Flagstaff Seasonal Outlook

Data: Monthly precipitation at Flagstaff Pulliam Airport (KFLG, station USW00003103) from January 1950 through . Ocean index data (PDO, AMO, Niño 3.4) from NOAA NCEI and CPC ERSSTv5 datasets.

Phase classification: Each year from 1950–2025 is classified by its annual mean PDO and AMO value. PDO and AMO are binary (positive or negative) following McCabe et al. (2004), who demonstrated these two indices explain over 52% of multidecadal drought frequency variance across the U.S. ENSO classification is seasonal, not annual: Winter (DJFM) analogs use the December–March mean Niño 3.4 anomaly; monsoon (JJAS) analogs use the June–September mean. This corrects the known limitation of annual-mean ENSO, where late-season events can be obscured. Threshold: ±0.4°C.

Composites: For each phase combination, the tool identifies all historical years with matching conditions (analog years) and displays the distribution of precipitation in those years — the full range, the middle half of outcomes, and the median. The long-term median is shown for comparison.

Probability bars: Show what fraction of analog years had precipitation below, near, or above the long-term median for that period. "Near normal" means within 15% of the long-term median.

How to use it forward-looking: The tool assumes the current PDO and AMO phase will persist — reasonable given that these indices shift on decadal timescales. ENSO is loaded automatically from current conditions using a seasonal mean: the DJFM average for winter outlooks, the JJAS average for monsoon outlooks. If you want to explore what happens under a different ENSO state (e.g., a CPC forecast for developing La Niña), note which analog years in the list correspond to that state and assess them directly.

Important limitations: This tool reflects historical tendencies, not a numerical weather forecast. Small sample sizes (fewer than 10 analog years) should be interpreted cautiously — the full list of analog years is shown so you can judge the evidence yourself. Climate is always variable; analog years span a wide range of outcomes.

New Zealand Index (NZI): The Current Conditions panel shows the most recent Jul–Sep (JAS) sea surface temperature for the New Zealand Index region (25°S–40°S, 170°E–200°E) from NOAA ERSSTv5. Mamalakis et al. (2018) identified this southwestern Pacific region as a stronger predictor of Southwest U.S. winter (Nov–Mar) precipitation than ENSO, with r ≈ −0.7 since 1982. Warm JAS SST in this region (above the 1950–present median of 16.575°C) is associated with drier Southwest winters; cool SST with wetter winters. This signal was weaker before ~1983. When Winter is selected, an annotation appears below the outlook showing the current NZI direction. NZI does not filter the analog years — sample sizes are too small for a reliable three-way split.

Niño 1+2 (monsoon context): The Current Conditions panel shows the most recent monthly Niño 1+2 anomaly (eastern equatorial Pacific, 0–10°S, 80–90°W) and the most recent Jul–Sep (JJAS) seasonal mean from NOAA CPC ERSSTv5. This region is the most volatile ENSO index — it warms first during developing El Niño events. Warm Niño 1+2 during monsoon season is associated with suppressed moisture flux into the interior Southwest (Adams & Comrie 1997; Higgins et al. 1999). The signal is weaker than ENSO's relationship with winter precipitation and does not filter the analog years.

Southern Oscillation Index (SOI) & teleconnection confidence: The Current Conditions panel shows the most recent monthly SOI (Australian Bureau of Meteorology, Tahiti-Darwin pressure difference) alongside the most recent complete June–November seasonal average — the lag window Redmond & Koch (1991) identified as predictive of the following October–March winter precipitation. SOI and Niño 3.4 normally move in opposite directions (BOM's convention: positive SOI = La Niña-like, negative = El Niño-like). McCabe & Dettinger (1999) found that when SOI and Niño 3.4 diverge from this expected relationship, ENSO's teleconnection to western US precipitation weakens or strengthens depending on the concurrent PDO phase. When Winter is selected, an annotation below the outlook states the current teleconnection confidence state (computed from SOI, Niño 3.4, and PDO — see the workbook's Climate_Indices sheet for the full classification logic). SOI does not filter the analog years or change the probability statement itself — it is a contextual qualifier, like NZI. A parallel note using the July–September SOI average appears for monsoon-season views.

References

1. McCabe, G.J., Palecki, M.A., and Betancourt, J.L. (2004). Pacific and Atlantic Ocean influences on multidecadal drought frequency in the United States. Proceedings of the National Academy of Sciences, 101(12), 4136–4141.
2. Mamalakis, A., Yu, J.-Y., Randerson, J.T., AghaKouchak, A., and Foufoula-Georgiou, E. (2018). A new interhemispheric teleconnection increases predictability of winter precipitation in southwestern US. Nature Communications, 9, 2332.
3. Adams, D.K. and Comrie, A.C. (1997). The North American Monsoon. Bulletin of the American Meteorological Society, 78(10), 2197–2213.
4. Higgins, R.W., et al. (1999). Influence of the North American monsoon system on the U.S. summer precipitation regime. Journal of Climate, 12(2), 406–420.
5. Mitchell, D.L., Ivanova, D., Rabin, R., Brown, T.J., and Redmond, K. (2002). Gulf of California sea surface temperatures and the North American monsoon: Implications for forecasting. Journal of Climate, 15(17), 2261–2281.
6. Redmond, K.T. and Koch, R.W. (1991). Surface climate and streamflow variability in the western United States and their relationship to large-scale circulation indices. Water Resources Research, 27(9), 2381–2399.
7. McCabe, G.J. and Dettinger, M.D. (1999). Decadal variations in the strength of ENSO teleconnections with precipitation in the western United States. International Journal of Climatology, 19(13), 1399–1410.