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Forecasting Convective Mode and Severity

Forecasting Convective Mode and Severity. Mark F. Britt National Weather Service St. Louis, MO. Why Am I Here?. A Basic Review of Severe Thunderstorm Forecasting. Examine moisture return,instability, and shear calculations.

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Forecasting Convective Mode and Severity

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  1. Forecasting Convective Mode and Severity Mark F. BrittNational Weather ServiceSt. Louis, MO

  2. Why Am I Here? A Basic Review of Severe Thunderstorm Forecasting. • Examine moisture return,instability, and shear calculations. • Examine how the amount and distribution of instability, vertical shear, and forcing interact to determine cell type, convective mode (linear or discrete), and coverage. • Determine what type(s) of severe weather to expect for a given environment.

  3. Using Numbers • There are NO “magic” numbers or thresholds. They are merely guidelines. • Best to look where several key parameters overlap instead of depending on one index. • You should look at skew-Ts and hodographs (observed and forecast) to better understand what the numbers mean. • Increase your situation awareness by using near storm environment data, but do not use it solely to make warning decisions.

  4. Objective Analysis • Available on AWIPS using MSAS, LAPS, or RUC40 analysis(Thompson/Edwards (2002)found RUC analysis is a reasonable proxy to observed soundings in supercell environments.) • Or, the SPC Mesoanalysis Page: http://www.spc.noaa.gov/exper/mesoanalysis/ • Displays three movable regions that is usually available by 20 minutes past each hour • Displays a robust set of hourly objective analysis datasets using the latest surface observations and upper air analysis from the RUC. Depicted contours highlight important “thresholds”. • Several new parameters available this year.

  5. Ingredients for Deep, Moist Convection • Moisture: (Gulf of Mexico, evapotranspiration) • Instability: (Steep lapse rates either from the Elevated Mixed Layer off the Rockies, or large scale “dry” ascent ahead of a trough.) • Forcing: (Surface frontal boundary, convective outflow, 900-800mb moisture convergence at nose of nocturnal low level jet, orographic lift over the eastern Ozarks)

  6. Moisture Return Lanicci and Warner (1991) • Look for rapid moisture advection from the Gulf of Mexico in strong pressure gradients ahead of a strong storm system. • Ridging associated with surface highs in or near the Gulf can inhibit moisture return.

  7. Assessing Instability Which is best? • SBCAPE • MLCAPE • MUCAPE

  8. From Peter Banacos, SPC (2003) SBCAPE: Surface Based. Uses the surface temperature and dew point. Will show large diurnal swings. Can give significant overestimates (an order of magnitude) in cases of shallow moisture and underestimates in cases of elevated convection.

  9. From Peter Banacos, SPC (2003) MLCAPE: Mean Layer. Uses the mean temperature and mean mixing ratio in the lowest part of the atmosphere (SPC uses lowest 100 mb). Less variable in time and space, and more conservative than MUCAPE when lower atmosphere is not well mixed.

  10. From Peter Banacos, SPC (2003) MUCAPE: Most Unstable Parcel. Uses most unstable parcel in lower atmosphere (SPC uses lowest 300mb). Helps with nocturnal or other types of elevated convection.

  11. CAPE vs. Parcel Selection April 22nd, 2004 Mean Layer CAPE Surface Based CAPE From Jon Davies Webpage

  12. Surface Based Parcels Violent tornado outbreak over western Missouri. May 4th, 2003

  13. Elevated Based Parcels Numerous Reports of Hailin Eastern NE/ Western IA May 4th, 2003

  14. How Tall is the CAPE? April 22nd, 2004 From Jon Davies Webpage (http://members.cox.net/jdavies1/)

  15. How Tall is the CAPE? April 22nd, 2004 From Jon Davies Webpage

  16. How Wide Is the CAPE? Larger differences between parcel temperature and the environmental temperature means stronger updrafts that are less susceptible to entrainment.

  17. Lapse Rates • Craven (2000) found in a study of 65 major tornado outbreaks that 6.7o C/km is a useful lower limit. He also found low shear environments that produce tornadoes have steeper lapse rates. • Steep mid level lapse rates (850-500 mb) have more conditional instability and increased CAPE. • Steep low level lapse rates (0-3km AGL) can give a better idea on how quickly convection will develop.

