The searches are being co-ordinated by volunteers from Foyle Search and Rescue, Search Team North West, along with family members and friends.
Over two days have passed and still there is no sign of the 42-year-old despite extensive searches by volunteers.
Foyle Search and Rescue say on its Facebook page this evening: “Helena Mc Elhennon has been missing since Sunday the 24th November and the PSNI are becoming increasingly concerned for her welfare.
“After tasking by the PSNI, volunteers from both Foyle Search & Rescue & the Community Rescue Service have been conducting ongoing search operations at various locations, in an effort to locate her.
“The combined efforts of FSR and Search Team Northwest along with a large family group and the public was evident last night as the search continued in the city.
“As of this evening search operations continue.
“Inspector John Moore described Helena as being 5″4 in height, of medium build with short, dark hair and possibly wearing a maroon coloured cardigan or long, brown dress.
“Helena was last seen in her black Mitsubishi Warrior vehicle on Sunday morning, November 24, at around 10.30 am.
“The vehicle was located at the lay-by close to the Foyle Bridge on the city side.
“Foyle Search & Rescue, would appeal to anyone who was in the vicinity on Sunday morning who may have observed Helena then or at any time since to contact police on 101 quoting reference number 771 24/11/19.
“FSR recommend all searchers read our search safety advice:
•Always stay in a group & let someone know the area you are searching and notify them when you have completed your search.
•Persons searching should wear appropriate clothing for the weather conditions.
•It is recommend that persons searching near the water’s edge wear a Life Jacket & that all other searchers wear high-vis clothing to enable boat crew to observe them on river banks.
•If you have access to a pole or walking stick please use it to test the ground in front of you.
•Always carry a mobile phone.
•Be careful along the river’s edge. Search between the river’s edge and high water line.
•If you approach rocks or large tress, try to walk around, not over, unless this means going to close to the edge.
•DO NOT enter the Glar/mud. You may become stuck & need rescued.
•DO NOT wade or enter the water.
•DO NOT cross over any railings under any circumstances.
•DO NOT cross over onto or walk along Rail Tracks
•In the event of finding any items of clothing etc. DO NOT TOUCH or remove. Please contact the PSNI on 101 who will in turn alert Foyle Search & Rescue on 02871 313800.
•Check the tidal tables and weather forecast prior to commencing the search. If you need any advice on tides please contact Foyle Search.
•People who are searching are reminded that the river banks are extremely hazardous after dark.
•Please when searching along railings or on any of the Bridges DO NOT lean over the railings as CCTV operators can
not differentiate between a genuine searcher or a person who may be in need of help requiring immediate activation of the emergency services.”