By Scott Borgioli, WeatherAg Chief Meteorologist
FALL/WINTER/SPRING 2024-25
General Summary (based on data at this time):
- Outlooks are always broad, represent “overall” probabilities for the seasons ahead, and should not be used as official forecasts. Starting November 15, these twice-monthly outlooks become daily official forecasts but will continue to include probabilistic predictions for the months/seasons ahead.
- La Nina is expected to peak in the high-end “weak” or low-end “moderate” category, influencing global weather patterns in Nov/Dec/Jan/Feb and adding large variables/uncertainties with predictions. Historical (rainfall) in the valley (since La Nina/El Nino records began in 1950):
- “Weak” La Nina events recorded: below average precipitation 50% of the time, near average at 17%, and above average at 33%
- “Moderate” strength La Nina events: below avg precipitation 50% of the time, average/above average 50%
- Keep in mind that there have not been enough recorded La Nina or El Nino events to establish a scientific conclusion about impacts; hence, possibilities are generalized and loosely probabilistic.
- October: Through Oct 17, high pressure is expected to “mainly” block / deflect low-pressure systems from the state. Temps above average through Oct 10 (highs 8-15°F above average, lows 6-13° above average). Oct 11-17, temps expected at 0 to +5°F of the mean. Around Oct 18-21 and through Oct 31, low-pressure systems are expected to more consistently pass through PacNW, increasing the potential for some to clip CenCal or get cut off and have a more direct aim. Temps likely at -3 to +4°F of avg Oct 20-31.
- Nov/Dec/Jan/Feb/Mar: Some signals lean towards above average precipitation for Nov and possibly Dec, but data consensus favors below average precipitation overall for the Nov-Jan period, then around average Feb-Mar. Besides La Nina, other weather features that enter into “active” phases will affect weather patterns, could result in periods of enhanced rainfall, and result in anomalies to long-range predictions. Temperatures are overwhelmingly predicted to be above to significantly above average through Feb, then average for March. Although cold air outbreaks are considerably less likely during this period across the far west (due to the anticipated overall pattern), atmospheric changes and displacement of warm/cold air could always result in anomalous cold air being directed into the valley.
Precipitation Probabilities (preliminary & conditional, based on current signals)
H = HIGH Probability, M = MODERATE Probability, L = LOW Probability
- Oct: H 40-70% of average due to expected “weak” strength La Nina
- Nov/Dec: H 40-70% of average due to expected “weak” or “moderate” strength La Nina
- Jan/Feb 2025: H 60-90% of average due “weak” or “moderate” strength La Nina starting to dissipate
- Mar/Apr 2025: M 70-100% of average – La Nina expected to be dissipated by this time (conditional on La Nina dissipating during January or February 2025)
Temperature Anomaly Probabilities (preliminary, based on current signals)
- Nov: Odds favor -1 to +6° of average mean temperatures (as a whole – conditional, preliminary)
- : Odds favor -2 to +5° of average mean temperatures (as a whole – conditional, preliminary)
- / Feb: Odds favor -3 to +4° of average mean temperatures (as a whole – conditional, preliminary)
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