The great Alaska warm-up of January 2009

During December 2008/January 2009 the Alaska Mainland experienced an extreme swing in temperatures of 70 degrees or more over a relatively short period of time. After two weeks of near continuous sub-zero temperatures, which bottomed out in the -20s to -50s (°F), tropical air was ushered northward causing temperatures to rise into the 40's and 50's (°F). At the onset of the pattern change, rapid warming aloft overrode the Arctic air at the surface causing precipitation to fall as freezing rain across much of Southcentral and Southwest Alaska. Conditions were particularly treacherous in the Anchorage area, where close to 300,000 people live. The combination of freezing rain, rain falling on ice, dangerous Chinook winds gusting up to 100 mph along the Anchorage hillside, and flooding due to ice blocked drainages made it nearly impossible to get around during the three day period January 14th-16th, whether on foot or by car. The impacts of this event, which were widespread and significant, include a high number of traffic accidents and vehicles in distress; injuries from people slipping on ice covered surfaces; and downed trees and power lines resulting in sporadic power outages. Closures of any type due to weather are an extreme rarity in Anchorage, but in this case there were a host of cancellations and closures, some of which lasted for up to three days.

The purpose of this study is to identify useful tools to aid in forecasting future high impact events. Temperature verification statistics for Southcentral Alaska were obtained using the BOIVerify program within the Graphical Forecast Editor (GFE) used by National Weather Service Forecast Offices. BOIVerify also provides bias-corrected guidance for temperature and dew point temperature based upon a 30-day regression fit analysis. The official forecasts of Maximum and Minimum temperature issued by the Anchorage office for the period January 13th-15th, 2009 were found to have a significant cold bias. This performance was closely mirrored by most of the bias-corrected guidance. The clear top performers for this event were the 2-meter temperatures from the GFS and NAM Models. This suggests that forecasters may have relied heavily on bias corrected guidance when producing the temperature forecasts.

Forecasts from the NCEP Global Ensemble Forecast System (GEFS) for the period of the warm-up showed a consistent depiction of a strong upper level ridge centered over Southeast Alaska with a surge of warm moist air up the west side of the ridge and over Mainland Alaska. A relatively small ensemble spread indicated high confidence in this solution. A review of standardized anomalies (Precipitable Water, 850hPa Temperature, and 850hPa v-wind) was performed to identify variables that may have given an indication to the significance of the event. In addition, Area Forecast Discussions issued by the Anchorage Forecast Office leading up to this event showed there was recognition of a major shift in the upper level pattern with potential for a freezing rain event.

The use of ensemble data, standardized anomalies and indentifying the strengths and weaknesses of bias-corrected guidance will be explored. It is hoped that the use of these parameters will lead to improvements in the forecasts of future high impact events.