Hurricane Arthur, July 2014: A case study in using disparate weather sources to reconstruct a weather event October 22, 2015 NWA Meeting Oklahoma City, OK

Who are we at Shade Tree Meteorology, LLC ? “A Full-Service Forensic Meteorology Firm Specializing in Severe Weather Event Reconstruction” Majority of our business involves weather event reconstruction for cases in litigation We also provide the following: Special event forecasts; weather risk assessment & planning; weather instrument siting

Hurricane Arthur: A brief review ▪ CAT2 landfall along NC coast ▪ Storm surge/flooding along Outer Banks ▪ Continued northeastward but stayed offshore ▪ ‘minor impacts’ to southeastern Massachusetts ▪ Extratropical transition by 1200 UTC 5 July Key time for our case

0000 UTC Jul 5 Surface Map

0000 UTC Jul mb

Case specifics: ▪ Located on north shore of Cape ▪ Located on a bluff (elevation 34 feet) ▪ 3.5 story house, windows facing Cape Cod Bay Key Questions to answer: ▪ How much rain? ▪ Wind speed/direction? ▪ Was wind-driven rain occurring, how long, and how much?

Issues: ▪ Data sources ▪ KBOX Radar (52 mi NW, beam elevation ~3934 ft/1.2 km) – only inbound/outbound ▪ KCHH Radiosonde (2x/day) ▪ ASOS (KHYA, KPVC) – missing data at KPVC ▪ Coop/CoCoRaHS ▪ Yarmouth (4 mi SW), Brewster (6 mi E) ▪ CWOP at Brewster (on a bluff elevation 39ft, few blocks in from shore) ▪ On-site anemometer shown in picture provided by client (N/A) ▪ Building code ▪ Timing ▪ Bluff speedup effect

1: Rainfall ▪ Radar ▪ Nearby obs

2. Winds ▪ Key issue: onshore vs. offshore flow ▪ Initially storm to south/southeast ▪ Wind from southeast ▪ As storm moves northeastward, wind shifts ▪ Northeast, then north, then northwest ▪ Recall: bluff elevated ~34 ft above shoreline

Key data: radar base velocity ▪ 52 miles northwest of site ▪ Beam height ~4000 ft ▪ Compare with winds in KCHH sounding

Radar velocity comparison 5 July 8:43 p.m.5 July 11:30 p.m.

Also important: CWOP at Brewster ▪ 5 miles east-northeast of site ▪ On a bluff (~39 ft) ▪ Compare with other ASOS obs (PVC and HYA) ▪ Use Durst curve to estimate wind gusts with known 2-min wind speed

End result: ▪ Cross compare all available observations of wind, both at the surface and aloft, to estimate wind speeds and directions at site ▪ Effective height of building is higher when wind is blowing from N than from S ▪ Take this into effect using power law to estimate faster winds aloft

The result: DateHour endingHourly Precip (inches) light rain = light green, moderate rain = dark green) Highest 2-minute average wind speed (mph) at 3.5 stories *direction is FROM which wind blows Wind gust range at 3.5 stories (42 ft for SE/E/SSE/ESE, 76 ft for NE, NW, NNW, N, WNW) 7/4/20141:00 p.m..1SE at 7 2:00 p.m..1SSE at 17 3:00 p.m..1SE at :00 p.m..1E at :00 p.m..1ESE at :00 p.m..1NE at :00 p.m..25NE at :00 p.m..3N at :00 p.m..3NNW at :00 p.m..25N at :00 p.m..25N at /5/201412:00 a.m..1NNW at :00 a.m..1NW at :00 a.m. NW at :00 a.m..1WNW at :00 a.m..1WNW at :00 a.m..1WNW at :00 a.m..25NW at :00 a.m..1NW at :00 a.m..1NW at :00 a.m..1NW at :00 a.m..1WNW at Total: 2.27 Building code: Must withstand 3 second gusts mph Conclusions: Strongest winds did not correlate with period of heaviest rain Fastest gusts were still at or below code level Question for client: window design? Client question for us: winds on ACK/MVY?

Last question from client: speedup effect ▪ When air flows across a bluff, a speedup occurs ▪ Does this effect matter in this case? FINDINGS: ▪ Speedup effect largest at bluff height at 1.27x sustained wind speed ▪ Speedup effect at 3.5 stories: 1.1x UAA Civil Engineering

The bottom line ▪ Used any and all available data to get information about observed wind speeds and gusts, both at the surface and aloft ▪ Carefully considered topography and shape of coastline, as well as known information about track of storm ▪ Considered the relationship between wind direction and ‘effective’ building height ▪ Investigated speed-up effect of winds blowing across a bluff

Thank You! Contact Information: Alicia Wasula Dick Westergard