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

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

5. 2007-2015 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

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

12z 15z 21z 00z 03z 06z 09z 12z18z

SVR/FF Event Criteria

Spatial Requirement: Reports cover area greater than 25,000 km2.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

“Parallel” Example

“Parallel” Example

12z 15z 21z 00z 03z 06z 09z 12z18z

“Parallel” Example

“Transition” Example

“Transition” Example

“Transition” Example

12z 15z 21z 00z 03z 06z 09z 12z18z

“Transition” Example

Northeast Climatology(2007–2015)

Climatology

• 43 Total Events– 35 Parallel– 8 Transition

Climatology

January

Febru

ary

March April

May

JuneJuly

August

September

October

November

December

0

2

4

6

8

10

12

14

Monthly Distribution

EventsParallelTransition

Month

Num

ber

of E

vent

s

Climatology• Sd

2007 2008 2009 2010 2011 2012 2013 2014 20150

1

2

3

4

5

6

7

8

Yearly Distribution

EventsParallelTransition

Year

Num

ber

of E

vent

s

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=43 Tornado Hail Wind Total SVRFlash Flood Flood Total FF/F

Average 2.00 15.37 68.49 85.86 18.60 6.09 24.70Standard Deviation 4.18 23.35 46.19 57.83 9.45 9.34 13.87Sum 86 661 2945 3692 800 262 1062Median 1 6 55 72 16 2 22Max 22 123 195 260 42 39 72Min 0 0 8 17 3 0 5

n=43 Deaths Injuries Damages FF Damage SVR DamageAverage 0.37 2.05 6817411.63 4798069.77 2019341.86Standard Deviation 1.00 5.67 10056994.78 8473514.59 5892701.72Sum 16 88 293148700 206317000 86831700Median 0 0 2046500 455000 600000Max 5 35 40004000 30350000 37775000Min 0 0 34000 0 8000

Parallel• Sd

n=35 Tornado Hail Wind Total SVRFlash Flood Flood Total FF/F

Average 2.06 16.31 70.49 88.86 19.57 6.29 25.86Standard Deviation 4.54 24.22 49.18 62.57 9.83 10.12 14.88Sum 72 571 2467 3110 685 220 905Median 1 7 55 66 20 2 24Max 22 123 195 260 42 39 72Min 0 0 8 17 4 0 5

n=35 Deaths Injuries Damages FF Damage SVR DamageAverage 0.43 2.00 7059462.86 4742085.71 2317377.14Standard Deviation 1.09 6.08 10226487.87 8272091.24 6507265.62Sum 15 70 247081200 165973000 81108200Median 0 0 2205500 585000 600000Max 5 35 40004000 30350000 37775000Min 0 0 34000 0 8000

Transition• Sd

n=8 Tornado Hail Wind Total SVRFlash Flood Flood Total FF/F

Average 1.75 11.25 59.75 72.75 14.38 5.25 19.63Standard Deviation 2.19 19.93 30.71 28.39 6.41 5.09 6.61Sum 14 90 478 582 115 42 157Median 1 1 56.5 77 14.5 3 17Min 0 0 30 32 3 0 11Max 6 55 115 117 26 13 30

n=8 Deaths Injuries Damages FF Damage SVR DamageAverage 0.13 2.25 5758437.50 5043000.00 715437.50Standard Deviation 0.35 3.65 9865047.93 9917477.17 532084.83Sum 1 18 46067500 40344000 5723500Median 0 0 1074500 222500 544250Min 0 0 223000 35000 153000Max 1 9 28303500 27650000 1797000