Alerting for the Threat of a High-Impact Sub-Advisory Snowfall with Adequate Lead-Time

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. However, even within 24 hours, small accumulations of snow (1” or less) from minor disturbances in the atmosphere are difficult to predict. These minor snow accumulations can result in major impacts to road transportation, if it coincides with a big city rush hour while road temperatures are well below freezing. Predicting the threat for these minor snows with enough advance notice to allow road crews time to respond to these particular conditions has historically been a challenge.

The evening rush hour on January 20^th 2016 in the Washington metropolitan area was one of these situations. While forecasters could see the threat of a rush hour disaster, they did not have the confidence until 7 hours before onset that a measurable amount of snow would fall. While that resulted in a Winter Weather Advisory with 7 hours of lead time, roads became icy during the evening rush hour, and societal impact from traffic delays and accidents was high.

During the summer of 2016, Weather Forecast Office (WFO) Sterling, Virginia, worked with the regional departments of transportation to develop a system where the region is notified not at the likelihood, or near certainty, of an event – which is typical for warnings and advisories – but for the threat of a winter commuting hazard. The alert was designed to be issued with about 24 hours of advance notice to give the departments of transportation enough time to plan and respond to the possibility of a commuting hazard. The alert is issued as a Special Weather Statement, and is headlined as a Potential Winter Commuting Hazard.

This presentation reports on the methodology developed. It also compares the January 20^th 2016 event, with how notification and decision-making would have had the opportunity to have been improved with this new system.