On March 29, 2021, about 1547 Pacific daylight time, a Piper PA-28-140 airplane, N665FL, was substantially damaged when it was involved in an accident near Yacolt, Washington. The pilot and pilot rated passenger sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The airplane was the subject of a Federal Aviation Administration Alert Notification (ALNOT) when the pilot reported a rough running engine and then radio contact was lost with the airplane. A few hours later, the airplane’s wreckage was located by Navy search and rescue in heavily forested terrain about 5 miles east of Yacolt, Washington. The pilot reported that while at cruise altitude about 8,000 ft mean sea level, and in instrument meteorological conditions (IMC), he felt a vibration in the airplane and initially thought that it was airframe icing. There was none and realized that the engine started to run rough. He applied carburetor heat and requested a lower altitude. However, the engine seemed to run worse and lose power, so the pilot requested vectors to the nearest airport. While being vectored and after exiting IMC, he elected to make a forced landing in a nearby clearing. The airplane impacted several trees and was substantially damaged during the landing. A review of the weather conditions in the area by a National Transportation Safety Board (NSTB) senior meteorologist revealed that a low-pressure trough extended from northern Idaho and across Washington around the time of the accident. The accident site was located ahead of the trough in an area of westerly winds and overcast cloud cover. An area of continuous snow was depicted north of the accident site near Tacoma, Washington. A sounding model run over the accident site revealed that the freezing level was about 3,227 ft mean sea level (msl) with broken to overcast clouds from about 3,000 ft to 8,000 ft msl and icing in the clouds. Additionally, AIRMET Zulu was current over the area for occasional moderate icing conditions from the freezing level, estimated at 3,000 ft, to 12,000 ft msl. Two jet aircraft reported and confirmed the moderate icing below 8,000 ft. Flying in moderate icing conditions would decrease the airplane’s performance and additionally potentially cause vibrations. Postaccident examination of the engine revealed that during manual rotation, thumb compression was not obtained on the Nos. 1 and 3 cylinders. Additionally, leaking air was heard from the exhaust piping. A compression check revealed that the No. 1 cylinder was 5 psi, and the No. 3 cylinder was 0 psi. Borescope examination of the cylinder’s interior revealed that the No. 3 cylinder exhibited scoring on the walls and had carbon deposits lodged in the area where the exhaust valve seats. Examination of the other cylinders revealed a similar built up of carbon deposits, except that the No. 2 cylinder had substantially less build up than the other two. No other anomalies were observed that would have precluded normal operation. The pilot reported that the last annual inspection was accomplished about 6 months before the accident and the airplane had accumulated 25 hours since that inspection. The engine had accumulated about 1,929 hours since overhaul.