I believe the 'justification' is JAROPS, based on that wonderful 'statistical analysis' - but I'm sure 'TEMPO' (and 'BECMG') have to be considered. PERSONALLY I exercise extreme disbelief in the 'PROB' side of the forecast, having been 3 times seriously caught by bad met. Many's the time I've arrived at a PROB 30 airfield to find PROB 100, but that is YOUR decision. Likewise whether you 'consider' a TEMPO or BCMG which claims to be outwith your operating window.