Actually, the forecasts have started coming true ahead of schedule.
The forecasts didn't say there would be a recession the day after the referendum. The forecasts were made to cover a wide time frame, but as most people believed Article 50 would be triggered the day after the referendum, they assumed the forecasts would be immediate too.
This was then sometimes mistakenly repeated by people, particularly by the Brexiteers when using it to mock the economists with their straw man argument.
It's also important to stress that the forecasts were not specifically that a recession would happen. The forecasts were basically a percentage hit to the UK economy. At the time the forecasts were made, the worldwide economic conditions meant that this would result in a recession.
However, worldwide economic conditions have improved significantly, which means that the forecast hit to the economy isn't big enough to push us into recession.
Whilst the UK economy is just trundling along, the rest of the world is booming. Right now, the UK should be booming too. The damage from Brexit means it's not.
We are already poorer than we would be if the referendum had resulted in remain vote.