The Severity Index Joshua Kastman1, James Nelson2, 1 Cooperative Institute for Research in Environmental Sciences (CIRES), Boulder, CO 2National Service Weather Prediction Center (WPC)

Background WSSI Components: The Severity Index (WSSI) is a prototype National Weather Service (NWS) product that is designed Amount Index: Snow Load Index: Blowing Snow Index: Ground Index: Flash Freeze Index: Ice Accumulation Index: Overall Impact to be a tool to assist NWS operational forecasters in PURPOSE: This PURPOSE: This PURPOSE: This component PURPOSE: This component is PURPOSE: This component component is designed to component is to highlight highlights areas where PURPOSE: The PURPOSE: The Summary graphic maintaining situational awareness of the possible to highlight areas where pre- was developed to account for highlight areas in which areas where the weight of blowing/drifting snow is component depicts is a composite of the maximum existing snow combined the combined effects of ice significance of winter weather related impacts based impacts, especially the snow could result in expected to occur and severity primarily to impact from any of the six with very strong winds accumulation and wind which transportation, could damage to trees and result in transportation components. This can be shown upon the current official forecast. results in ground blizzard transportation of can produce widespread tree become overwhelmed powerlines. In general, related problems. In to highlight the areas where the conditions, which result in a situations where damage, transportation This tool is designed to communicate a general level of due to either: the lower the snow-liquid general, the blowing snow most impact is expected significant impact to temperatures rapidly shutdowns and other impacts. ratio (SLR) is and the significance increases as potential societal impacts and their spatial distribution. • The total amount of transportation. fall below freezing greater the total snow the SLR and winds both snow. Useful for: accumulation, the higher increase. Prior blowing during or just after • Snow climatology the index. snow research indicates . • NWS to better communicate potential impacts to taken into account that in general it takes just for each location Forest type is taken into partners under 20 mph of wind to • The rate at which the account start to move snow • Media & public to identify areas that could be snow is falling. around. significantly impacted

Impact Scale and Delimitations WSSI Use and Visibility (2019-20) WSSI Website Motivation – To Better Depict • Amazon Use https://www.wpc.ncep.noaa.gov/wwd/wssi/wssi.php Aspects of Winter • Lead intelligence Analyst reached out for an explanation on product • Reported they use it daily for decision making at shipping and Current NWS Procedures: – Winter weather distributions centers Watches/Warnings/Advisories are raised based • Washington Post’s Capital Weather Gang wrote a positive article on the primarily on “yes/no” thresholds of accumulation WSSI and generally at the level of individual counties. • Weather Channel also wrote an article on the WSSI • Pivotal Weather now has the tool on its website Reality of Winter Weather: – Severity/impacts from • TV station use winter weather are due to more than just amounts • KMLs are being (one 5” snowstorm is not like the next 5” used to create graphics snowstorm) Great variation in weather conditions • Example from TX frequently occur with individual counties.

WSSI Example and How-to Interpret 2019-20 Survey Results - Public Survey Word Top Left: WSSI depiction of all Future Development threats. Top Right: The snow amount • Probabilistic WSSI (encircled in light blue) component • Currently the Snow Amount component has an prototype matches the total WSSI around webpage showing probability of reaching impact southern VT, western MA and NY. threshold out through three days Bottom Left: The snow load • Currently developing a fully probabilistic version of the component (encircled in dark blue) WSSI matches the WSSI for central MA • Each member will calculate WSSI component output and southeast NH. • Output can be thought of as X% of reaching minor, moderate etc. Bottom Right: The ice accumulation component (encircled in pink) • Model Generated WSSI matches the WSSI for southeastern • Currently developing a version of the WSSI which will MA and northern RI. work with numerical weather prediction forecast models Final interpretation: Expect the Minor Threshold to forecast WSSI out further in time primary impacts to come from ice • The National Blend of Model (NBM) is the first accumulations across northern RI choice northeastward toward Boston, MA. Expect impacts to come from heavy Acknowledgments snowfall for VT and NY. There is a major threat for impacts from snow Special thank to Jim Nelson for his vision and work overseeing load across central MA through the WSSI Project. southeast NH. Special thank goes to Andy Nash the original creator of the WSSI. He and his team laid the foundation for which this work has been built upon.