Slater, who is from Oswaldtwistle in Lancashire, was last in contact on the morning of Monday, June 17. He called a friend to say he had no water and only 1% battery left on his phone.
He was on vacation with friends and had returned to a rented house in a rural area of the island with people he met at a festival.
On Friday, the Guardia Civil asked volunteer groups, including firefighters and rugged terrain experts, to sign up for a large planned search of the rocky area near where the 19-year-old disappeared.
Angel Sanz Coronado, the police chief of Tenerife's Guardia Civil, said, "After the 19-year-old British man disappeared on June 17 in the Masca area of Buenavista del Norte, the Civil Guard is ready to conduct a large search."
"Since the area is steep, rocky, and full of uneven terrain, ravines, and trails, we ask for help from all volunteer groups for this planned and coordinated search."
"This large search will start on Saturday, June 29, at 9:00 AM. We will meet at the Mirador de la Cruz de Hilda in Masca to begin the search in an organized way along the many paths and ravines in Masca."
David Cameron, the UK foreign secretary, said he was "thinking of and worrying about" Slater and that the family was receiving consular support.
During a campaign visit to Rolls-Royce in Barnoldswick, Lancashire, on Thursday, Lord Cameron said, "I am obviously thinking about and worrying about the family and this young man. Consular officials are in Tenerife talking to the family and local authorities, and are very eager to make progress and find out what happened."
The Masca gorge, where the search is happening, has already been searched by police with dogs, helicopters, and drones, but they found no trace of Slater.
He is not the first person to go missing in the area, and locals mentioned that it sometimes takes months to find the bodies of missing people.
On Friday, Slater's friend Brad Hargreaves said on ITV's This Morning that he had been on a video call with Slater before he disappeared and heard him mention going off the road.
He explained, "He was on the phone, walking along a road, and he veered off a bit—not a big drop, just a small one—and he was going down. He said, 'I'll call you back, I'll call you back,' because I think someone else was trying to reach him."
He mentioned seeing his friend's feet "sliding" down the hill and could hear the sound of him walking on gravel.
"At the time, I didn't think much of it. I just assumed he was going to take a bus or a taxi home, as he often says he will."
Hargreaves mentioned they were both laughing at that moment.
He told the program that he was "hoping" for him to come home.
Slater's mother, Debbie Duncan, who has been searching on the island for her son for over a week, said she still believes he could be found alive and hasn't lost hope.
A GoFundMe page created to support the search for Slater raised over £40,000 by Friday. Duncan mentioned that the funds would be used for mountain rescue, accommodation, and food costs.
In an update on the page, initiated by Slater's friend, she wrote: "I want to let you know that this money will help the mountain rescue teams who are working hard to find Jay. Also, because we need to stay longer in Tenerife, the funds will cover our accommodation and food costs."
"I'm with amazing people who are here with me, away from their families. We'll use some of this money to bring them to Tenerife so we can support each other during this difficult time."
SOURCE: The Guardian