SUM column B but if B contains blanks, sum it with column A for these rows?

Good afternoon everyone,

I've ready through several other posts from other users requesting help with SUM, SUMIF, and IF/AND/OR formulas but I can't see anything which quite fits with my query here.

I have a spreadsheet which is forecasting tonnage into our site for the week. I enter the estimated tonnage by weight p/day (lets call this column A) on a Friday for the following week. As we progress through the week, I enter the actual tonnage received for the previous day (column B).

I am trying to determine a formula that will calculate the expected tonnage for the week, but leaning predominantly towards the actual tonnage.

So basically, at the start of the week on a Monday, the calculation would simply show the estimated tonnage total (column A). As I fill in column B through the week, I need the formula to update to use this figure, as opposed to it's estimated figure in column A.

I hope you are all still following at this point. I've tried various permutations on the formula but I just don't know if this is even possible and I'm asking too much? I thought I'd found the answer on an Excel support page which showed:

=SUMIF(B13:31,"<>",A13:A31)

but this returned a value which didn't seem to correlate with anything?

Can any of you excel whizz's on here possibly help me?

Many thanks!

Assuming you are only calculating for Monday - Friday, and Monday Forecast starts in cell A2, and Monday Actual starts in cell B2 then use this formula,
This also assumes fridays Forecasting and Actuals data is in cell A6 and B6 respectively.. [Change cell references if necessary]

=SUM(IF(B2="",A2,B2),IF(B3="",A3,B3),IF(B4="",A4,B4),IF(B5="",A5,B5),IF(B6="",A6,B6))

SUM(if(B13:b31="",a13:a31,B13:B31)) with shift ctrl enter, its an array

or
=SUMIF(B13:B31,"",A13:A31)+SUM(B13:B31)