The 22-year-old requested his month-long spell at Blundell Park be terminated after the Mariners completed the loan signing of Josh Lillis from Scunthorpe on Tuesday.
Forecast joined the League Two club on August 14 after first-choice shot-stopper Nick Colgan suffered a groin injury, and he started the four subsequent league games.
However, Lillis' arrival put his place in the team into question and he has gone back to St Mary's meaning Lillis will make his full debut against Port Vale on Saturday.
Town manager Mike Newell said: "It was difficult for Tommy coming into a side which was not performing, but unfortunately it didn't work out.
"Tommy said 'thanks very much for the opportunity' and wished us all the best. That was the right response, he's a terrific lad. Sometimes it works out and sometimes it doesn't, and you have to hold your hands up and cut your losses."