Joe Villani WFO Albany NY Northeast Regional Operational Workshop November 23 2016 Motivation Using downed trees primary method of verifying severe thunderstorm wind damage in the eastern US ID: 537242
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Severe Thunderstorm Wind Damage Criteria – Is it time for a change?
Joe VillaniWFO Albany, NYNortheast Regional Operational WorkshopNovember 2-3, 2016Slide2
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):Slide3
Motivation
Issuance criteria inconsistent with NWS mission since damage from T-storm winds (downed trees or tree limbs) can occur with winds < 58 mph70 of 73 missed reports in 2016 (ALY) due to downed tree(s) or wiresPresent evidence and basis for needed change of issuance criteria…Slide4
History Lesson
From Washington Post (Capital Weather Gang) article – July 9, 2014:
Legacy criteria for wind damage based on aviation from 1970!Slide5
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:Slide6
Rationale for Change
At ALY, in 2016, 70 of 73 missed (or unwarned) reports were from tree(s) or wires downEquates to 96% of missed reports!Over the past 5 years, 220 of 242 missed reports were tree(s) or wires down91% of missed reports from 2012-2016!
Receiving more
reports than 5+ years
ago…Social
MediaSlide7
Rationale for Change
NWS Meteorologists currently look for radar signatures that indicate wind around or > 50 ktFor example: strong reflectivity gradients on leading edge of lines/bows“wall of wind” signatures in base velocity dataHowever, most storms that produce damage to weakened/rotted trees occur with subtle signaturesSlide8
Severe Radar Signatures Examples for Wind
“Wall of wind” from 0.5° base velocity
60-70 kt
Strong reflectivity gradient on leading edgeSlide9
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 trunkSlide10
Background Information
Case study from Frelich and Ostuno (2012): July 4, 2003 Grand Rapids, MIA dozen trees or large tree limbs downed by 45-55 mph windsStorm survey revealed all but one tree was rotted/diseasedSlide11
Background Information
Even occurrences such as drought can have an effect on the health of trees and can increase their susceptibility of fallingSlide12
Scientific Basis for Change
From Enhanced Fujita Scale Report (McDonald and Mehta 2006):Lower bound for tree limb damage found to be 48 mph Slide13
Scientific Basis for Change
From Enhanced Fujita Scale Report (McDonald and Mehta 2006):Lower bound for tree limb damage found to be 48 mph Slide14
Proposed Change
Impossible to account for condition of trees when issuing warningsChange 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 ScaleSlide15
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.Slide16
Acknowledgements/References
Acknowledgements: Neil Stuart, Brian Frugis (ALY)References: NWS Directiveshttp://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.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.Slide17
Hopefully the discussion continues…
Goal: Get people talking about this issue and discuss potential solutions!Contact info: Joe.Villani@noaa.gov
Questions/Comments?