Notes:

• Data is obtained from reforecast database. Consists of 15 ensemble members (control and 7 +/- bred pairs). Ensemble mean is NCEP/NCAR reanalysis. Forecasts are performed at T62 resolution, but forecast data is given in T35. Data is converted to 5 x 5 degree grid for calculations.
• Dates are determined from Casola and Wallace (2007, JAMC) clustering technique. PNW cold-air outbreaks are correlated with the Alaska Ridge pattern. All of the above dates are within these pentads.
• EOF patterns are generated by removing the mean from the 500 hPa height pattern, scaling the perturbations by the square-root of cos(latitude), and regressing the ensemble perturbations onto the first principle component. In all figures, the EOF patterns are scaled by the maximum value over the domain.
• EOF are computed seperatly for the entire Northern Hemisphere north of 20 N and over the Pacific sector (PAC), which spans 20N to 90N and 120E to 90W.
• Error bars for EOF amplitudes is determined via North et al. (1982, MWR) criterion, assuming 50 degress of freedom (Wang and Shan, 1999 JoC).
• Last forecast day is center date of pentad.