1. 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)

2. 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

3. 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

4. Result (Instead Of This)

5. Result (We Get This)

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

7. 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

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

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

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

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

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

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

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

15. 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?)

16. 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

17. 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

18. Winter Storm Severity/Impact Index Categories

19. Winter Storm Severity/Impact Index Example

20. 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

21. 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

22. The Components

23. The Components

24. January 2016 Mid-Atlantic Storm Examples

25. 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

26. Season Results

27. 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”

28. 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 ?)

29. Questions? Thank You!!
Contact Us: Andy Nash – Mike Muccilli – Website: weather.gov/btv/wsii

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

31. Additional Info
Example of Urban Areas

32. 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

33. 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)