Helicopter
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Rotorcraft
Situational Awareness; Time Pressure
Inflight Event / Encounter VFR In IMC; Inflight Event / Encounter Weather / Turbulence
Flight Crew Became Reoriented
Company Policy; Weather; Human Factors
When accepting the flight the Wx (Weather) report at base/sending hospital was 10 sm and clear. Wx at receiving hospital was 6 sm and clear. Although cameras showed haze down the mountain I was confident with the visibility reported and knew that the sun set made it look worse. Descending down the mountain the layer of haze did look more dense than forecasted and arrived at receiving hospital without issues. I checked all weather before return back to base and all were still reported the same. Now it was fully dark and cameras gave a clear view of the basin. I chose the return leg through ZZZ as this gives a good indication of visibility due to the lights up the mountain and all the way. Due to 0% moon there was no reference than the lights on the ground. I climbed to 8;000 ft. which provided proper clearance along that flight path. From ZZZ I could see the lights at the airport from about 10 sm. When I got to ZZZ1 valley the visibility rapidly decreased and within seconds we were IMC. I started my climb to about 9;000 ft. and because my crew did a fantastic job immediately giving me report on visibility left and right; I made a left turn which got us out of the clouds fairly quickly. My crew kept on briefing me their view/visibility on both sides which got us back on track on the north side of the lake. Here we could see the haze/fog/clouds spilling through and over the south mountain range. From that position we had a clear path to the airport and landed without further incident. Weather reporting in our area is spotty. We have wx reporting up in the mountain at the airport and down the mountain at two other locations. The weather 'out of control' factor here is the mountain ranges. There are several tools like cameras that supply information and you make your best judgment; but if the weather is very dynamic you can be caught by surprise like we did on this incident. The additional factor of 'no-moon illumination' added to the fact that we did not see the forecasted clouds rolling in. The 'in-control' weather factor here was that I could have been severely cautious and decline the flight on the haze shown on the cameras. Although I kept positive control over the aircraft when initiating IIMC [Inadvertent IMC] protocol I do want to mention that an autopilot would be a great tool in circumstances like this. When practicing the IIMC procedures in the SIM I am spoiled with the situational awareness and added 'second pilot' of the autopilot. Due to the fact that we are dealing with mountain obscuration and often weather that is not reported and can only be identified with visual cues; we should be first in line for a helicopter with these kind of capabilities.
Medical transport helicopter pilot reported inadvertent IMC caused unforecast mountain obscuration.