  18. Mid Level Lapse Rates

  19. Assess Vertical Shear • Distribution of vertical shear will determine dominant thunderstorm type. • Can be determined using either: • Traditional fixed layers (0-6km bulk shear, 0-1km SRH) • “Effective” shear which accounts forsounding dependentinflow layer through CAPE and CIN constraints. (Large sample testing suggests that “effective layer” is best defined by >100 J kg-1 CAPE and <250 J kg-1 CIN. (Thompson et al, 2004a & b)) • Low level curvature can determine if right-movers, left-movers, or both kinds of splits are favored.

  20. Storm Type: Ordinary Cells • Dominant Type in Weak Shear Environments • Pulse Type Severe Storms.

  21. Storm Type: Multicells Moderate to strong shear is confined mainly to the lower levels (0 to 3 km AGL)

  22. Organized Multicells • >40kt 0-6 km shear • >30kt 700-500mb wind • Dry (low theta-e) midlevel air (strong cold) • Downshear SBCAPE max • System relative convergence acting downshear to enhance forward propagation

  23. Storm Type: Supercells • A storm that possesses a persistent mesocyclone that can be sustained on the order of tens of minutes. • 90% of this type associated with some kind of severe weather (Burgess and Lemon, 1991)

  24. 0-6 km Shear Magnitude • General deep layer shear “thresholds”: • 40+ kt suggests: if storms develop -- supercells are likely (provided convective mode favors cellular activity) • 30-40 kt: supercells also possible if environment is very or extremely unstable as storm can augment local shear (>5,000 J/kg (Burgess (2003)) • About 15-20 kt:shear needed for organized convection (multicell or supercell) with mid level winds at least 25 kt • While 0-6km shear is a good discriminator between cell types, it isn’t a good tornado forecast tool (Thompson et al, 2002).

  25. 0-6 km Shear Magnitude Supercells Non-Supercells From Thompson et al (2002)

  26. BRN Shear (Weisman and Klemp (1982), and Thompson (2000 &2002) BRN Shear is the vector difference between the density weighted mean winds in the lowest 6 km and the lowest 500 m above ground level. BRN shear can be a used as a good predictor of storm type and severity. Supercells Non-Supercells 40 m2/s2 35 m2/s2 From Thompson et al (2002)

  27. Supercell Composite Parameter (Thompson et al, 2003) • The Supercell Composite Parameter (SCP) is a multi-parameter index that includes 0-3 km SRH, CAPE, and BRN Shear. Each parameter is normalized to supercell “threshold” values. • SCP = (muCAPE/1000 J/kg) * (0-3km SRH/100 m**2/s**2) * (BRNShear/40 m**2/s**2) • Computed every hour on the SPC Mesoanalysis Page.

  28. Supercell Composite Parameter (Thompson et al, 2003) May 4th, 2003

  29. What Causes Supercell Type Rasmussen and Straka (1998) found in an observational study of 43 isolated supercells that supercell type is much more dependent precipitation efficiency based on its ingestion of hydrometeors.

  30. Classic Supercells • The real “value” of a CL supercell is that it appears to be the most efficient of the three types to produce significant tornadoes. • Can occur nearly anywhere in U.S. when NSE supports them.

  31. High Precipitation (HP) Supercells

  32. High Precipitation (HP) Supercells • Lower mid-level and anvil-relative flow. • Interactions with other storms – “seeding”, more storms can occur with weak caps. • Typically associated with weaker tornadoes, but can produce significant tornadoes (Plainfield IL). • More of a severe wind (Pakwash), hail, and flash flooding threat. • Are the more-common supercell type east of the Mississippi owing to NSE conditions there (weaker caps, etc.), and may be the most common type everywhere in the U.S.

  33. Pond Bank PA Falcon Co Hurr. Opal Cone of Silence Horizontal Dimension “mini” “mini” “large” Vertical Dimension “low-topped” “high-topped” “low-topped” Supercell Dimensions Burgess (2003)

  34. Supercell Movement Bunkers et al (2000) A physically based, shear-relative, and Galilean invariant method based on 290 supercell hodographs.

