Severe Thunderstorm Wind Damage Criteria Is it time

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

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 10 -511):

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…

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

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 10 -1605:

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 2012 -2016! • Receiving more reports than 5+ years ago…Social Media

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

Severe Radar Signatures Examples for Wind 60 -70 kt “Wall of wind” from 0. 5° base velocity Strong reflectivity gradient on leading edge

Background Information • Frelich and Ostuno (2012) found damage can occur in healthy trees at 55 -60 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

Background Information • Case study from Frelich and Ostuno (2012): July 4, 2003 Grand Rapids, MI • A dozen trees or large tree limbs downed by 45 -55 mph winds • Storm survey revealed all but one tree was rotted/diseased

Background Information • Even occurrences such as drought can have an effect on the health of trees and can increase their susceptibility of falling

Scientific Basis for Change • From Enhanced Fujita Scale Report (Mc. Donald and Mehta 2006): • Lower bound for tree limb damage found to be 48 mph

Scientific Basis for Change • From Enhanced Fujita Scale Report (Mc. Donald and Mehta 2006): • Lower bound for tree limb damage found to be 48 mph

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

Proposed Change • For the more intense or high-end storms producing wind damage, use thresholds provided in Warn. Gen 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.

Acknowledgements/References • Acknowledgements: Neil Stuart, Brian Frugis (ALY) • References: • NWS Directives • http: //www. nws. noaa. gov/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. • Mc. Donald, J. and K. C. Mehta, 2006: A Recommendation for an Enhanced Fujita Scale (EFScale), Revision 2. Wind Science and Engineering Research Center, Texas Tech University, Lubbock, TX, 111 pp.

Hopefully the discussion continues… • Goal: Get people talking about this issue and discuss potential solutions! • Contact info: Joe. Villani@noaa. gov • Questions/Comments?