DETROIT— March will be one of the most crucial months for the U.S. auto industry in years. Sales were slower than expected in January and February, and the number of unsold cars on dealer lots grew. Some automakers had to resort to juicy discounts to lure reluctant buyers. Most industry executives, dealers and analysts blame the historic cold and snowfall and expect warmer weather to restore consumers’ enthusiasm for car buying. Most still expect annual sales to exceed 16.1 million, which would be the highest since 2006.