Historical skill · May 2026
How forecasts made with the same method scored in past years against climatology (the observed frequency of each tercile in the training years, about one-third each). The main number is the cross-validated 1993–2016 score: each year is forecast with parameters fitted without that year. Later years are exploratory: they were seen while the method was chosen.
Takeaway — May 2026
Cross-validated 1993–2016 skill is +0.035 (interval +0.009 to +0.060, above zero): a modest improvement over climatology. The exploratory 2017–2025 result (−0.006) is encouraging but those years were inspected during method selection. The 2026 verification (+0.033) is one season and does not override the historical uncertainty.
| Target | Cross-validated 1993–2016 (main) | In words | Exploratory 2017–2025 |
|---|---|---|---|
| May | +0.035 (+0.009 to +0.060) | Modest improvement over climatology (interval above zero) | −0.006 (−0.089 to +0.051) |
| FMAM | +0.036 (−0.003 to +0.071) | Small estimated improvement; skill uncertain | +0.014 (−0.048 to +0.080) |
| Feb | +0.026 (−0.006 to +0.059) | Small estimated improvement; skill uncertain | +0.025 (−0.003 to +0.057) |
| Mar | +0.001 (−0.012 to +0.012) | Small estimated improvement; skill uncertain | +0.011 (−0.004 to +0.024) |
| Apr | −0.002 (−0.020 to +0.013) | No demonstrated improvement over climatology | +0.001 (−0.027 to +0.028) |
RPSS +0.05 means a 5% lower ranked probability score than climatology. An interval that includes zero means the gain is not established.
Technical detail: years better, p-values and blend weights
| Target | Years better (cross-validated) | p (one target / Holm) | Years better (exploratory) | Blend weight λ |
|---|---|---|---|---|
| FMAM | 18 of 24 | 0.043 / 0.171 | 3 of 9 | 0.34 |
| Feb | 16 of 24 | 0.067 / 0.202 | 7 of 9 | 0.76 |
| Mar | 12 of 24 | 0.451 / 0.901 | 5 of 9 | 0.86 |
| Apr | 12 of 24 | 0.564 / 0.901 | 5 of 9 | 0.76 |
| May | 19 of 24 | 0.009 / 0.044 | 6 of 9 | 0.59 |
Each interval and one-target p-value describes that target alone. Because 5 targets are examined, the Holm-adjusted p is the stricter test: May remains significant after adjustment. λ is the weight given to climatology in the blend (higher = closer to climatology).
Historical verification explorer
The evidence behind the scores above, for May 2026 in this area. Choose the evaluation period; all diagnostics use the blended probabilities against climatology.
Performance by year
Probability skill (RPSS) of each year; better than climatology in 19 of 24 years. Weakest years: 1997 (−0.154), 2001 (−0.084), 2010 (−0.075).
Reliability
Points on the dashed diagonal are reliable: events forecast with probability p happen p of the time. Points flatter than the diagonal mean over-confident probabilities. Open points have fewer than 134 cell-years (0.5% of the sample, at least 50) and are not joined. Hover a point for its count.
Reliability table (forecast bins, observed frequency, counts)
| Category | Bin | Mean forecast | Observed frequency | Cell-years |
|---|---|---|---|---|
| below normal | 10%–20% | 17% | 27% | 1,293 |
| below normal | 20%–30% | 27% | 23% | 7,040 |
| below normal | 30%–40% | 35% | 35% | 14,685 |
| below normal | 40%–50% | 44% | 57% | 3,676 |
| below normal | 50%–60% | 52% | 56% | 186 |
| near normal | 10%–20% | 18% | 20% | 556 |
| near normal | 20%–30% | 26% | 23% | 9,463 |
| near normal | 30%–40% | 35% | 32% | 12,118 |
| near normal | 40%–50% | 43% | 44% | 3,636 |
| near normal | 50%–60% | 54% | 41% | 960 |
| near normal | 60%–70% | 61% | 35% | 147 |
| above normal | 10%–20% | 18% | 10% | 99 |
| above normal | 20%–30% | 27% | 23% | 9,203 |
| above normal | 30%–40% | 35% | 40% | 13,610 |
| above normal | 40%–50% | 44% | 45% | 3,775 |
| above normal | 50%–60% | 53% | 36% | 193 |
How strong are the signals?
Share of cell-years by the leading tercile probability.
| Category | Forecast | Climatology | Brier skill |
|---|---|---|---|
| below normal | 0.2185 | 0.2265 | +0.036 |
| near normal | 0.2083 | 0.2122 | +0.018 |
| above normal | 0.2190 | 0.2268 | +0.034 |
Rainfall amounts
Historical rainfall-amount diagnostics (CRPS, CRPSS and amount errors) have not been generated for May 2026 in this area and period; this needs a separate cross-validated amount evaluation. Single-season amount scores are in the Verification section.
Performance by year
Probability skill (RPSS) of each year; better than climatology in 6 of 9 years. Weakest years: 2021 (−0.280), 2020 (−0.012), 2017 (−0.003).