  35. Supercell Movement Bunkers and Zeitler (2000) • There are some caveats to this method: • Stronger deep-layer vertical wind shear (0-6 km) leads to a stronger mesocyclone and thus to greater deviation from the mean wind. • Weaker mid-level storm-relative winds allow for a stronger cold pool, and thus a tendency for the supercell to move rapidly downshear. • Depth of thunderstorms need to be considered. • Supercell motion can be altered by wind shear from boundaries and orography.

  36. Storm Coverage and Mode • What’s the Problem? • Evans (2003) noted Strong Forcing Derechoes and discrete, significant tornadic supercells (F2-F5) can occur in similar environments. • Unfortunately, differences can be very subtle and difficult to diagnose operationally.

  37. What Controls Storm Coverage?(Thompson, 2004) • Widespread coverage expected with: • Rich moisture influx and steep lapse rates • Combination of Q-G and mesoscale ascent • (Differential CVA and WAA with surface frontogenesis) • Little CIN (Everything goes up.) • Isolated (or no) storms with: • Marginal moisture and lapse rates (weak CAPE) • Neutral to subsident large-scale environment (Rely on small-scale/shallow processes for initiation) • Large CIN (Confine storms to “strongly forced” or in areas of most persistent ascent)

  38. What Causes Convective Mode? DiscreteSquall Line stronger and/or deeper (or confined near boundary) less greater more parallel weaker more weaker more perpendicular Strength and Depth of Boundary Forcing Amount of CIN compared to Boundary Forcing Potential for Cold Pool Shear Vector w.r.t. Boundary Orientation

  39. Initiating Boundary w.r.t. Deep Layer Flow(Bluestein and Weisman, 2000; Dial and Racy, 2004) • Parallel: (lines dominate, with end supercells) • 45o: (discrete supercells, little storm interaction) • 90o: (colliding storm splits, but depends on storm spacing and hodograph shape)

  40. Progressive Trough May 4th 2003 Tornado Outbreak, Progressive Flow Aloft 0-6 km shear across dryline, and storm motion faster than boundary motion From Rich Thompson, SPC

  41. High Amplitude Trough April 6th 2001 Great Plains “High Risk” Squall Line 0-6 km shear largely parallel to dryline, and storm motion slower than boundary motion From Rich Thompson, SPC

  42. Derechoes or Tornadoes? Anvil SR Winds may show some discrimination (Evans 2003).

  43. Surface Pressure Changes • 1-2 hourly pressure changes help identify: • Mesolow /mesohigh couplets and boundaries • Concentrated fall/rise couplet enhance low- level convergence/shear by backing surface winds (enhancing tornado threat) • Clouds associated with surface pressure falls may be linked to a dynamical feature • Implications on thermal advection • Rise/Fall couplets may indicate severe wind threat in marginal CAPE environments

  44. From DLOC “Hazards Assessment” Convective Severity

  45. Tornado Parameters • Mesocyclonic Tornadoes • Low Level Shear Vector and Storm Relative Helicity • Low Level Thermodynamic Profile • Height of LCL • Height of LFC • Low Level CAPE and CIN • Boundaries • Non-Mesocyclonic Tornadoes

  46. 0-1km Shear Vector Brooks and Craven (2002) Supercells associated with significant tornadoes Non-Tornadic 20 kts 15kts • Markowski et al (2002) states this is a measure of the amount of horizontal vorticity available near the earth’s surface. • The shear magnitude in the lowest 1 km discriminates well between tornadic and non-tornadic supercells, and is a good proxy for 0-1km helicity (Thompson et al, 2002). • Does not require knowledge of storm motion.

  47. 0-1km Storm Relative Helicity Thompson et al (2002) Supercells associated with significant tornadoes Non-Tornadic • SRH can vary up to two orders of magnitude within 100km and 3 hrs. • No good threshold, but 100 m2/s2 is considered a good lower number with increasing threat as the numbers grow. Outbreaks 200-300 m2/s2. (Rasmussen and Blanchard, 1998 and Thompson et al, 2002).

  48. 0-1km Storm Relative Helicity May 4th, 2003 April 22nd, 2004 From Banacos (2003) From Jon Davies Webpage

  49. Height of the LCL (Mean Layer) Markowski (2000) speculates that lower LCL heights mean high boundary layer RH and increased buoyancy in the RFD.

  50. Height of the MLLCL From Thompson et al (2002) From Brooks and Craven (2002)

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