“Foreboding” means “ominous,” as in “The sky was a foreboding shade of gray” (i.e. predictive of a storm). The prefix “fore-” with an E, often indicates futurity, e. g. “forecast,” “foreshadowing” and “foreword” (a prefatory bit of writing at the beginning of a book, often misspelled “forword”). A forbidding person or task is hostile or dangerous: “The trek across the desert to the nearest latte stand was forbidding.” The two are easily confused because some things, like storms, can be both foreboding and forbidding
“Formidable,” which originally meant “fear-inducing” (“Mike Tyson is a formidable opponent”) has come to be used primarily as a compliment meaning “awe-inducing” (“Gary Kasparov’s formidable skills as a chess player were of no avail against Deep Blue”).
See also fearful/fearsome.