Deterministic Skill of Subseasonal Precipitation Forecasts for the East Africa-West Asia Sector from September to May

The East Africa-West Asia (EA-WA) sector is a region where the skill in forecasting rainfall is unusually high in the subseasonal-to-seasonal (S2S) time range and where ENSO and MJO signals have significant impacts. Much of regional rainfall intra-seasonal variability is related to tropical-temperate interactions on synoptic time scales, and we examine whether the skill of dynamical S2S forecasts for the region exceeds the predictability associated with ENSO and the MJO. Deterministic skill (Spearman's rank correlation) of multimodel ensemble forecasts of weekly and Week 3-4 precipitation averages is examined for starts in September-April, based on three ensemble prediction systems (EPS) from the S2S prediction project over the common 1999-2010 reforecasts period. The skill of weekly forecasts decreases with increasing lead but remains statistically significant out to Week 3 and Week 4, a level of skill that has not been previously reported at such leads. The skill of Week 3-4 forecasts is higher than that of Week 3 or Week 4 forecasts. The prediction skill of weekly and 2-week precipitation averages from ENSO and the MJO is estimated using a multiple linear regression and found to be less than that achieved by the dynamical forecasts for Week 3, Week 4, and Week 3-4 targets. This is evidence for achievable forecast skill that extends beyond typical weather-scale lead times and that is not simply related to ENSO or MJO signals. The opportunity for skillful predictions is illustrated through successful forecasts up to 4 weeks in advance of extreme rainfall over the Arabian peninsula related to strong tropical-temperate interactions during weak ENSO and MJO conditions.