HOUSTON, Texas (KTRK) -- A Silver Alert has been issued for a missing 77-year-old man last seen in northwest Harris County. According to the Harris County Sheriff's Office, Leon Howard was last seen on Tuesday in the 8100 block of Enchanted Forest Drive around 7 p.m. Howard has been diagnosed with a cognitive impairment and authorities believe his disappearance may pose a threat to his health and safety. He's being described as a Black man with grey hair and black eyes standing at 5 feet 10 inches and weighing 130 pounds. According to authorities, he was last seen wearing a green shirt with white stripes, blue jeans with one black shoe and one brown shoe.