A Winter Storm Severity Index Supporting a Weather-Ready Nation Andy Nash, NWS Burlington VT Mike Muccilli, NWS Baltimore/Washington DC Nathan Foster, USFS (formerly NWS Las Vegas)

Motivation Current NWS Watches/Warnings/Advisories for winter storms are “yes/no” thresholds based primarily on accumulation Reality: severity/impacts from a winter storm due to totality of much more than just accumulations one 5” snowstorm is not like the next 5” snowstorm

Motivation Watches/Warning/Advisories primarily issued only at the county level. But great variation exists at much higher spatial resolution Let’s use our own data! National Digital Forecast Database at 2.5km resolution – exists only because of forecasters blood, sweat and tears

Result (Instead Of This)

Result (We Get This)

Don’t We Already Have A Storm Severity Classification? YES RSI – Regional Storm Index NESIS – Northeast Snowfall Impact Scale

Don’t We Already Have A Storm Severity Classification? YES RSI – Regional Storm Index NESIS – Northeast Snowfall Impact Scale BUT…. Those are calculated AFTER the fact Heavily weighted toward populated areas Give a single value descriptor for the overall storm – local impacts will vary

Which Storm Was More Significant? October 29-30, 2011 Heavy/Wet Snow

Survey Says….. Out of 58 ranked snow storms since 1956… Rank: #50 Score: 1.75 Classification: Notable (Not historic, epic or ridiculous)

Which Storm Was More Significant? February 1979: “President’s Day” Storm

Survey Says….. Out of 58 ranked snow storms since 1956… Rank: #24 Score: 4.77 Classification: Major (Not crippling as the headline says)

Which Storm Was More Significant? February 2007: Valentines Day Blizzard

Survey Says….. Out of 58 ranked snow storms since 1956… Rank: #15 Score: 5.63 Classification: Major

Which Storm Was More Significant? January 2016 – Mid Atlantic “Blizzard”

Survey Says….. Out of 58 ranked snow storms since 1956… Rank: #4 Score: 7.66 Classification: Crippling (Wait! Media headline used “Epic”. Wasn’t Oct 2011 also described as epic?)

What do they all have in common? They were historic, at least for some communities Caused severe societal impact for those in the path of the storm But also had minimal impact for those on the fringes of the storm A single rating doesn’t capture everything

What The Winter Storm Severity/Impact Index Is A tool to assist NWS operational forecasters in maintaining situational awareness of the possible significance of weather related impacts based upon the current official forecast. A tool to help communicate a general level of potential societal impacts and their spatial distribution “Actionable Information” for partners in their goal to mitigate problems due to winter storms

Winter Storm Severity/Impact Index Categories

Winter Storm Severity/Impact Index Example

What The Winter Storm Severity/Impact Index Is NOT: It is not a specific forecast for specific impacts. categorization of “moderate” does not mean schools will or have to close. It does not account for antecedent conditions or combined effects of separate weather events. 30 mph winds occur two days after ½” of ice accumulated on trees

How Is It Created? Combine NDFD with non-meteorological data sets in a GIS environment Incorporate climatology, land use/type, urban or rural area, pre-existing snowpack characteristics along with forecast data Deconstruct the storm into primary components and separately model component severity Re-assemble into a final “1 to 5” severity index

The Components

The Components

January 2016 Mid-Atlantic Storm Examples

2015-16 Season Results 20 NWS offices evaluated WSII in real-time and provided post-event verification and feedback “In the ballpark” vast majority of the time Several offices provided WSII to trusted core partners for additional feedback Majority of partners found it useful increased their confidence in the forecast and reinforced decisions

2015-16 Season Results

What’s Upcoming Becomes an official “Experimental” NWS product this winter! weather.gov/btv/wsii (now, but will change) More focus on partner feedback Association with NWS’ “Hazard Simplification” project Need to implement timing components Website mods to ensure users can find answers to: Why it’s “red” and when it’s “red”

Further Down The Road Possibilities Impact/Severity Index is just an engine Other data sources can be used as the inputs (eg: model ensembles) Interest at NCEP/Weather Prediction Center to use our “engine” concept to produce longer range probabilistic outlooks (testing for 2017-18?)

Questions? Thank You!! Contact Us: Andy Nash – andy.nash@noaa.gov Mike Muccilli – michael.muccilli@noaa.gov Website: weather.gov/btv/wsii

Additional Info Severity Index only based on 3” of snow everywhere – strong influence of climatology

Additional Info Example of Urban Areas

Additional Info Example of Land Use Type Raw Data Land Use Classifications (Dozens of types) Reclassified into 5 types for use in blowing snow. More “open” the area is, the more blowing snow there will be and a higher factor is used

Additional Info Logic flow used for blowing snow component Equations: Blowing Snow  = (Wind Gust category value) x (snow ratio) x (snow amount) x Land Use factor Cumulative Blowing Snow = blowing snow value + (50% x prior 6 hour blowing snow value)+ (25% x prior 12 hour blowing snow value)