My problem is a small sample of quarterly macro data with only about 55 observations. During the observed period there were several shocks, one of which happened four years ago and was rather huge, affecting all variables. Now I am trying to forecast GDP growth based on 20 other macro/financial variables, but the forecasts are simply anticipating the huge plunge.
Introducing a break does not solve the problem, as it would only make sense in the case where the independent variables were not so severely affected. I tried combination forecasts using quadratic optimization, MSFE and DMSFE, I even tried restricting the lag structure of ADL or introducing a cubic Hermite term, but each time there was a downward trend after the 4th or 5th step ahead. The data seems to beg for smoothing, only I cannot use a simple mean/median over the problematic period for all the variables as it would mean losing a lot of information.
Would a spline term be a good solution? I am not looking for an opinion, but for an advice based on experience with modelling shocks.