Regional scale ensemble forecast of the Lake Effect snow event of February 2007

The causes of lake effect snow (LES) bands are widely known and understood. Regional models are capable of forecasting the conditions conducive to the formation of LES bands. However, the details related to the timing of heavy snow and the locations most likely to receive heavy snow are often elusive. In an attempt to improve LES band forecasts downwind of lakes Erie and Ontario, a regional ensemble forecast system was developed using the workstation Weather Research and Forecast (WRF) model at National Weather Service forecast offices surrounding the Great Lakes. The WRF members were set up over the same domain with varied physics and initial conditions to create a set of ensemble forecasts.

All members were run at 12km resolution and covered a large enough domain to capture not only lakes Erie and Ontario, but also the potential upstream connection to the Great Lakes farther to the west. One advantage to running the WRF locally is that hourly output is accessible whereas the operational guidance from National Centers, such as the National Centers for Environmental Predictions (NCEP) is only available in 3-hour increments. This facilitated the examination of hourly data and the ability to ensemble hourly forecasts to time the onset, development, and areas impacted by the larger and more persistent LES bands. Products produced included accumulated precipitation in hourly increments and the probability of exceeding discrete forecast thresholds in hourly, 3,6,12 and 24 hour increments. The ensemble WRF forecasts were limited to 24-hours in duration. Longer range forecasts relied on boundary conditions from the 3-hour data from the 32-km NCEP short range ensemble prediction system.

The lack of significant cold weather this past November, December, and early January left the Great Lakes unseasonably warm. The warm lakes were susceptible to produce a significant LES event should the correct conditions arrive. In early February, an arctic air mass with 850 hPa temperatures below -20C and strong westerly winds moved over the region. This produced a period of potentially record breaking lake effect snows in New York State. This event facilitated the testing of a regional 12km ensemble forecast system specifically designed to forecast such events.

This paper will present an ensemble of 12km WRF forecasts of the LES event of February 2007. The goal is to show products that help define the areas most likely to be impacted by the significant lake effect event. The efficacy of a detailed regional ensemble to forecast mesoscale phenomena for 0-24 hour events is presented.