Early Return Stratus			SFO Fog			SFO Post-Frontal Cu			Sierra Thunderstorms

• 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 25/05 = -1.19

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

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