Search continues for 5-year-old boy swept away by floodwaters in storm-hit California

More than 100 National Guard troops are helping search for a young boy who was swept away by raging floodwaters in central California several days ago.

The "first wave" of troops arrived on Wednesday "to assist with the search as conditions permit," according to the San Luis Obispo County Sheriff's Office, which is leading the efforts to find 5-year-old Kyle Doan. By Thursday, there were approximately 120 members of the California National Guard assisting with the massive search operations, in addition to the search and rescue teams and divers from the sheriff's offices of San Luis Obispo County, Santa Barbara County, Ventura County, Sacramento County, Santa Clara County, and Kern County.

There were also six K9 teams along with members of the California Rescue Dog Association, three hand crews from the California Department of Forestry and Fire Protection, air operations from the California Highway Patrol, drone teams from both the San Luis Obispo County Sheriff's Office and Grover Beach Police Department as well as members of the California Governor's Office of Emergency Services.

In total, some 200 personnel were involved in the search operations that day.

"The search is being conducted in extremely challenging conditions with mud and debris hampering the efforts," San Luis Obispo County Sheriff's Office said in a statement Thursday. "But this is a comprehensive effort to search every brush pile and area of debris."

Kyle Doan was traveling in a truck with his mother when their vehicle became stranded in floodwaters on Monday just before 8 a.m. local time near San Miguel, a small town inland from California's central coastline. Bystanders were able to rescue the mother but floodwaters swept the little boy out of the truck and downstream, authorities said. There was no evacuation order in the area at that time.

Later that day, at approximately 3 p.m. local time, an underwater search and rescue team was forced to call off their efforts after several hours "because the extreme weather conditions became too unsafe for first responders," according to San Luis Obispo County Sheriff's Office spokesperson Tony Cipolla.

An unrelenting series of powerful storms wielding torrential rain, heavy snow and gusty winds have walloped California over the past week and are expected to continue to do so until Jan. 18. At least 17 people have died in the state due to the recent stormy weather, according to California Gov. Gavin Newsom.

A break in the powerful storms allowed the search to resume Tuesday morning, though authorities warned that the conditions "remain extremely dangerous."

"The water level is high and continues to be fast-moving," the San Luis Obispo County Sheriff's Office said in a statement Tuesday. "The public is strongly cautioned not to conduct self-initiated searches and put themselves in harm’s way and become a victim requiring resources that would otherwise be used for searching."

Search operations were conducted for most of Tuesday, utilizing various resources, including the San Luis Obispo County Sheriff's search and rescue team, underwater search and rescue team, drone team, deputies, detectives and air operations.

On Wednesday, other local law enforcement agencies pitched in, providing dive team members, search and rescue team members and K9 units from the sheriff's offices of Ventura and Santa Barbara counties, as well as personnel with sonar equipment from the Sacramento County Sheriff's Office. Approximately 80 personnel were involved in the search operations that day.

"Search efforts will continue the rest of the day and will continue tomorrow as well if warranted," the San Luis Obispo County Sheriff's Office said in a statement Wednesday.

Kyle Doan is a kindergartener at Lillian Larsen Elementary School in San Miguel. His mother, Lindsy Doan, is a teacher at the same school, according to San Miguel Joint Union School District Superintendent Karen Grandoli.

"San Miguel is a small, very close-knit community and everyone has the family in their thoughts and prayers during this unfathomable tragedy," Grandoli told ABC News in a statement on Tuesday. "The family is continuing to search for him in hopes that he might be found. Staff is beginning to organize support for the family. We are determining what their immediate needs are at this time. The school will have counselors on campus to support students and staff as soon as the school reopens which will hopefully be tomorrow. Like most districts in San Luis Obispo County, we are closed today due to the severe weather and dangerous road conditions."

The missing boy, whom authorities have not declared dead, is described as having short dirty blond hair and hazel eyes. He is 4 feet tall and weighs 52 pounds. He was last seen wearing a black puffer jacket with a red liner, blue jeans and blue and gray Nike sneakers, according to the San Luis Obispo County Sheriff's Office.

ABC News' Marilyn Heck and Alyssa Starr contributed to this report.