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Cool Season Strong Tornadoes in the Southeast United States

Jared L. Guyer NOAA/NWS Storm Prediction Center E-mail: Jared.Guyer@noaa.gov. Cool Season Strong Tornadoes in the Southeast United States. March 1, 2007 – National Severe Weather Workshop. Early Morning Florida Tornadoes February 2, 2007. Lady Lake Church.

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Cool Season Strong Tornadoes in the Southeast United States

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  1. Jared L. Guyer NOAA/NWS Storm Prediction Center E-mail: Jared.Guyer@noaa.gov Cool Season Strong Tornadoes in the Southeast United States March 1, 2007 – National Severe Weather Workshop

  2. Early Morning Florida Tornadoes February 2, 2007 Lady Lake Church The Villages / Lady Lake Tornado EF-3 Tornado with a 16.5 mile track and 8 Fatalities 3:08 a.m. – 3:25 a.m. EST Mobile Homes Demolished in Lady Lake Images courtesy NWS Melbourne

  3. Early Morning Florida Tornadoes February 2, 2007 Motor Home Destroyed at Lake Mack Lake Mack / Deland Tornado EF-3 Tornado with 26 mile track and 13 Fatalities 3:37 a.m. – 4:10 a.m. EST Mobile Home Frame at Lake Mack Images courtesy NWS Melbourne

  4. Introduction • An ongoing SPC study to help identify a set of patterns, parameters, and conditions that are commonly associated with the development of cool season tornadoes in the Southeast States, with a focus on strong (F2+ greater) tornadoes. • Will also discuss some non-meteorological aspects of these tornado occurrences

  5. Study Introduction • 68% of all December through February tornadoes in US occur in southeast (Galway and Pearson 1981) • Cool-season conditions • Favorable wind shear common, however… • Instability usually questionable ingredient for supercell development • Uncertainty regarding low-level moisture and atmospheric instability can make forecasting such events quite challenging for operational forecasters (Vescio and Thompson 1993).

  6. Project Definitions • “Cool-Season” • October 15 – February 15 • Examined 20 cool seasons • 1984-1985 through 2003-2004 • “Strong tornado” • F2+ intensity • Focus on non-tropical systems

  7. Cool Season F2+ Tornadoes Tornado Tracks in Red All F2-F5 Tornadoes between October 15 - February 15 from 1984-1985 to 2003-2004 239 F2+ Tornadoes (F2 76%, F3 20%, F4 4%, F5 0%) from 100 tornado days Long term yearly average: 12 F2+ tornadoes amongst 5 tornado days each year

  8. Research Methodology • Past Upper Air Data • North America Regional Reanalysis (NARR) data at 6 hour intervals (00 UTC, 06 UTC, 12 UTC, 18 UTC) • Mandatory level winds, geopotential heights, and temperatures • Precipitable water, surface temperature, surface dewpoint • MSLP, 0-3 km helicity, lowest 180 mb most unstable CAPE (MUCAPE) • Proximity Upper Air Soundings • Constraints of 200 km and +- 3 hr • 57 soundings amongst 50 tornado days • Modified for nearest surface inflow observation of temperature, dewpoint, and wind speed/direction with 0-1 hr preceding the tornado

  9. Research Findings – 500 mb Patterns • 72% of cases associated with the anticyclonic south fringe of a 500 mb jet • 20% of cases coincided with the 500 mb jet nose and/or beneath the jet axis • Strength of the mid/upper level jet did not appear to discriminate between isolated strong tornado events vs. larger outbreaks Example 500 mb - 12 UTC 24 November 2001 14 F2+ Tornadoes

  10. Example Upper Air Sounding Observed RAOB preceding (23z XMR) the February 2, 2007 Tornadoes Height (km) Temperature Extremely strong wind fields favorable for rotating storms Each Bar = 10 kt Each Flag = 50 kt Dewpoint Deep moisture 68 F Temp 66 F Dewpoint Modest Convective Instability (CAPE)

  11. Southeast Cool Season F2+ Tornadoes 90th Percentile 75th Percentile 50% 80% Mean Value 25th Percentile 10th Percentile Precipitable Water (inches) Surface Dewpoints (°F) Derived from upper air soundings in proximity to F2+ Tornado Events

