Ensemble predictions of the 2007 Valentines Day Winter Storm

A winter storm brought heavy snow from Nebraska to New England with a swath of significant ice accumulation south of the region of heavy snow during the 12 February to 14 February 2007 period. The heaviest snow of the event was observed in northeastern Pennsylvania and eastern New York State on 14 February 2007 (Valentines Day) where upward of 2 to 3 feet of snow was observed. The combination of heavy snow and ice led to unprecedented road closures in northeastern Pennsylvania. This paper will qualitatively assess the performance of ensemble forecast systems in forecasting this event.

The storm was relatively well forecast 2-3 days in advance by the National Centers for Environmental Prediction global ensemble forecast system and the short range ensemble forecast system. The convergence of solutions included good agreement and low spread in the 500 hPa heights, mean sea level pressure, quantitative precipitation, and 850 hPa temperature fields in the ensemble prediction systems. The ensemble systems also forecast mean 850 hPa u-winds to be 3 to 5 standard deviations below normal, and 250 hPa u-winds to be 2 to 3 standard deviations below normal, signals indicative of a long-duration, heavy precipitation event in the eastern United States. The low- and upper-level wind anomalies contributed to the unusually high degree of confidence for a significant winter storm 36 to 72 hours in advance of the storm. This included some confidence in delineation of the precipitation transition zone.

Key ensemble products highlighting the performance of the ensemble forecast systems will be presented at the conference. Ensemble products presented will include plume diagrams, mean and spread charts, anomaly forecasts, and the probabilities for QPF exceeding 1.00” liquid equivalent. Such products will be used to highlight the utility of using ensemble forecast systems in real-time forecasting operations.