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Determining Favorable Days for Summertime Severe Convection in the Deep South

Determining Favorable Days for Summertime Severe Convection in the Deep South. Chad Entremont NWS Jackson, MS. Situation / Concerns. During the months of June, July and August, the primary mode of severe weather is damaging winds from microbursts.

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Determining Favorable Days for Summertime Severe Convection in the Deep South

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  1. Determining Favorable Days for Summertime Severe Convection in the Deep South Chad Entremont NWS Jackson, MS

  2. Situation / Concerns During the months of June, July and August, the primary mode of severe weather is damaging winds from microbursts. During this time, large synoptic scale forcing is pushed well to the north and the southern states are left with very subtle features to drive the daily convection.

  3. Distribution of Severe Weather (May-Sep)Compliments of NWS Birmingham

  4. Goal To differentiate between the days there will be severe convection to the days of just general thunderstorms.

  5. Methodology • Used 12z soundings from KJAN & KBMX from 1999 to 2003 • Sample size is 326 soundings • Excluded strongly forced or frontal forced systems as well as tropical systems

  6. Methodology • Modified the 12z soundings to determine certain instability parameters • Used initial conditions and 0-12 hr model forecasts to locate favorable synoptic patterns that support convection

  7. Methodology • Collected Severe reports from Stormdata Database to determine “event” days vs “Null” days • Event days are defined as any day with at least one severe report • Null days are days with no severe reports • There were 137 event days (42%) • Reports were collected within the CWA of interest for the local sounding

  8. Sounding Modification Used 12z soundings from KJAN and KBMX from 1999 to 2003 during the months of June, July and August. Modified the 12z sounding for temperature and dew point, utilizing a sounding application called N-Sharp. The temperature and dew point used were taken from hourly observations just before and closest to thunderstorms or what best represented the area.

  9. Key Parameters • Surface Based Cape (SBCAPE) • Lifted Index (LI) • Precipitable Water (PW) • 850-500mb temperature difference or Vertical Totals • Downdraft Cape (DCAPE or DAPE) • 0 - 6 km Shear

  10. SBCAPE • Box indicates 75th and 25th percentiles • Whiskers indicate 90th and 10th percentiles

  11. Lifted Index (LI)

  12. Precipitable Water (PW) • Used a “slot” range for favorable values • Values between 1.6 and 1.95 inches are favorable • 67 % of all severe reports occur in this range

  13. 850 - 500mb Temp Difference

  14. DCAPE • Favorable values > 900 J/kg when the SBCAPE was > 3000 J/kg • This was done because the DCAPE could be very high but the instability would not be sufficient enough to support deep convection. • Using the above “favorable values”, an “event” occurred on 71 of 104 days. The frequency of occurrence was 68 % • This N-sharp program also tells you what level the parcel descended from • Any parcel descending from a level above 600mb should be given extra attention

  15. 0 - 6 km Shear • Favorable value, > 8 m/s when SBCAPE was > 3000 J/kg • Using the above “favorable values”, an “event” occurred on 31 of 40 days • The frequency of occurrence was 78 % • When the shear was < 9 m/s and SBCAPE > 3000 J/kg, an “event” occurred 38 of 69 days, a frequency of occurrence of 55 % • When the shear was > 8 m/s and SBCAPE < 3000 J/kg, an “event” occurred only 1 of 81 days

  16. Quantifying the Parameters • Use a checklist style application to organize each parameter. • Determine favorable ranges for each parameter and weight the values (point scale) • Define a daily potential with the point totals. • At JAN we use: Extreme, Likely, Chance, Slight Chance and Little Chance

  17. Microburst Checklist and Potential Scale

  18. Distribution of the Daily PotentialNumber of Days that Occurred in each Potential (326 total)

  19. Distributions and Events

  20. Avg # of Events for each Potential

  21. Synoptic AnalysisImportant Fields to Consider • 850 mb Theta E ridges, moisture transport and lapse rates (vertical totals) • 500 mb winds, looking for subtle veering with time (weak trough passage), increases in wind speed and positions of any shear axis • 250 mb jet positions, diffluent areas, jet streaks, shear axis and divergence • Combine sfc instability (CAPE, LI) and moisture convergence with the favorable regions described above

  22. July 13, 2003 Case • 12z JAN sounding • 850 and 500 mb RUC forecast at 18z • 250 mb winds and divergence • radar picture of a downburst • Severe weather plot

  23. 12z, July 13 JAN Modified Sounding

  24. Checklist Results of July 13 Case • SBCAPE = 4152 J/kg • PW’s = 1.74 in • Lifted Index = -11 • Lapse Rate = 28 C • DCAPE = 1056 J/kg • 0 - 6 shear = 10 m/s • Checklist Total = 10 Likely • Events in CWA = 13

  25. 18z 850mb RH (img), Winds, Theta E, Moisture Transport

  26. 18z 500mb Winds (isotachs), Vorticity (img)

  27. 18z 250mb Winds (isotachs), Divergence

  28. 21z 250mb Winds (isotachs), Divergence

  29. 18z SBCAPE, LI, Sfc Winds

  30. 19z Visual Sat Image, LI, Sfc Obs

  31. Downburst just North of the GWX radar • Microburst 8 mi N of GWX • 50 kts at 470 ft AGL • Amazingly, no official severe weather reports!

  32. Severe Weather Reports, Hail, Wind

  33. Loop of the Visual Sat Image and 5 min Lightning

  34. Summary • Try and quantify instability and sounding parameters • A checklist that distributes points to certain parameters works well. • Instability does not work alone, synoptic and model analysis need to be done to identify favorable regions • The collocation of favorable synoptic patterns and strong instability point you in the right direction

  35. Parameter Summary • Values > 3500 J/kg are favorable for SBCAPE • Values between 1.6 and 1.95 are good for PW • Values < -8 are favorable for Lifted Index • Values > 27 C and especially >28.4 C are favorable for 850-500mb temp difference • Values > 900 J/kg and especially > 1200 J/kg or favorable for DCAPE when SBCAPE is > 3000 J/kg (give extra consideration when the parcel descends from a level <600mb) • Values > 8 m/s for 0-6 shear are favorable when SBCAPE is > 3000 J/kg

  36. Future Work • Continue to experiment with new parameters, emphasis on atmospheric moisture, mainly mid and upper levels • Investigate radar data to try and improve on traditional radar techniques that are precursors to damaging wind and severe weather during the warm season • Incorporate lightning data and trends with radar (using SCAN, storms with CG rates >17 strikes/min should strongly be considered for warnings)

  37. Another Microburst! • Intense microburst near DGX, Brandon radar, on Aug 6th 2003 • Several pixels of >62 kts, max output of 68 kts! • Numerous trees and powerlines were blown down. A billboard was also blown down along I-20. Twenty homes sustained minor damage

  38. The End

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