1. 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 12222 Steven J. Weiss NOAA/NWS/Storm Prediction Center Norman, OK 12233 Northeast Regional Operational Workshop XVI Wednesday 4 November 2015 Support Provided by: NSF-AGS-1240502

2. 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.

3. 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.

4. Outline 1. Objectives 2. Methodology 3. “Parallel” Event Example 4. “Transition” Event Example 5. 2007-2015 NE US Climatology

5. 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)

6. Methodology

7. 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)

8. Northeast Domain 12z15z21z00z03z06z09z12z18z

9. 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.

10. 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”

11. “Parallel” Example

13. 12z15z21z00z03z06z09z12z18z

14. “Parallel” Example

37. “Transition” Example

39. 12z15z21z00z03z06z09z12z18z

40. “Transition” Example

75. Northeast Climatology (2007–2015)

76. Climatology 43 Total Events – 35 Parallel – 8 Transition

77. Climatology

78. Sd

79. All Events Sd

80. Parallel Sd

81. Transition Sd

82. 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)

83. All Events Sd n=43TornadoHailWindTotal SVR Flash FloodFloodTotal FF/F Average2.0015.3768.4985.8618.606.0924.70 Standard Deviation4.1823.3546.1957.839.459.3413.87 Sum86661294536928002621062 Median16557216222 Max22123195260423972 Min00817305 n=43DeathsInjuriesDamagesFF DamageSVR Damage Average0.372.056817411.634798069.772019341.86 Standard Deviation1.005.6710056994.788473514.595892701.72 Sum168829314870020631700086831700 Median002046500455000600000 Max535400040003035000037775000 Min003400008000

84. Parallel Sd n=35TornadoHailWindTotal SVR Flash FloodFloodTotal FF/F Average2.0616.3170.4988.8619.576.2925.86 Standard Deviation4.5424.2249.1862.579.8310.1214.88 Sum7257124673110685220905 Median17556620224 Max22123195260423972 Min00817405 n=35DeathsInjuriesDamagesFF DamageSVR Damage Average0.432.007059462.864742085.712317377.14 Standard Deviation1.096.0810226487.878272091.246507265.62 Sum157024708120016597300081108200 Median002205500585000600000 Max535400040003035000037775000 Min003400008000

85. Transition Sd n=8TornadoHailWindTotal SVR Flash FloodFloodTotal FF/F Average1.7511.2559.7572.7514.385.2519.63 Standard Deviation2.1919.9330.7128.396.415.096.61 Sum149047858211542157 Median1156.57714.5317 Min0030323011 Max655115117261330 n=8DeathsInjuriesDamagesFF DamageSVR Damage Average0.132.255758437.505043000.00715437.50 Standard Deviation0.353.659865047.939917477.17532084.83 Sum11846067500403440005723500 Median001074500222500544250 Min0022300035000153000 Max1928303500276500001797000