my buttercream frosting keeps seperating

this is a recipe I found from

I've made the frosting, refrigerated till ready to use. I frosted my cake today, but it keeps seperating when it is sitting out.

I have 10 cakes to make for a bakesale Sat, and I was hopping this recipe would hold up to potential warm weather.

what am I doing wrong?

thanks in advance for any advice.

Diem

2/3 cup water 4 Tablespoons Meringue Powder 12 cups sifted confectioners' sugar 1-1/4 cups shortening 3/4 teaspoon salt 3/4 teaspoon No Color Almond Extract 3/4 teaspoon Clear Vanilla Extract 1/2 teaspoon No Color Butter Flavor