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. 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.

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

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

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

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

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

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.

Key Parameters Surface Based Cape (SBCAPE) Surface Based Cape (SBCAPE) Lifted Index (LI) Lifted Index (LI) Precipitable Water (PW) Precipitable Water (PW) mb temperature difference or Vertical Totals mb temperature difference or Vertical Totals Downdraft Cape (DCAPE or DAPE) Downdraft Cape (DCAPE or DAPE) km Shear km Shear

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

Lifted Index (LI)

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

mb Temp Difference

DCAPE Favorable values > 900 J/kg when the SBCAPE was > 3000 J/kg 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. 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 % 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 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 Any parcel descending from a level above 600mb should be given extra attention

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

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

Microburst Checklist and Potential Scale

Distribution of the Daily Potential Number of Days that Occurred in each Potential (326 total)

Distributions and Events

Avg # of Events for each Potential

Synoptic Analysis Important Fields to Consider 850 mb Theta E ridges, moisture transport and lapse rates (vertical totals) 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 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 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 Combine sfc instability (CAPE, LI) and moisture convergence with the favorable regions described above

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

12z, July 13 JAN Modified Sounding

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

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

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

18z 250mb Winds (isotachs), Divergence

21z 250mb Winds (isotachs), Divergence

18z SBCAPE, LI, Sfc Winds

19z Visual Sat Image, LI, Sfc Obs

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

Severe Weather Reports, Hail, Wind

Loop of the Visual Sat Image and 5 min Lightning

Summary Try and quantify instability and sounding parameters Try and quantify instability and sounding parameters A checklist that distributes points to certain parameters works well. 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 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 The collocation of favorable synoptic patterns and strong instability point you in the right direction

Parameter Summary Values > 3500 J/kg are favorable for SBCAPE Values > 3500 J/kg are favorable for SBCAPE Values between 1.6 and 1.95 are good for PW Values between 1.6 and 1.95 are good for PW Values < -8 are favorable for Lifted Index Values < -8 are favorable for Lifted Index Values > 27 C and especially >28.4 C are favorable for mb temp difference Values > 27 C and especially >28.4 C are favorable for mb 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 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 Values > 8 m/s for 0-6 shear are favorable when SBCAPE is > 3000 J/kg

Future Work Continue to experiment with new parameters, emphasis on atmospheric moisture, mainly mid and upper levels 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 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) Incorporate lightning data and trends with radar (using SCAN, storms with CG rates >17 strikes/min should strongly be considered for warnings)

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

The End