Found a blue hole thermal, 4-5ms, rode it up above cloud base where it started forming around me. Decided to stick with it as I could see the outline of the sun above me. After a few hundred more feet I realised I couldnt see it anymore and the lift was increasing. I aimed downwind and stuck with it for a short while but it started getting seriously rough and still lifty. I started a spiral still in whiteout, and realised I had lost a sense of the horizon, and wasn't too sure how to safely exit. After only 1 turn wasn't sure if I was going up or down, and how steep my spiral was, but felt a sharp airspeed and G increase. My right arm basically made the decision for me, and reserve came out quick but invisibly as it was cloud colour (looking back and not seeing it as my wing feel in front of me was awful). For about 20 seconds, my vario was reading -1ms under reserve but then came out the bottom and went to -5. I wouldn't call the deployment 'panicked' but instinctual instead.