Seattle's Big Snowstorms: PNA is Key

[donsutherland1]

Based on data for the date on which 6" or greater snowstorms commenced in Seattle, the PNA is key. Although the largest share of snowstorms commenced when ENSO Region 3.4 anomalies were -0.50°C or colder, there were also a fairly high number of those that occurred when ENSO Region 3.4 anomalies were +0.50°C or warmer. Instead, the PNA was the most important factor. 21/22 (95.5%) commenced when the PNA was < 0. A large cluster (68.2%) occurred when the PNA was -1 or below. Unlike with major East Coast snowstorms, the state of the AO made little difference.

Data Summary:

ENSO:

R3.4 Anomalies of -0.50°C or colder: 10 (45.5%)

R3.4 Anomalies between -0.49°C and +0.49°C: 5 (22.7%)

R3.4 Anomalies of +0.50°C or warmer: 7 (31.8%)

AO:

AO < 0: 12 (54.5%)

AO > 0: 10 (45.5%)

PNA:

PNA < 0: 21 (95.5%)

PNA > 0: 1 (4.5%)

PNA of -0.5 or below: 20 (90.9%)

PNA of -1 or below: 15 (68.2%)

Scatter Diagram: