant Group, which owns Taco Cabana; Zoe's Kitchen; Outback Steakhouse owner Bloomin' Brands; and Ruby Tuesday.