Chris Gorham wrote...
....
I can get part of the "forecast" function to work using "indirect" to
reference the beginning and end of the data sequence - I can therefore
control the range that "forecast" covers. It is now simply a matter of
detecting the beginning and end of the sequence.
....

If you have a variable number of positive values in, say, B3:IV3, the
index of the first one would be given by the array formula

=MATCH(TRUE,B3:IV30,0)

and the index of the last one would be given by

=MATCH(2,1/(B3:IV30))

So the dynamic range from the first to the last could be given by

INDEX(B3:IV3,MATCH(TRUE,B3:IV30,0)):INDEX(B3:IV3, MATCH(2,1/(B3:IV30)))

which would need array entry in formulas.