Reliability
Points on the dashed diagonal are reliable: events forecast with probability p happen p of the time. Points flatter than the diagonal mean over-confident probabilities. Open points have fewer than 50 cell-years (0.5% of the sample, at least 50) and are not joined. Hover a point for its count.
Reliability table (forecast bins, observed frequency, counts)
| Category | Bin | Mean forecast | Observed frequency | Cell-years |
|---|---|---|---|---|
| below normal | 10%–20% | 17% | 15% | 452 |
| below normal | 20%–30% | 28% | 15% | 2,473 |
| below normal | 30%–40% | 34% | 29% | 5,643 |
| below normal | 40%–50% | 43% | 21% | 1,441 |
| below normal | 50%–60% | 52% | 4% | 71 |
| near normal | 10%–20% | 19% | 31% | 13 |
| near normal | 20%–30% | 27% | 35% | 2,480 |
| near normal | 30%–40% | 34% | 40% | 5,977 |
| near normal | 40%–50% | 42% | 47% | 1,152 |
| near normal | 50%–60% | 55% | 30% | 385 |
| near normal | 60%–70% | 62% | 33% | 73 |
| above normal | 10%–20% | 20% | 55% | 67 |
| above normal | 20%–30% | 26% | 38% | 3,126 |
| above normal | 30%–40% | 34% | 34% | 6,002 |
| above normal | 40%–50% | 42% | 51% | 885 |
How strong are the signals?
Share of cell-years by the leading tercile probability.
| Category | Forecast | Climatology | Brier skill |
|---|---|---|---|
| below normal | 0.1891 | 0.1900 | +0.004 |
| near normal | 0.2430 | 0.2421 | −0.004 |
| above normal | 0.2380 | 0.2346 | −0.014 |
Rainfall amounts
Historical rainfall-amount diagnostics (CRPS, CRPSS and amount errors) have not been generated for May 2026 in this area and period; this needs a separate cross-validated amount evaluation. Single-season amount scores are in the Verification section.
Historical performance by rainfall region
Probability skill by rainfall region (full regional matrix)
| Region | FMAM | Feb | Mar | Apr | May |
|---|---|---|---|---|---|
| All Ethiopia | +0.032 (−0.003 to +0.067) 18/24 yrs | +0.027 (−0.002 to +0.055) 16/24 yrs | +0.002 (−0.009 to +0.012) 13/24 yrs | +0.001 (−0.014 to +0.014) 15/24 yrs | +0.028 (+0.005 to +0.051) 17/24 yrs |
| R0 arid / marginal | +0.051 (−0.012 to +0.113) 14/24 yrs | +0.026 (−0.023 to +0.076) 14/24 yrs | +0.002 (−0.024 to +0.025) 15/24 yrs | −0.019 (−0.050 to +0.007) 11/24 yrs | +0.033 (0.000 to +0.066) 15/24 yrs |
| R1 western unimodal | −0.003 (−0.053 to +0.043) 13/24 yrs | +0.040 (+0.016 to +0.062) 18/24 yrs | +0.003 (−0.008 to +0.013) 14/24 yrs | +0.013 (−0.004 to +0.030) 15/24 yrs | −0.014 (−0.041 to +0.014) 10/24 yrs |
| R2 Belg–Kiremt | +0.061 (+0.011 to +0.113) 17/24 yrs | +0.039 (+0.010 to +0.066) 18/24 yrs | +0.008 (−0.001 to +0.017) 16/24 yrs | −0.008 (−0.036 to +0.014) 14/24 yrs | +0.045 (+0.014 to +0.075) 19/24 yrs |
| R3 Gu–Deyr | +0.036 (−0.012 to +0.078) 18/24 yrs | +0.008 (−0.032 to +0.048) 13/24 yrs | −0.003 (−0.022 to +0.015) 10/24 yrs | +0.002 (−0.018 to +0.020) 14/24 yrs | +0.047 (+0.004 to +0.088) 19/24 yrs |
| FMAM rainfall contribution domain | +0.036 (−0.003 to +0.071) 18/24 yrs | +0.026 (−0.006 to +0.059) 16/24 yrs | +0.001 (−0.012 to +0.012) 12/24 yrs | −0.002 (−0.020 to +0.013) 12/24 yrs | +0.035 (+0.009 to +0.060) 19/24 yrs |
| FMAM main-season rainfall domain (Belg / Gu / Ganna, >=40% of annual) | +0.041 (−0.008 to +0.083) 17/24 yrs | +0.006 (−0.034 to +0.047) 13/24 yrs | −0.002 (−0.021 to +0.014) 10/24 yrs | +0.001 (−0.021 to +0.020) 13/24 yrs | +0.050 (+0.007 to +0.092) 20/24 yrs |
Positive values mean lower RPS than climatology. Intervals resample whole years; regions are the fixed 1993–2025 rainfall regimes, and the last rows are this cycle's rainfall domains. Regional results are noisier than national ones.