Aaron. The easiest way would be to use a helper column (which you can hide later if you like) wherein you assign numerical values to your Low, Medium and High (presumably 1, 2 and 3 respectively). Then you sort using the numerical column. It's fairly easy to write a one-cell array formula that would assign the numerical values to your labels. The numerical column need not be beside the label column; it can be any column.
Without seeing an actual sample sheet, I can't show you. But hopefully the concept is clear, and you can take it from there.
Added description after sheet was shared:
In the example sheet, Sheet1 Column A contained the Priority in words (Low, Medium, High) and Column B contained "other data." I placed the following array formula into C1:
=ArrayFormula({"Priority Val";IF(A2:A="","",VLOOKUP(A2:A,Data!A:B,2,FALSE))})
The formula is an array formula, hence the ArrayFormula() wrap.
Inside this are curly brackets {} which allow the building of arrays that are not "of a type." In this case, the header is listed first ("Priority Val"). The semicolon means "place the next part underneath." Then a VLOOKUP references every value in Column A (i.e., the priority words) against a simple chart in a second sheet named "Data." In that "Data" sheet, Column A simply lists 1, 2, 3 and Column B lists your exact words: Low, Medium, High. The IF() function just checks to see if a row in Sheet1!A:A is blank. If so, a null is assigned before trying the VLOOKUP; otherwise, every blank row would show an #NA error.
If you want to make it even more air tight, it's good practice to wrap VLOOKUP in IFERROR(), just in case you misspell something in Sheet1!A:A. That would look like this:
=ArrayFormula({"Priority Val";IF(A2:A="","",IFERROR(VLOOKUP(A2:A,Data!A:B,2,FALSE)))})
And you can avoid misspelling by applying data validation to Sheet1!A2:A, referencing Data!A:A as the only allowable answers. This is not strictly necessary; but I have done it in the sample sheet to show you.
