Seasonal to Interannual Forecasts
Extreme Weather Forecasts: Methods
We constructed frequency distributions from daily weather records for U.S. stations during 1948-1998 (October-March only), sub-sampled according the phase of the daily PNA index (the positive phase category for PNA > +1, the negative phase category for PNA < -1, and a neutral category otherwise). Extreme events were defined as temperature, precipitation, snowfall, or wind gusts that exceeded ± 1.5 standard deviations from the long-term mean for each calendar day. The method used in this study was adopted from Thompson and Wallace (1998) work examining the impacts of the Arctic Oscillation on daily weather extremes.
Distinct regional variations in the relative frequencies of extreme weather events associated with phase of the PNA pattern include:
- For positive phase (PNA > +1):
- Increased frequency of extremely warm days in coastal Alaska and the contiguous Pacific Northwest states (WA, OR, ID, MT)
- Decreased frequency of extremely warm days around the Great Lakes
- Decreased frequency of extreme heavy precipitation days in the US Pacific Northwest (PNW)
- Increased frequency of extreme heavy precipitation days for coastal Gulf of Alaska
- Significant reduction in the frequency of subfreezing temperatures along the west coast, but an increased frequency of freeze events in the southeast US
- Significant reduction in the frequency of heavy snowfall events in the western US, but an increased frequency of heavy snowfall events around the Great Lakes
- For negative phase (PNA < -1), shifts in the frequencies of extreme weather events are generally opposite those detailed above.
Results from our analysis of the impacts of daily PNA variations on the statistics of extreme weather events are available online. (That site also examines relationships between extreme PNW weather events and two other climate patterns: the Northern Hemisphere annular mode and the El Niño/Southern Oscillation.)
Predictability of the PNA
The PNA pattern has been identified (Renwick and Wallace 1995) as the northern Hemisphere's most predictable atmospheric circulation pattern at lead times of 5 to 10 days. NOAA's National Center for Environmental Prediction routinely issues 7 to 14 day lead-time ensemble forecasts for the PNA index. These forecasts have shown promising and perhaps surprisingly good skill during the past two winter seasons.