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  • Redwood City WSFO Study
    • Conducted study in 1991
    • Noted challenge of forecasting afternoon/evening return of stratus at SFO
    • Significant impacts can result on afternoons/evenings with high air traffic
    • Study focused on 22Z SFO-SAC and SFO-ACV gradients
      • Finding 1: SFO-SAC >= 3.6MB and SFO-ACV >= -3.4MB and upstream trigger(vort max, trough, front) => 75% chance stratus return by 03Z
      • Finding 2: No upstream trigger = 19% chance for early return by 03Z

  • ZOA CWSU Early Return Study
    • Goal to build upon Redwood City Study
      • Focused only on days where stratus was adjacent to coast at 01Z with no ceiling over the approach
      • Used 01Z pressure gradients which would provide 1hr+ lead-time for highest traffic at 02Z-03Z
      • Used 00Z forecaster-estimated marine layer depth from multiple sources
      • Looked at other pressure gradients to include nearby Buoy-SFO gradient
      • Looked at SFO 01Z relative humidities and daily high temperatures
      • Split early returns into three categories
        • Permanent Early Return (once stratus arrives it does not diminish until after 06Z)
        • Temporary Early Return (once stratus arrives it lasts for an hour or less)
        • No Early Return
  • Initial findings 2008 summer season(using 00Z-01Z data)
    • No Early Return Likely
      • Finding 1: 92% chance no early return when SFO Hi temperature reaches 70F(only one temporary return with 70F Hi)
      • Finding 2: 90% chance no early return when ratio SFO-ACV and SFO-SAC <= -1.1
      • Finding 3: 83% chance no early return when SFO-ACV <= -5.0MB
    • Early Return Likely
      • Finding 4: 100% chance permanent early return when SFO-SAC >= 4.0MB and SFO-ACV >= -5MB and Marine Lyr >= 1100ft
      • Finding 5: 67% chance permanent early return when SFO-SAC >= 4.0MB and Marine Lyr Depth >= 1100ft
      • Finding 6: SFO-SAC correlation to early return was highest at 0.42
      • Finding 7: Marine layer depth correlation to early return was next highest 0.17
      • Finding 8: SFO relative humidity correlation to early return was -.07
  • Decision Aids
    • Current Ratio of SFOACV to SFOSAC as of 23/11 = -6.83

  • Status
    • Will continue to add to study during/after each warm season

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Fremont, CA 94536-6531

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