Update on Conveying Potential Winter Storm Threats in Days 3-7 Using Multi-Model Ensembles
Steven Zubrick, NWS Baltimore/Washington WFO, Sterling, VA
Brian LaSorsa, NWS Baltimore/Washington WFO
Christopher Strong, NWS Baltimore/Washington WFO
James E. Lee, NWS Baltimore/Washington WFO

Forecasts of snow and its impacts on commerce (particularly travel) have generally improved in the short range forecast period (days 1-2) over the last decade. Improved observational networks (satellite, terrestrial, radar), and continued improvements in numerical models and data assimilation techniques, have played a large role in these improvements. For the medium range forecast period (days 3-7), snowfall forecasting is more difficult than in the short range, because of inherently less predictability of wintertime precipitation at these longer time ranges. Despite the challenge, there is increased demand by users placed on meteorologists for identifying potential winter impacts in the medium range forecast period.

The growing availability of guidance from various ensemble systems of models, has allowed development of probabilistic approaches in forecasting wintertime precipitation. Novak, et .al. (2014) examined the approach used at the National Weather Service (NWS), Weather Prediction Center (WPC). Their approach is based upon developing percentile snow accumulations. These accumulations are derived from probabilistic cumulative distribution functions (CDFs) applied to model ensemble forecasts to produce forecast snowfall range forecasts that vary dynamically, with the resultant ranges specified by the spread of the ensemble forecasts. Similarly, WPC produces probabilistic guidance for snow and freezing rain threats for Days 1-3.

During the 2015-16 winter season, Weather Forecast Office (WFO) Sterling, Virginia, implemented (on January 4, 2016), an experimental system that leveraged WPC medium range probabilistic winter precipitation outlooks for days 4 -7. This system provided guidance (via web-based graphics) on potential winter impactful events spanning days 4-7. Starting for the 2016-17 winter season, the winter medium range outlook guidance threat system run by WFO Sterling was expanded to cover winter threats in days 3-7 for both snow and freezing rain. All of this guidance is based upon WFO-Sterling’s staff expertise and experience working with key users to assess impacts of wintertime precipitation events, and the staff’s expertise in local meteorology in the mid-Atlantic region. The WFO Days 3-7 Winter Storm Threat product is designed to be easy to interpret for the general public and media, but also displays more information for decision makers than just a standard sentence or two in the NWS hazardous weather outlook.  This presentation reports on the methodology developed along with results from the 2015-2016 winter season.


David R. Novak, Keith F. Brill, and Wallace A. Hogsett, 2014: Using Percentiles to Communicate Snowfall Uncertainty. Wea. Forecasting, 29, 1259–1265.