predicting change or predicting absolute values

we define
forecasting as testing model on data not utilised to develop model and
predicting as testing model on data which is obtained from observing the system in a future time.

in that case, the above graph shows a prediction of algal biomass one hour ahead of time. the system was developed to emulate the natural function relating (water quality parameters) and (chlorophyll one hour ahead in time) as observed in a period of 388 hrs, which is around 16 days and 4 hrs. it is validated over the next (approximately) 16 days and is tested over the next 32 days. The figure below summarises this.

what we find is that we follow trends well, but base value is lost. which suggests that we might as well try to predict 'change' in algal biomass. we could experiment by defining change as a vector value - with a magnitude and a direction.

One reason why this idea has not been persued yet is also because the above graph collapses to almost gibberish when time gap is increased further - see below.

To be fair, the above graph is not ALWAYS the case, the exact graph changes often enough - in one case it was tracing ok for a while and then turned into a straight line. but in all cases the regression coeff drops to something like .3 and the graph whatever be their flaw they all have this in common that they ARE NOT ACCURATE!

Predictions and Forecasts

Typically, the terms are used as synonyms - and it is important to remember that.
Principles of Forecasting, which is linked to by Journal of Forecasting, defines

Forecasting
Estimating in unknown situations. Predicting is a more general term and connotes estimating for any time series, cross-sectional, or longitudinal data. Forecasting is commonly used when discussing time series.

Prediction
A statement regarding future events or events that are unknown to the forecaster. Generally used as synonymous with forecast. Often, but not always used when the task involves forecasting with cross-sectional data (e.g., personnel predictions).

Forecast
A prediction or estimate of an actual value in a future time period (for time series) or for another situation (for cross-sectional data). Forecast, prediction, and prognosis are typically used interchangeably.

However since there are predictions and then there are predictions and then there are ofcourse forecasts - many people use these terms as 'jargon' with subtle differences; usually these are not obvious. Google gives some results which i will put when i update.

The defintion that is of significance to me was found on the NASA website where it talks in context of fluid dynamics here

Prediction.
Prediction is defined as

Use of a CFD model to foretell the state of a physical system under conditions for which the CFD model has not been validated. (AIAA G-077-1998)

Prediction is going beyond the validation database and performing simulations of untested systems.

The rest of the definitions at the NASA site are quite relevant too.