Place rice in a fine mesh strainer and rinse under cold water until water runs clear. Tap the bottom of the strainer to remove excess water.

Transfer rice to a medium saucepan along with 2 1/4 cups water. Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes. Remove from heat, keep covered, and let rest for 10 minutes.

In a small saucepan, combine 4 Tbsp of vinegar with sugar and salt. Heat over medium heat, whisking until the sugar dissolves. Let cool while rice is resting, then pour the vinegar mixture over the rice and toss to coat.

In a small mixing bowl, whisk together mayonnaise and sriracha. Transfer to a sandwich-size resealable bag and set aside.

In another small mixing bowl, whisk together soy sauce and remaining 1 Tbsp vinegar, then set aside.

In a large mixing bowl, gently toss together crab meat, cucumber, carrots, nori, ginger, and avocado or keep separate and divide into sections over rice in serving bowls.

Divide prepared rice among 4 or 5 bowls. Top with the crab mixture.

Spoon the soy sauce mixture over each serving. Cut a small tip from one corner of the resealable bag and drizzle sriracha mayo over each serving. Serve shortly after preparing.