1. STC Climatology (1979–2010) ~3 STCs/year Strong TT Weak TT Trough induced N = 105

2. STC Climatology (1979–2010) N = 34 Strong TT Weak TT Trough induced

3. STC Climatology (1979–2010) N = 56 Strong TT Weak TT Trough induced

4. STC Climatology (1979–2010) N = 15 Strong TT Weak TT Trough induced

5. Identify the most common upper-tropospheric features linked to STC formation in 1979–2010 climatology Separate STCs included in 1979–2010 climatology into five clusters representing the most common upper-tropospheric features linked to STC formation: 1) PV Streamer, 2) Cutoff, 3) Midlatitude Trough, 4) Subtropical Disturbance, and 5) Debris Perform a cyclone-relative composite analysis of the upper- tropospheric features linked to STC formation within each cluster Upper-Tropospheric Precursors

6. Clusters: 1) PV Streamer, 2) Cutoff, 3) Midlatitude Trough, 4) Subtropical Disturbance, and 5) Debris

7. Upper-Tropospheric Precursors Clusters: 1) PV Streamer, 2) Cutoff, 3) Midlatitude Trough, 4) Subtropical Disturbance, and 5) Debris AWB 350K isentropic surface STC formation is associated with a PV streamer injected into the subtropics by a precursor anticyclonic wave breaking (AWB) event PV streamer maintains a clear connection with the midlatitudes

8. Upper-Tropospheric Precursors Clusters: AWB 350K isentropic surface STC formation is associated with a region of relatively high upper- tropospheric PV cut off in the subtropics by a precursor AWB event Upper-tropospheric cutoff is entirely removed from midlatitude flow 1) PV Streamer, 2) Cutoff, 3) Midlatitude Trough, 4) Subtropical Disturbance, and 5) Debris

9. Clusters: Upper-Tropospheric Precursors 350K isentropic surface STC formation is associated with a broad midlatitude trough moving progressively toward the southeast Broad midlatitude trough is not associated with a precursor AWB event 1) PV Streamer, 2) Cutoff, 3) Midlatitude Trough, 4) Subtropical Disturbance, and 5) Debris

10. Clusters: Upper-Tropospheric Precursors 350K isentropic surface H + STC formation is associated with a small-scale PV filament propagating around the northern edge of a subtropical anticyclone PV filament is smaller than streamers, cutoffs, and midlatitude troughs 1) PV Streamer, 2) Cutoff, 3) Midlatitude Trough, 4) Subtropical Disturbance, and 5) Debris

11. Clusters: Upper-Tropospheric Precursors 350K isentropic surface STC formation is associated with residual PV debris deposited in the subtropics from a previous AWB event PV debris moves westward on southern edge of a broad subtropical ridge 1) PV Streamer, 2) Cutoff, 3) Midlatitude Trough, 4) Subtropical Disturbance, and 5) Debris

12. Debris Cutoff Streamer Unclassifiable 7.62% 11.43% 20.95% 29.52% N = 105 (8) (22) (31) (12) Upper-Tropospheric Precursors Subtropical Disturbance 9.52% (10) Midlatitude Trough

13. N = 105 Upper-tropospheric Precursors Streamer Cutoff Midlatitude Trough Subtropical Disturbance Debris Unclassifiable

14. Streamer = 0.0% Cutoff = 0.0% Midlat. Trough = 0.0% Subtrop. Dist. = 20.0% Debris = 73.3% Unclassifiable = 6.7% Streamer = 1.8% Cutoff = 16.1% Midlat. Trough = 7.1% Subtrop. Dist. = 30.4% Debris = 33.9% Unclassifiable = 10.7% Upper-Tropospheric Precursors Weak TTTrough InducedStrong TT Streamer = 20.6% Cutoff = 38.2% Midlat. Trough = 17.6% Subtrop. Dist. = 5.9% Debris = 2.9% Unclassifiable = 14.7% Upper-level disturbance with strong lower-level thermal gradients Upper-level disturbance with moderate lower-level thermal gradients Upper-level disturbance without appreciable lower-level thermal gradients N = 34N = 56N = 15