The West Coast of New Zealand’s South Island can be a wet place. More forecasts of heavy rain (>100 mm/day) are issued for this region than any other in New Zealand. The period over which the National Centers for Environmental Prediction’s (NCEPs) ensemble prediction system (EPS) shows ability in detecting heavy rain events along the West Coast has been investigated. Verifications from MetService’s Severe Weather Warnings (SWWs) for the period June 2001 to May 2002 were compared against probability forecasts of 24 hour accumulated precipitation from the NCEP ensemble at the single grid point closest to Hokitika on the West Coast. The utility of the forecasts was assessed using signal detection theory.

It was found that there was little bias in amount, or drift in time of arrival, in the precipitation forecasts from the NCEP AVN model on which the ensemble system is based. Overall, the EPS probability of exceeding 13 mm/day showed the most potential to identify heavy rain events, with marginally useful forecasts possible out to about 7 days. However the overall level of skill in these forecasts compared to those issued by human forecasters was low. This may be due to the inability of the coarse resolution ensemble system to adequately detect convective events.