The cupcakes are for a party that will be outside, by the ocean, in the middle of April. This year, it's really difficult to predict what the temperature will be; could be anywhere from 65 to 80 degrees... The plan is for the cupcakes to be placed on a tables outside but in the shade. Do you have advice for ingredients or a type of frosting based on that? Thank you so much in advance!
Cupcakes themselves should be fine. I'd chill after decorating. Place in air tight container. If stored in boxes, then wrap the box with plastic wrap, Long refrigeration can dry a cake out, but overnight shouldn't be an issue is in air tight containers.
Regarding buttercream, You'll need to use either a Italian meringue buttercream or Rose Levy Beranbaum's mousseline buttercream. They are stabilized with boiled sugar. I've used both.
Rose Levy Beranbaum's claims her mousseline buttercream is stable In ambient temperature up to 90°. I've used it in temperatures in the low 80s without issue. I lived in Orange County at the time. She has both recipe and video on her website.
If the weather forecast for the week of the event is both high heat and humidity, you may want to consider using an Italian meringue buttercream with a blend of butter and shortening. Gretchen's Bakery website has a recipe and video. Her recipe calls for 1 cup of shortening, but you can probably reduce that to 1/2 a cup, and increase the butter. While I'm not a fan in the least of shortening based icings, an all butter buttercream is not an option where there's a combination of high heat and humidity.
If you use shortening, then Sweetex brand if you can buy it. It's a commercial high ratio shortening that is superior to supermarket shortening. It's produced for the trade so it can be difficult to purchase in small quantities. But I've seen it offered in 3 lbs tubs (re-packaged by the retailer). Amazon, Kitchen Kraft, or Global Sugar Arts would be a couple of places to look for it. You could call Surfas Culinary District in Costa Mesa. They are a restaurant supply company; since they offer a lot of classes for the home cook, they re-package bulk ingredients that are produced to the trade.