April 20-28 has seen no extreme cold. The closing two days of April are not likely to see such cold either. Instead, on April 22, 2005, Seattle had a record warm temperature.
Why was there no extreme cold near the end of April 2005?
The 2005 Pattern Did Not Fit the Classic Pattern for Such Cold:
If one examines past bouts of record cold during the 4/20-30 timeframe, one finds that the classic pattern consists of far above normal height anomalies just west of the Gulf of Alaska, much below normal height anomalies over the Pacific Northwest/western Canada, and above normal height anomalies in the eastern United States. The 4/25-27/2005 period saw an almost oppositive configuration.
See below:
The PDO and PNA:
If one examines past bouts of record cold during the April 20-30 timeframe, one finds:
• 9/11 (82%) occurred when the April PDO was negative
• 2/2 (100%) of the PDO+ occurrences saw a PNA below -2.000
• 7/9 (78%) occurred when the PNA was negative
• 0/11 (0%) occurred when the April PDO was positive and the PNA was positive
April 20-27 saw a positive PNA setup. The 4/28 PNA was -0.407. Per SSTAs, the April PDO overwhelmingly appears to be positive. Using the 4/28 0z GFS ensembles, not one member brings the 4/29 or 4/30 PNA to -2.000 or below.
Overall, the PDO/PNA setup was also much against the outbreak of extreme cold for the April 20-30 period.