East coast winter storms of long duration are characterized by either very light winds or easterly winds at 300 hPa, signaling a disruption of the typical westerly 300 hPa flow that contributes to the eastward progression of the storms. Additionally, negative U wind anomalies at 850 hPa represent strong low-level winds that advect moisture off the Atlantic Ocean, provide low-level convergence, and enhance low-level frontogenesis, resulting in increased precipitation production.
Historical snowstorms (widespread observed snowfalls ³18 inches) were characterized by negative U wind anomalies greater than 2 Standard Deviations (SD) at 300 hPa and greater than 3 SD at 850 hPa. Reanalysis wind anomalies from recent snowstorms, from 2000 to present, were compared to operational Numerical Weather Prediction (ETA and GFS) anomaly values. The finer-resolution model data produces slightly larger departures from normal compared to the coarser reanalysis data used to compute the climatological means and standard deviations. Gridded forecasts were obtained from the NCEP stepped-terrain Eta.
Cases were selected to show how these data could be used operationally to add value to real forecast problems. Forecast anomalies that meet or exceed the threshold values for historical snowstorms can alert a forecaster to potentially significant weather that may not otherwise be resolved in NWP guidance, and forecasts can be modified accordingly.