An Overview of Environmental Conditions and Forecast

Implications of the 3 May 1999 Tornado Outbreak

Richard L. Thompson and Roger Edwards

Presentation by Chris Fisher

Overview

• Brief Introduction of the Outbreak• Synoptic Scale Characteristics• Moisture and Instability• Vertical Shear• Convective Initiation• Conclusion

Introduction

• Late Afternoon and Evening of May 3, 1999• 69 Tornadoes, 10 which were Supercells• Central/Northern Oklahoma and Southern

Kansas• Violent tornadoes occurred in the Oklahoma

City and Wichita metropolitan areas• Poor Numerical Model Forecast meant that

outbreak was not initially predicted.

•500 mb heights•Trough located over the Western United States•Short Wave trough progresses from Arizona to Western Oklahoma and Kansas

•Surface dew points in the middle to upper 60’s degrees F•Drylines present•Fronts become more occluded

•Strong dew points and high daytime surface temperatures in southwest Oklahoma contributed to large values of CAPE and weak values of CIN.

•Moist boundary layer in the lowest 1km•Elevated mixed layer from 825mb to 600mb•CAPE value of 2434 J/kg at Norman Ok at 1200UTC on May 03

•General 20-45kt strengthening of the flow in the 4-10km layer•According to previous research, an observed strengthening of the lower and middle tropospheric flow during the afternoon, generally below 8km, is an indication of enhanced supercell potential.•Increasing mid-upper tropospheric winds resulted in sufficient deep layer vertical shear.

•BRN values of 55 m^2/s^2 in Frederick, OK at 2042 UTC on May 3rd

•BRN values of 166 m^2/s^2 in Purcell, OK at 0200 UTC on May 4th

•SRH values of 338 m^2/s^2 in Purcell

•Convection formed under a gap in the cirrus canopy between the two drylines•Storm A moves across the confluence boundary 20 minutes prior to the first significant tornado of the outbreak•Storm A and B combined produced a total of 35 tornadoes, including an F5 that moved across southern Oklahoma City

•Eta Model Forecasts of 300mb (a,b) and 500mb (c,d)•Model showed the possibility of Severe Weather but the tornado outbreak was not anticipated due to poor forecasts of the mid-upper tropospheric winds

•Eta Model initial analyses of 300-mb geopotential height (dam), divergence (solid lines 1 × 10−5 s−1, dashed lines represent convergence), and negative absolute geostrophic vorticity (shaded area, with gradations at 0, −1 × 10−5 s−1, −5 × 10−5 s−1, and −10 × 10−5 s−1) at (a) 1800 UTC 3 May 1999, and (b) 0000 UTC 4 May 1999. The shaded areas of negative absolute geostrophic vorticity represent areas of inertial instability. (Courtesy of D. Schultz, National Severe Storms Laboratory.)

CONCLUSION

• General Characteristics of severe weather outbreak were observed

• Trough located over the 4 corners region and deep low pressure in the high plains

• Unstable warm sector• Mid-upper jet moved eastward from the

trough during the afternoon of the outbreak

CONCLUSION

• Large area of high clouds overspread the drylines and warm sector complicating forecasts for convective development

• Tornado threat was not realized due to poor model forecasts of the mid and upper level flow, and deep layered vertical shear

• Observational data suggested a greater threat of supercells.

CONCLUSION

• Sufficient CAPE values (3000-5000 J/Kg)• Sufficient vertical shear (excess of 20 m/s)• BRN shear greater than 40 m^2 s^2• 0-3 km SRH values of 150-300 m^2s^2• Initial storms did not evolve into a squall line

meaning that supercells remained in a favorable environment, allowing them to produce a large number of tornadoes (due to weak convergence along the dryline)

QUESTIONS????