What does the Special METAR observation "SPECI KBOI 091854Z" signify?

The Special METAR observation "SPECI KBOI 091854Z" indicates a significant weather change that has been reported. METARs are routine aviation weather reports, whereas SPECIs are special reports issued when there are notable changes in weather conditions.

The 'SPECI' keyword in the observation alerts pilots and meteorologists to important changes that could impact aviation operations. The presence of "rain" and "mist obstructing visibility" in the correct answer highlights the specific weather conditions affecting visibility. The detail about when the rain began—at 1812Z—provides crucial intel for flight operations because it tells pilots how fresh the precipitation is and helps them anticipate visibility changes due to rain.

Understanding the implications of rain and mist is vital for flight safety as they can significantly impact visibility and aircraft performance. This context around visibility and precipitation timing is critical in aviation, making this answer appropriate in the context of the question.