2. Soundings and Adiabatic Diagrams for Severe Weather Prediction and Analysis Continued

3. Shear vs. CAPE Need a balance between Shear and CAPE for supercell development Without shear: single, ordinary, airmass thunderstorm which lasts 20 minutes If shear is too strong: multicellular t-storms (gust front moves too fast)

4. Shear Classification: Two Main types: Directional and Speed Bulk Shear: The Boundary Layer through 6 km (or higher) above ground level shear vector denotes the change in wind throughout this height. Usually given in units of knots Thunderstorms tend to become more organized and persistent as vertical shear increases. Supercells are commonly associated with vertical shear values of 35-40 knots and greater through this depth Doesn’t take into account elevated parcels: Effective Shear (kts) Bulk Richardson Number shear (m^2/s^2)

5. CAPE and Shear

6. Shear Just Right 2-D equilibrium: squall line develops 3-D equilibrium: right moving and left moving supercells AB A B Right Mover Left Mover

7. Bulk Richardson Number BRN = CAPE ½ (U z 2 ) Where U z = the vertical wind shear (averaged over 3-6km layer) In general: 15-40 favors supercell development >40 favors multicellular type storms Explains the balance between wind shear and convective energy

8. Bulk Richardson Number (BRN) BRN= CAPE 1 / 2 U z 2 (where U z is a measure of the vertical wind shear)

9. Hodographs Wind speed Draw wind vectors in direction they are going This is opposite of how the wind barbs are drawn V U

10. Example

11. Straight Line Shear Storm Splitting: –R and L storm cells move with mean wind but drift outward 1000 900 850 700 500

12. Curved Hodograph Emphasizes one of the supercells –Veering (clockwise curve): right moving supercells warm air advection in northern hemisphere –Backing (counter clockwise curve): left moving supercells warm air advection in southern hemisphere 1000 850 900 700 500 300

13. Straight Line Hodograph Curved hodograph

14. Helicity Can be thought of as a measure of the “corkscrew” nature of the winds. Higher helicity values relate to a curved hodograph. –large positive values--> emphasize right cell –large negative values--> emphasize left cells Values near zero relate to a straight line hodograph. H = velocity dotted with vorticity = V ζ = u (d y w - d z v) - v (d x w - d z u) + w (d x v - d y u)

15. CAPE and Helicity Plainfield, IL tornado: CAPE=7000 Helicity=165 Energy Helicity:

16. Tornado Outbreak Missouri, Illinois, Indiana 3/08/09

17. Synoptic Setup: 300mb

18. 500mb

19. 700mb

20. 850mb

21. 925mb

22. Surface

23. SPC really jumps on the bandwagon…Yee Haw!!!

25. Springfield, Missouri

26. Lincoln, Illinois

27. 1600z SPC Mesocale Discussion

28. Radar Loops http://vortex.plymouth.edu/nids.html

33. What to take away from this event…? Dynamics/Shear dominate heat energy! Strong mid/upper level shortwave trough Strong dynamical forcing/frontal forcing Strengthening surface low Cold temperatures: <70F Very small CAPE values: <1000J/kg Bulk Shear: 80+ kts !!! Helicity: 500+ m^2/s^2 !!! Clockwise Hodographs favoring right moving cells Perfect situation for low topped discrete supercells capable of producing tornadoes near triple point.

34. Stability Indices

35. K Index This index uses the values for temperature (t) and dew point temperature (t d ), both in o C at several standard levels. K = t 850 - t 500 + t d850 - t 700 + t d700 K valueT-Storm Probability <150% 15-20<20% 21-2520-40% 26-3040-60% 31-3560-80% 36-4080-90% >40>90%

36. Vertical Totals VT = T 850 - T 500 A value of 26 or greater is usually indicative of thunderstorm potential.

37. Cross Totals CT =T d850 - T 500 CTT-Storm Potential 18-19Isolated to few moderate 20-21scattered moderate, a few heavy 22-23scattered moderate, a few heavy and isolated severe 24-25scattered heavy, a few severe; isolated tornados 26-29scattered to numerous heavy, few to scattered severe, a few tornados >29numerous heavy, scattered showers, scattered tornadoes

38. Total Totals (TT) TT = VT + CT =T 850 + T d850 - 2 T 500 TTT-Storm Potential 44-45Isolated to few moderate 46-47scattered moderate, a few heavy 48-49scattered moderate, a few heavy and isolated severe 50-51scattered heavy, a few severe; isolated tornados 52-55scattered to numerous heavy, few to scattered severe, a few tornados >55numerous heavy, scattered showers, scattered tornadoes

39. SWEAT (severe weather threat) Index SWI = 12D + 20(T - 49) + 2f 8 + f 5 + 125(S + 0.2) where: D=850mb dew point temperature ( o C) (if D<0 then set D = 0) T = total totals (if T < 49 then set entire term = 0) f 8 =speed of 850mb winds (knots) f 5 = speed of 500mb winds (knots) S = sin (500mb-850mb wind direction) And set the term 125(S+0.2) = 0 when any of the following are not true 1.850mb wind direction is between 130-250 2.500mb wind direction is between 210-310 3.500mb wind direction minus 850mb wind direction is positive 4.850mb and 500mb wind speeds > 15knots

40. SWEAT (severe weather threat) Index SWI = 12D + 20(T - 49) + 2f 8 + f 5 + 125(S + 0.2) <300Non-severe thunderstorms 300-400Severe thunderstorms possible >400Severe thunderstorms, including possible tornados

41. Lifted Index (LI) Compares the parcel with the environment at 500mb. LI = (T env -T parcel ) 500 Lifted IndexThunderstorm Potential >+2No convective activity 0 to +2Showers probable, isolated thunderstorms possible -2 to 0Thunderstorms probable -4 to –2Severe thunderstorms possible < -4Severe thunderstorms probable, tornados possible

42. Best Lifted Index –Uses the highest value of  e or  w  in the lower troposphere. –Use the highest mixing ratio value in combination with the warmest temperature. SELS Lifted Index –Use the mean mixing ratio and mean  of the lowest 100mb –If using a 12z sounding add 2 o –Start parcel at 50mb above the surface

43. Showalter Index (SI) Compares a parcel starting at 850mb with the environment at 500mb. SI = (T env -T parcel ) 500 SIThunderstorm Possibility > +3No convective activity 1 to 3Showers probable, isolated thunderstorms possible -2 to 1Thunderstorms probable -6 to –2Severe thunderstorms possible < -6Severe thunderstorms probable, tornados possible

44. Supercell Index Weights various parameters which are indicative of possible supercell development

45. Important Points to Remember Severe weather is more dependent on dynamical forcing than instability! No one parameter tells the full tale! 12z soundings usually predict afternoon convection better than 00z soundings predict evening convection.

46. Links http://www.geocities.com/weatherguyry/swx2.html http://avc.comm.nsdlib.org/cgi-bin/wiki.pl?Severe_Weather_Indices http://www.theweatherprediction.com/severe/indices/ http://www.theweatherprediction.com/habyhints/315/ http://www.spc.noaa.gov/exper/mesoanalysis/ http://mocha.meteor.wisc.edu/table.12z.html