Shocking, startling, jarring, and surprising are a few synonyms.
intimidating, alarming, frightening, unnerving, disconcerting
perturbing, troubling, worrisome, distressing, upsetting, alarming, unsettling.
Scary, frightening, alarming, unsettling, disquieting, disturbing, freaky, ghostly, nightmarish
soothingly
terrifying, horrifying, scary, paralyzing, alarming,apalling... The synonym you should use depends on the context.
Other words for startling are alarming, astonishing, shocking, surprising, unexpected and frightening.
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Another word for "yikes" is "whoa." It is commonly used to express surprise or alarm because it is a short, informal exclamation that conveys a sudden reaction to a surprising or alarming situation.
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Marie with some alarm called the Police after hearing the tree brush against the window.
It was alarming how fast he grew.
scared and worried terrifying, shocking, alarming, startling, horrifying, menacing, fearful