STC Climatology ( 1979–2010)

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

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. Upper-Tropospheric Precursors • 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

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

7. Upper-Tropospheric Precursors 1) PV Streamer, 2) Cutoff, 3) Midlatitude Trough, 4) Subtropical Disturbance, and 5) Debris Clusters: 350K isentropicsurface AWB • 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 1) PV Streamer, 2) Cutoff, 3) Midlatitude Trough, 4) Subtropical Disturbance, and 5) Debris Clusters: AWB 350K isentropicsurface • 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

9. Upper-Tropospheric Precursors 1) PV Streamer, 2) Cutoff, 3) Midlatitude Trough, 4) Subtropical Disturbance, and 5) Debris Clusters: 350K isentropicsurface • 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

10. Upper-Tropospheric Precursors 1) PV Streamer, 2) Cutoff, 3) Midlatitude Trough, 4) Subtropical Disturbance, and 5) Debris Clusters: 350K isentropicsurface + 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

11. Upper-Tropospheric Precursors 1) PV Streamer, 2) Cutoff, 3) Midlatitude Trough, 4) Subtropical Disturbance, and 5) Debris Clusters: 350K isentropicsurface • 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

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

13. Upper-tropospheric Precursors N = 105 Streamer Cutoff Midlatitude Trough Subtropical DisturbanceDebrisUnclassifiable

14. Upper-Tropospheric Precursors Strong TT Weak TT Trough Induced N = 34 N = 56 N = 15 Streamer = 20.6% Cutoff = 38.2% Midlat. Trough = 17.6%Subtrop. Dist.= 5.9% Debris = 2.9% Unclassifiable = 14.7% Streamer = 1.8% Cutoff = 16.1% Midlat. Trough = 7.1%Subtrop. Dist. = 30.4% Debris = 33.9% Unclassifiable = 10.7% Streamer = 0.0% Cutoff = 0.0% Midlat. Trough = 0.0%Subtrop. Dist.= 20.0% Debris = 73.3% Unclassifiable = 6.7% Upper-level disturbance with strong lower-level thermal gradients Upper-level disturbancewith moderate lower-level thermal gradients Upper-level disturbancewithout appreciable lower-level thermal gradients