Chipotle Mexican Grill (NYSE:CMG) has been on a tear this year. Comparable-restaurant sales growth has accelerated, and digital sales are soaring as the company's efforts to streamline operations and improve online ordering are paying off. This impressive business momentum, of course, has pleased investors. Shares have surged, rising 73% year to date.

Given Chipotle's strong performance recently, investors will be keeping a close eye on the fast-casual burrito chain when it reports second-quarter results later this month. Ahead of Chipotle's second-quarter update on July 23, here's a preview of some key areas investors will want to watch.

Chipotle burrito, chips, and guacamole

Image source: Chipotle Mexican Grill.

Comparable-restaurant sales

Chipotle's comparable-restaurant sales, or the change in sales at restaurants in operation for 13 months or longer, have seen significant improvement recently. The momentum became particularly evident in the fourth quarter of 2018, when the company returned to transaction growth. Comparable-restaurant sales increased 6.1% year over year during the quarter, helped in part by a 2% increase in restaurant transactions. Then comparable-restaurant sales jumped sharply in Q1, rising 9.9%, driven primarily by a 5.8% increase in transaction growth.

When Chipotle reports its second-quarter results later this month, investors will be looking for more strong growth in this metric. For context, management guided for full-year comparable-restaurant sales to increase at a mid- to high-single-digit rate compared to 2018.

Digital sales

Perhaps even more impressive than Chipotle's comparable-restaurant sales recently is the company's accelerating digital sales growth. Digital sales increased 48%, 66%, and 101% year over year in the company's third quarter of 2018, fourth quarter of 2018, and first quarter of 2019, respectively. Even more, digital sales accounted for 16% of first-quarter sales, up from 11% of sales two quarters ago.

Digital strength has been fueled by growth in delivery, order ahead, catering, enhanced capabilities on Chipotle's app, and improved conversion on the company's website, management explained in the company's first-quarter earnings call.

Restaurant-level operating margin

Highlighting the strengthening of Chipotle's economic model, the burrito maker reported its highest restaurant-level margin in almost four years in Q1. The metric came in at 21%, up from 19.5% in the year-ago quarter and 17% in the fourth quarter of 2018.

"The improvement was driven primarily by comparable restaurant sales increases and lower repair and maintenance expense," said Chipotle in the company's first-quarter earnings release, "partially offset by wage inflation, increased marketing and promotional cost, and delivery expense associated with increased delivery sales."

Look for a similar 150 year-over-year basis-point improvement in Chipotle's restaurant-level operating margin in Q2. This would put the metric at about 21.2% for the quarter.

Chipotle reports its second-quarter results after market close on Tuesday, July 23.