  12. Southeast Cool Season F2+ Tornadoes CAPE (J/kg) Derived from upper air soundings in proximity to F2+ Tornado Events

  13. Southeast Cool Season F2+ Tornadoes 25 F2+ Tornadoes in November-February associated with < 500 J/kg MLCAPE in 2003-2005 Storm environment data provided by Schneider and Dean, SPC

  14. Southeast Cool Season F2+ Tornadoes Storm Relative Helicity (m2/s2) Vertical Shear (kt) Derived from upper air soundings in proximity to F2+ Tornado Events

  15. Gulf Buoy Data Preceding Cool Season F2+ Tornadoes Gulf of Mexico Buoy data (#42002) Preceding F2+ Tornadoes in the Cool Season Surface Dewpoint with Tornado Buoy 42002 Dewpoint Preceding Tornado Event Time Preceding the Tornado Evans and Guyer 2006

  16. Meteorological CaveatsParameters Alone Don’t Tell the Whole Story 18 UTC BMX 16 Dec 2000 18 UTC BMX 16 Feb 2001 Tornadic supercell (F4 at TCL) Bow Echo/ Derecho Slide courtesy Steve Weiss, SPC

  17. Meteorological Summary • Similar to previous studies, confirmed the predominance of high shear/low instability regimes with cool season Southeast U.S. F2+ tornadoes • Pronounced speed shear component in the lowest few km • 850 mb wind speeds typically >40 kt • 0-1 km bulk shear >25 kt • Only modest amounts of convective instability is typically needed • CAPE values may be 500-1000 J/kg or less • Given a favorable synoptic system (and associated wind fields), measures of low level moisture (dewpoints and/or mixing ratios) appear to be paramount for anticipating Southeast U.S. F2+ tornadoes during the cool season.

  18. Cool Season F2+ Tornadoes Midnight All F2-F5 Tornadoes between October 15 - February 15 from 1984-1985 to 2003-2004

  19. Overnight Cool Season Tornadoes • Since November 2006, 88% of tornado fatalities have occurred during the overnight hours • 30 nighttime fatalities (of 34 total) associated with 5 killer tornadoes • Lady Lake, FL and Paisley/DeLand, FL on February 2, 2007 (20 fatalities total) • Gentilly, LA on February 13, 2007 (1 fatality) • Riegelwood, NC on November 16, 2006 (8 fatalities) • Montpelier, LA on November 15, 2006 (1 fatality) • Since 1998, the biggest single killer tornado was November 6, 2005 in Evansville, IN at 2:00 am CST (25 fatalities)

  20. Non-Meteorological Factors • A tendency for events to occur during the late evening/overnight hours during the cool season in the southeast United States • Getting the message out before most people go to sleep – late evening news? • What late night precautions do people take? • NOAA Weather Radio • Even in more modest scenarios (i.e. not a larger outbreak), the potential may exist for strong tornadoes in a more isolated sense given the conditions (i.e. strong wind shear) • A low probability and/or coverage, but it may be a high end event • Scenario leads to an inherent tendency for occasional false alarm • Expected severe coverage may not be particularly high, but conditional potential exists for a strong tornado • Public education of the caveats associated with such cool season/nocturnal scenarios • A large part of a tornado watch may not end up experiencing widespread severe weather, but there is conditional potential for strong tornadoes where supercells occur in the watch

  21. Summary • Future Research: • Focus on solely hail/wind, and possibly weak tornado, events to derive a separate proximity sounding dataset for comparative purposes. (Summer 2007) • Closer examination of moisture return trends via buoy data and surface data. • My coauthors on this study: • David A. Imy, NOAA/NWS Storm Prediction Center • Amanda Kis, University of Wisconsin • 2005 NOAA Hollings Scholarship program • Kar’retta Venable, Jackson State University • 2005 Research Experiences for Undergraduates (REU)

  22. Jared L. Guyer NOAA/NWS Storm Prediction Center E-mail: Jared.Guyer@noaa.gov Cool Season Strong Tornadoes in the Southeast United States March 1, 2007 – National Severe Weather Workshop

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