# Presentation: 735843

## April 20, 2004Tornado Outbreak

*Nature Can Still Surprise...*

- Paul L. SirvatkaProfessor of MeteorologyCollege of DuPageGlen Ellyn, IL

- 2005 National Severe Weather Workshop Paul Sirvatka

**Notes:**
April 20 2004 is the date of one of the most underforecast tornado outbreaks in the modern era.

## Event Overview

- April 20, 2004 in Iowa, Illinois and Indiana
- 30 Tornadoes
- 8 Deaths in Utica IL
- Milestone Tavern Collapsed
- Sought Safer Shelter

- 2005 National Severe Weather Workshop Paul Sirvatka

## Northeast Illinois

- 2005 National Severe Weather Workshop Paul Sirvatka

**Notes:**
Event Overview
Number of Tornadoes - 30 in 4.5 hours
Number of Deaths 8 Milestone Tavern - two crushed, 6 asphyxiated

## Photo courtesy of Jim Krancic

- 2005 National Severe Weather Workshop Paul Sirvatka

## Photo courtesy of Jim Krancic

- 2005 National Severe Weather Workshop Paul Sirvatka

## Photo courtesy of Jim Krancic

- 2005 National Severe Weather Workshop Paul Sirvatka

## Photo courtesy of Jim Krancic

- 2005 National Severe Weather Workshop Paul Sirvatka

## Photo courtesy of Jim Krancic

- 2005 National Severe Weather Workshop Paul Sirvatka

## Granville, IL

- Photos courtesy of Steve Drews

- 2005 National Severe Weather Workshop Paul Sirvatka

## Granville, IL

- Photos courtesy of Steve Drews

- 2005 National Severe Weather Workshop Paul Sirvatka

## Utica, IL

- Photos courtesy of Steve Drews

- 2005 National Severe Weather Workshop Paul Sirvatka

## F -3 Damage to Milestone Tavern

- Photos courtesy of Steve Drews

- 2005 National Severe Weather Workshop Paul Sirvatka

## Video from CBS-Chicago and NWS- LOTJim Allsopp and Al Pietrycha

- 2005 National Severe Weather Workshop Paul Sirvatka

## Synoptic Scale Set-up

- 2005 National Severe Weather Workshop Paul Sirvatka

## Storm Reports 4-20-04

- 53 tornado reports nationwide
- Very few hail reports in Northern Illinois
- Largest hail occurred in warm sector but there was tennis ball hail with the LaSalle County Storm

- 2005 National Severe Weather Workshop Paul Sirvatka

## 5 Supercells in Northeast Illinois

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## 20Z Convective Outlook

- 2005 National Severe Weather Workshop Paul Sirvatka

## Why the Miss?

- Completely missed the forecast instability
- Pattern recognition was incorrect
- Stratiform rain and clouds cleared in a narrow band along warm front
- All numerical guidance was poor (RUC and ETA)

- 2005 National Severe Weather Workshop Paul Sirvatka

## Favorable for Severe

- Incredible low-level shear 0-1KM north of warm front
- Clearing in narrow zone north of front
- Sufficient moisture
- Favorable storm motion with respect to front motion

- 2005 National Severe Weather Workshop Paul Sirvatka

## VAD Profile from LOT

- 2005 National Severe Weather Workshop Paul Sirvatka

## Unfavorable for Severe

- CAPE was limited
- Rain-cooled air in spring almost always kills the threat of surfaced-based severe in NE IL
- Initiation mechanism in warm sector was uncertain
- Despite strong SRH in 0-1km, it won’t be realized if storms are elevated

- 2005 National Severe Weather Workshop Paul Sirvatka

## 21 Z Surface Map

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## Visible Satellite Imagery

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## Radar Review

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## Watch Number 95 - 5:40 PM CDT

*DISCUSSION...FAVORABLE SHEAR PROFILES ON NWD MOVING WARM FRONT FOR TORNADIC SUPERCELLS. STORMS WILL CONTINUE TO DEVELOP AS THE WARM FRONT SHIFTS NWD INTO NRN IL/IND.*

- 2005 National Severe Weather Workshop Paul Sirvatka

## The result...?

- 2005 National Severe Weather Workshop Paul Sirvatka

## Storms Spun like Tops!

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## Convergent Rotation

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## Mini-Supercells

- TBSS (Three-body Scatter Spike)

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## Mini-Supercells

- Tornadic cases have a great deal of CAPE below 500mb
- Lower total CAPE and lower EL
- Significant shear over the depth of the storm (30-35k feet)

- 2005 National Severe Weather Workshop Paul Sirvatka

## 22Z RUC Analysis (PIA ua and SPI sfc)

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## 00Z DVN Sounding

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## 100 mb vs. 50 mb MLCAPE

- 2005 National Severe Weather Workshop Paul Sirvatka

## Summary of April 20th Events

- Warm front provided source of lift and additional vorticity
- Low LCL’s, high helicity and adequate moisture and shear
- CAPE appeared to be lacking but was more than sufficient with limited sunshine
- Speed of storms and front matched, allowing for storms to stay in “sweet zone” for a long period of time
- Position of 850mb warm front

- 2005 National Severe Weather Workshop Paul Sirvatka

## Summary of April 20th Events

- Forecasters missed the event in terms of surface-based convection
- The response to what was *actually* happening was quick, decisive and showed a good state of readiness - situational awareness was strong

- 2005 National Severe Weather Workshop Paul Sirvatka

## 100% POD!

- Warnings were issued prior to all tornadoes
- FAR = .58  CSI = .42
- Average lead time is 15.6 minutes
- Utica tornado had a lead time of 26 minutes
- Excellent spotter reports from chasers, spotters and law enforcement officers

- 2005 National Severe Weather Workshop Paul Sirvatka

## So What Can We Learn?

- We need more skill in weather forecasting - models are not 100% reliable
- Spring warm fronts can produce very dangerous weather events
- Lift, moisture, instability, wind shear: what is missing? Watch for the last ingredient to show up!
- High CAPE is overrated!

- 2005 National Severe Weather Workshop Paul Sirvatka

## Questions to Answer

- Why did the models miss the CAPE so badly? Should forecasters have anticipated the miss?
- Why did the air clear north of the early spring warm front?
- Would we anticipate this event in the future?

- 2005 National Severe Weather Workshop Paul Sirvatka

## The “No-duh” info that we need to hear over and over...

- Supercells come in many shapes and sizes but they are all dangerous
- Be prepared in all circumstances
- Meteorology can always find ways to surprise us and teach us
- Tornadoes are cool!!!!

- 2005 National Severe Weather Workshop Paul Sirvatka

## Acknowledgements

- Al Pietrycha and Jim Allsopp of theChicago National Weather Service
- Jon DaviesSteve DrewsJim Krancic
- College of DuPage Meteorology Staff

- 2005 National Severe Weather Workshop Paul Sirvatka

## Close-up of Joliet Tornadic Storm

- 2005 National Severe Weather Workshop Paul Sirvatka

## Amazing RFD

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## LaSalle County Flyover

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## Kankakee Tornado

- 2005 National Severe Weather Workshop Paul Sirvatka