Environmental Conditions Associated with Cool Season Significant Tornadoes over the North Central United States Mark F. Britt and Fred H. Glass National Weather Service St. Louis, MO

Hey Mark, It’s Winter! Why are you talking about tornadoes? Based on the LSX dataset, 43% of all tornadoes are >F2 during the cool season compared to 26% throughout the year.

Produced by Jonathan Finch, WFO DDC,

Why Am I Here? To increase situational awareness and increase knowledge of parameters of winter tornado events by examining past event’s: Temporal and spatial distribution. Thermodynamic and shear parameters. Synoptic scale characteristics.

The Database Was compiled using the SPC Storm Events Database and Storm Data. “Cool Season” is defined as November 16 th – end of February. Covers the years from 1979 to A total of 43 strong and violent tornadoes were identified over 18 separate tornado days.

Plotted using SeverePlot v2.0, 1999, Hart & Janish, NWS/NCEP/SPC Area Studied Environmental parameters were calculated using the tornadoes in RED.

Environmental Data Sounding data was extracted from North American Regional Reanalysis (NARR) using NSHARP. Every 50mb from the lowest model level to 100mb The NARR is a long-term, consistent, high-resolution climate dataset for the North American domain, as a major improvement upon the earlier global reanalysis datasets in both resolution and accuracy (Mesinger et al., 2006) Uses the old 32km Eta Model with the Regional Data Assimilation System (RDAS) 29 Vertical Levels Analysis every three hours (eight times a day)

Environmental Data Thermodynamic and kinematic parameters currently deemed important to severe storm and tornadic environments were computed for each sounding. A composite sounding and hodograph were generated using the means of temperature, dewpoint, wind speed and wind direction from each level.

Composites 87 The Skew-T depicts a sounding with a relatively moist lower troposphere, small CAPE, and a low equilibrium level. Most of the instability is below 500mb. The hodograph shows some veering of the winds from south to southwesterly in the lowest kilometer. Above that, winds are generally southwesterly, unidirectional, and increase in speed with height.

Thermodynamic Parameters 90 th Percentile 75 th Percentile Median 25 th Percentile 10 th Percentile Median from Thompson et al. (2003) J/kg # of Cases

Thermodynamic Parameters 16

Thermodynamic Parameters 79

18

Thermodynamic Parameters Little convective inhibition in these environments with median values to ML=17 J/kg and MU=6 J/kg. Middle level ( mb) lapse rates were in a narrow range between 6.0 and 6.6 o C/km.

Kinematic Parameters 1512 (m/s)

Kinematic Parameters 14 Bunker’s RM Storm Motion = 237 o /47kts.

Mean Composites Images provided by the NOAA-CIRES Climate Diagnostics Center, Boulder Colorado from their Web site at Composites of three-hourly mean and daily mean anomalies for sixteen of the cases were generated from the NARR dataset.

300 mb Mean Zonal Winds T-24T-12 Tornado TimeAnomaly

500 mb Mean Heights T-24T-12 Tornado TimeAnomaly

700 mb Mean Omega T-24T-12 Tornado TimeAnomaly

850 mb Mean Meridional Wind T-24T-12 Tornado TimeAnomaly

850 mb Mean Temperature T-24T-12 Tornado TimeAnomaly

850 mb Mean Specific Humidity T-24T-12 Tornado TimeAnomaly

1000 mb Mean Heights T-24T-12 Tornado TimeAnomaly

Climatology Summary A larger percentage of tornadoes that occur in the cool season are strong or violent compared to the year as a whole. Strong and violent tornadoes can occur at anytime during the winter months, including during the late evening and overnight hours.

Sounding Summary When comparing these cases to the significant tornadic supercells in the Thompson et al dataset. The MLCAPE in these cases are significantly lower. The 0-1km and 0-6km bulk shear is noticeably higher, and the 0-1km SRH is similar. The lower ML LCLs imply a relatively high amount moisture in the sub cloud layer. The mb lapse rates occur in a narrow range between the o C/km. All of these cases have their CAPE concentrated below 500mb. Bunker’s Supercell Motion (2000) is 240 o at 24 m/s.

Composite Summary All values were anomalously high for the time of year. These cases occurred with strong large scale forcing (upward omega) caused by coupled jet structure, relatively deep trough, and low-mid level warm air advection. This likely offsets the weak thermodynamics. The wind fields for these cases are quite strong which contributes to the observed enhanced vertical shear. The mean zonal wind speeds associated with jets streaks is 30 m/s at 500 mb and 42 m/s at 300 mb.

Composite Summary A strong low level jet is responsible of for significant lower tropospheric moisture transport and warm thermal advection. The mean 1000 mb height field suggests that surface lows move northeastward from the Central Plains into the Great Lakes. The low-level means of temperature and specific humidity are indicative of each of the individual cases; which possess a sharply baroclinic, low level boundary.

References Matthew J. Bunkers, Brian A. Klimowski, Jon W. Zeitler, Richard L. Thompson and Morris L. Weisman. 2000: Predicting Supercell Motion Using a New Hodograph Technique. Weather and Forecasting: Vol. 15, No. 1, pp. 61–79. Mesinger, Fedor, DiMego, Geoff, Kalnay, Eugenia, Mitchell, Kenneth, Shafran, Perry C., Ebisuzaki, Wesley, Jovi, Dušan, Woollen, Jack, Rogers, Eric, Berbery, Ernesto H., Ek, Michael B., Fan, Yun, Grumbine, Robert, Higgins, Wayne, Li, Hong, Lin, Ying, Manikin, Geoff, Parrish, David, Shi, Wei, 2006: North American Regional Reanalysis Bulletin of the American Meteorological Society, 87, Thompson R.L., Edwards, J. A. Hart, K. L. Elmore, and P. M. Markowski, 2003: Close proximity soundings within supercell environments obtained from the Rapid Update Cycle. Wea. Forecasting, 18,