1. Severe Thunderstorm Wind Damage Criteria – Is it time for a change?
Joe Villani
WFO Albany, NY
Northeast Regional Operational Workshop
November 2-3, 2016

2. Motivation
Using downed trees primary method of verifying severe thunderstorm wind damage in the eastern U.S.
Official issuance criteria for a Severe Thunderstorm Warning does not match NWS mission (from NWSI ):
Occasional structural damage for significant severe winds and few measured wind reports.

3. Motivation
Issuance criteria inconsistent with NWS mission since damage from T-storm winds (downed trees or tree limbs) can occur with winds < 58 mph
70 of 73 missed reports in 2016 (ALY) due to downed tree(s) or wires
Present evidence and basis for needed change of issuance criteria…
Persistent trend for majority of missed events from wind damage. More stats on ALY missed events later.

4. History Lesson
From Washington Post (Capital Weather Gang) article – July 9, 2014:
Legacy criteria for wind damage based on aviation from 1970!

5. Food for Thought
Does the 58 mph issuance criteria for wind damage best serve our NWS customers in 2016?
Fact: Tree damage can and does occur with wind speeds < 58 mph!
Condition of tree(s) a factor:
From NWSI :
Constrained by 58 mph issuance criteria, even when assessing damage.

6. Rationale for Change
At ALY, in 2016, 70 of 73 missed (or unwarned) reports were from tree(s) or wires down
Equates to 96% of missed reports!
Over the past 5 years, 220 of 242 missed reports were tree(s) or wires down
91% of missed reports from !
Receiving more reports than 5+ years ago…Social Media
Conversely, only 9% of missed reports over the past 5 years were from large hail. Indicative of increasing difficulty in warning for wind due to tree damage. Damage not consistent with radar signatures.

7. Rationale for Change
NWS Meteorologists currently look for radar signatures that indicate wind around or > 50 kt
For example:
strong reflectivity gradients on leading edge of lines/bows
“wall of wind” signatures in base velocity data
However, most storms that produce damage to weakened/rotted trees occur with subtle signatures

8. Severe Radar Signatures Examples for Wind
60-70 kt
“Wall of wind” from 0.5° base velocity
Strong reflectivity gradient on leading edge

9. Background Information
Frelich and Ostuno (2012) found damage can occur in healthy trees at mph, but unhealthy trees (diseased, rotting, not planted properly) can be damaged with lower wind speeds
Example of tree planted too deep
Example of rotted trunk

10. Background Information
Case study from Frelich and Ostuno (2012): July 4, 2003 Grand Rapids, MI
A dozen trees or large tree limbs downed by mph winds
Storm survey revealed all but one tree was rotted/diseased
Events with “sub-severe” wind can still have a significant impact.

11. Background Information
Even occurrences such as drought can have an effect on the health of trees and can increase their susceptibility of falling
Several factors that compromise the health of trees.

12. Scientific Basis for Change
From Enhanced Fujita Scale Report (McDonald and Mehta 2006):
Lower bound for tree limb damage found to be 48 mph
Scientific basis for change – Enhanced Fujita Scale (for healthy trees)

13. Scientific Basis for Change
From Enhanced Fujita Scale Report (McDonald and Mehta 2006):
Lower bound for tree limb damage found to be 48 mph
Scientific basis for change – Enhanced Fujita Scale (for healthy trees)

14. Proposed Change
Impossible to account for condition of trees when issuing warnings
Change the issuance criteria for Severe Thunderstorm wind damage to:
wind gusts >= 48 mph or producing damage to trees including downed limbs
48 mph threshold = lower bound of tree limbs broken in Enhanced Fujita Scale

15. Proposed Change
For the more intense or high-end storms producing wind damage, use thresholds provided in WarnGen warning template:
Wind speeds > 60 mph, > 70 mph, > 80 mph, etc.
NWS moving towards impact-based warnings:
Puts focus on impact of wind, such as trees or tree limbs damaging houses/cars, power outages, etc.

16. Acknowledgements/References
Acknowledgements: Neil Stuart, Brian Frugis (ALY)
References:
NWS Directives
Frelich, L. E., and E. J. Ostuno, 2012: Estimating wind speeds of convective storms from tree damage. Electronic J. Severe Storms Meteor., 7 (9), 1–19.
McDonald, J. and K. C. Mehta, 2006: A Recommendation for an Enhanced Fujita Scale (EF-Scale), Revision 2. Wind Science and Engineering Research Center, Texas Tech University, Lubbock, TX, 111 pp.

17. Hopefully the discussion continues…
Goal: Get people talking about this issue and discuss potential solutions!
Contact info:
Questions/Comments?