A Hornbill Skyways Beech B200GT was performing a non-scheduled communication flight from Mukah Airfield (WBGK) to Kuching International Airport (WBGG), Sarawak with 4 passengers and 3 crew on board. Upon landing on runway 25 at Kuching International Airport, the aircraft experienced runway excursion to the right of the runway pavement. The weather was cloudy and heavy downpour with cross-wind from the left during the final phase of the landing and also during landing roll. The aircraft travelled approximately 600 meters on the soft ground parallel to the runway before coming to a stop parallel to the runway heading of 255 degrees. The nose gear collapsed just before the aircraft came to a complete halt position. Probable Cause 1 Sudden Increase in the intensity of rain while flaring to land resulted in the significant reduction of the PF visual reference. Without the visual contact with runway centerline, the PF was not able to detect the lateral movement or heading change of the aircraft, therefore, the PF was unable to correct the displacement away from the runway centerline. The drift was also compounded by the sudden increase of strong crosswind from the left. 2 The PF had likely lost his positional and directional awareness with reference to the runway due to the degraded visibility, hence, he did not exert sufficient and timely rudder application to regain the runway centerline before departing the paved portion of the runway. Contributory Factors 1 Intensity of rain and wind speeds were increasing over the airfield throughout the approach and landing. 2 Inadequate risk assessment on the prevailing weather conditions made by the flight crews through the established TEM briefing. 3 Lack of alertness in recognizing the abnormal situation and assertiveness by the PM in getting the attention of the PF to the developing and impending deviation from centerline. 4 Currently, there is no standard callout in OM (B) in regards to calling out for runway centerline deviation.