Forecast Issues Related to the Unprecedented Severe and High Wind Event of December 1, 2006

A late season severe weather event affected the northeastern United States on 1 December 2006. Between 1200 UTC 1 December 2006 and 1200 UTC 2 December 2006, over 130 reports of severe weather were tallied, including 3 tornadoes in Pennsylvania. Most of the severe reports occurred between 1500 UTC 1 December and 0100 UTC 2 December 2006, with an additional 46 severe weather reports occurring on the 30th of November, most after 0300 UTC on 1 December. In addition, strong non-convective high winds produced widespread damage behind the frontal system.

This paper examines forecast issues related to this unprecedented event. National Centers for Environmental Predictions (NCEP) operational models and ensemble prediction systems (EPS) are examined, with the focus on the ability of these systems to forecast the conditions associated with this unusually strong late season severe weather and high wind event.

It will be shown that the NCEP guidance was able to forecast both the surge of anomalously high precipitable water (PW) and strong low-level southerly winds into the affected region ahead of the cold front. PW anomalies were on the order of 2 to 3 standard deviations above normal. Behind the front, strong westerly winds associated with the non-convective high wind event were also well forecast.

Due to timing issues at longer forecast ranges, anomalies in the NCEP short-term EPS and global ensemble forecast systems (GFS) were markedly smaller than those indicated by individual deterministic model runs. The strength of ensembles in the forecast process at longer ranges is to indicate confidence and relative probabilities of events/outcomes occurring. In this case, the ensembles showed the high probability of an event, which gave forecasters confidence in the anomalous details depicted by the deterministic GFS forecasts. Products from the EPS and GFS will be presented to illustrate their use in successfully forecasting this historic event.