What does the Special METAR observation SPECI KBOI indicate when it reports rain and mist?

The choice that rain began at 1812Z with visibility impaired is correct because a SPECI report is a special aviation weather observation that is issued when there are significant changes in weather conditions. When a METAR is updated to a SPECI due to rain and mist, it typically indicates that precipitation has started, which affects visibility. The timestamp (1812Z) indicates when the rain began, and the mention of visibility impairment suggests that the rain is impacting how far one can see, making it relevant information for flight operations and safety.

The other options do not accurately represent what the SPECI indicates. The presence of rain and mist does not imply that thunderstorms are ongoing, and calm winds would not typically be mentioned in conjunction with rain and mist in such an observation. Lastly, the report directly contradicts the idea that no precipitation is expected in the next hour, as the report is specifically detailing ongoing rain conditions.