An Examination of “Parallel” and “Transition” Severe Weather/Flash Flood Events Kyle J. Pallozzi and Lance F. Bosart Department of Atmospheric and Environmental Sciences University at Albany/SUNY Albany, NY Steven J. Weiss NOAA/NWS/Storm Prediction Center Norman, OK Northeast Regional Operational Workshop XVI Wednesday 4 November 2015 Support Provided by: NSF-AGS

Definitions “Parallel” events are defined as cases where traditional forms of severe weather (tornadoes, damaging winds, large hail) are ongoing at the same time during which severe flooding is also occurring within a given area. “Transition” events are characterized by a shift in the main threat type with respect to time. Usually this shift is from classical forms of severe weather to flash flooding at a later time as initial discrete supercells grow upscale into Mesoscale Convective Systems (MCSs) with training elements.

Motivation Severe thunderstorms produce multiple hazards all of which have high societal impacts. –Classical severe weather (tornado, hail, damaging wind) –Flash flooding When both threats are possible it presents a major challenge for operational forecasters –Communication of dual threat to general public –Accurate prediction of convective initiation, upscale growth process and “transitions” in primary threat type with respect to time associated with changes in storm environment.

Outline 1. Objectives 2. Methodology 3. “Parallel” Event Example 4. “Transition” Event Example NE US Climatology

Objectives Create a 10-year climatology of these events nationally: –Where do these events occur? –When do they occur (diurnally and seasonally)? Identify synoptic scale and mesoscale environmental features which favor the development of these events (future work): –End goal is to improve the predictability of these events –Composite analysis of environmental variables –Selected case studies –Numerical simulation (if time permits)

Methodology

Study time period: 2007–2015 Data –Storm reports from NOAA Storm Data publication –Archived radar data Study domain: CONUS east of the Front Range –Focus on Northeast climatology –“Northeast” was defined as area bounded by 39.0– 47.5 N latitude and 66.5–81.0 W longitude (see next slide)

Northeast Domain 12z15z21z00z03z06z09z12z18z

SVR/FF Event Criteria Spatial Requirement: Reports cover area greater than 25,000 km 2. Temporal Requirement: Event at least 3 hours in duration. Minimum Report Threshold: At least 30 Severe (SVR) and 10 Flash Flood/Flood (FF) Reports. Exception: If an event resulted in either a fatality, 10 or more injuries, or greater than $10 million in damages the minimum report threshold was lowered to 15 SVR and 5 FF reports.

Methodology Report plots, color coded hourly report plots and report time series were created for each day using the storm reports Combined SVR/FF events were identified using the plots mentioned above, the criteria on the previous slide and archived radar data SVR/FF events were subjectively classified as either “Parallel” or “Transition”

“Parallel” Example

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“Parallel” Example

“Transition” Example

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“Transition” Example

Northeast Climatology (2007–2015)

Climatology 43 Total Events – 35 Parallel – 8 Transition

Climatology

Sd

All Events Sd

Parallel Sd

Transition Sd

Conclusions/Future Work “Parallel” events most common in Northeast US Average ~ 5 combined events per year Seasonal Distribution is as expected Delineation between “parallel” and “transition” events may be of greater benefit in other regions of the United States Future Work: –Expansion of 10-year climatology to remainder of study area –Composite analysis of environmental variables –Selected case studies –Numerical simulations (if time permits)

All Events Sd n=43TornadoHailWindTotal SVR Flash FloodFloodTotal FF/F Average Standard Deviation Sum Median Max Min n=43DeathsInjuriesDamagesFF DamageSVR Damage Average Standard Deviation Sum Median Max Min

Parallel Sd n=35TornadoHailWindTotal SVR Flash FloodFloodTotal FF/F Average Standard Deviation Sum Median Max Min n=35DeathsInjuriesDamagesFF DamageSVR Damage Average Standard Deviation Sum Median Max Min

Transition Sd n=8TornadoHailWindTotal SVR Flash FloodFloodTotal FF/F Average Standard Deviation Sum Median Min Max n=8DeathsInjuriesDamagesFF DamageSVR Damage Average Standard Deviation Sum Median Min Max