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How to Interpret Advanced Severe Weather Data Sets EAX Severe Weather Seminar March 1, 2006

How to Interpret Advanced Severe Weather Data Sets EAX Severe Weather Seminar March 1, 2006. Objectives. To provide an overview of resources for anticipating severe weather severe thunderstorm forecasting parameters. SPC Outlook Products.

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How to Interpret Advanced Severe Weather Data Sets EAX Severe Weather Seminar March 1, 2006

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  1. How to Interpret Advanced Severe Weather Data SetsEAX Severe Weather SeminarMarch 1, 2006

  2. Objectives • To provide an overview of • resources for anticipating severe weather • severe thunderstorm forecasting parameters

  3. SPC Outlook Products • Convective Outlooks (www.spc.noaa.gov/products/outlook) • Categorical • Probabilistic • Mesoscale Discussions • Convective Outlooks and Mesoscale Discussions can also be found in the “Watch and Warning” link on the EAX homepage

  4. Categorical Convective Outlook

  5. Probabilistic Convective Outlooks25 mile radius

  6. Probabilities You May See In The Probabilistic Outlooks: • Day 1 Tornadoes • 2%, 5%, 10%, 15%, 30%, 45%, 60% • Large Hail • 5%, 15%, 30%, 45%, 60% • Damaging Wind • 5%, 15%, 30%, 45%, 60% • Day 2 Any severe weather • 5%, 15%, 30%, 45%, 60% • Day 3 Any severe weather • 5%, 15%, 30%, 45%

  7. Anticipating Severe Weather Mesoscale discussions • When conditions appear favorable for the development of severe storms • Ideally issued 1 to 3 hours before a watch is issued.

  8. CAPE LOW LEVEL SBCAPE MLCAPE MUCAPE DCAPE CIN LAPSE RATE 850 MB –700 MB 700 MB - 500 MB SHEAR EFFECTIVE SHEAR BRN SHEAR 0-6 km 0-3 km 0-1 km SRH 0-3 KM 0-1 KM 0-3 KM EFFECTIVE Severe Weather Parameters Instability Low Level Shear

  9. Severe Weather Parameters, cont. • STORM RELATIVE WINDS • 0-2 KM • 4-6 KM • 9-11 KM • ANVIL LEVEL • LCL • LFC • EHI • 3 KM • 1 KM • VGP (3 KM) • SUPERCELL COMPOSITE PARAMETER • SIGNIFICANT TORNADO PARAMETER • CRAVEN SIGNIFICANT SEVERE COMPOSITE INDICES

  10. Lifted Condensation Level (LCL) and Level of Free Convection (LFC) • LCL is the level at which the cloud base forms. • (the level at which unsaturated parcel lifted from surface becomes saturated) • Tornadoes often occur with low LCL’s - less than 1500 m. • Level of free convection is level at which air is warmer than its environment and rises on its own. • Thunderstorms “blossom” rapidly when rising air reaches the LFC.

  11. CAPECONVECTIVE AVAILABLE POTENTIAL ENERGY • Is a measure of instability through the depth of the atmosphere • Directly related to updraft strength in thunderstorms • Values • > 1000 J/kg – “weak instability” • 1000-2500 J/kg-”moderate instability” • 2500-4000 J/kg-”strong instability” • > 4000 J/kg- “extreme instability” Higher number  stronger storm

  12. CAPE, cont. • Variations • Low level Cape - Depicts buoyancy for SR-inflow (0-3km) and RFD. Large Cape favors strong low-level stretching. • SBCAPE- Calculates buoyancy from LFC on up. Parcel originates at the surface. • MUCAPE- Calculates buoyancy of most unstable parcel in the lowest 3 km. • MLCAPE- Calculates buoyancy utilizing average T/Td in lowest layers in atmosphere (< 1 km). • DCAPE- Estimates potential strength of the downdraft

  13. CIN • CIN is a quantitative measure of the amount of work necessary to lift a parcel above LFC Larger CIN (> 50)  less threat for thunderstorms.

  14. Lapse Rates • The rate of temperature change with height (C/km) • 850-500 mb (4500-18000 feet) • 700-500 mb (10000-18000 feet) • Values • < 5.5 – 6 C/km – stable conditions • ~ 9.5 C/km – absolutely unstable

  15. Hodographs of Thunderstorm Types Typical wind hodographs for (a) single cell, (b) multicell, and (c) supercell thunderstorms. Adapted from Chisholm and Renick, 1972.

  16. Deep Layer Wind Shear Parameters • BRN shear – uses a difference between the low level wind and a density-weighted mean wind through the mid levels. • Values of 35-40 m^2/s^2 or greater have been associated with supercells • Effective Bulk Shear – Similar to 0-6km bulk shear. Most unstable layer lifted parcel upward to 40-60 % of EL. • 6 km shear - the “boundary layer” to 6 km above ground level • Supercells commonly associated with values of 35-40kt and greater through this depth • Supercells more probable as value increases through the 25-40kt range and greater.

  17. BL-6 KM shear (5/4/03)

  18. Low Level Shear • 0-1 KM Shear vector - 15-20 kts or greater favors supercells.

  19. Bulk Richardson Number • A measure of buoyancy to shear ratio. • BRN 10-50: range for supercell storms. • BRN >50: multi-cell lines or clusters.

  20. Storm Relative Helicity • Used to assess potential for storm rotation. Higher number  greater potential for tornadoes.

  21. Sfc-1 km SR Helicity • Sfc-1 km SR helicity 100 m2/s2 indicates and increased threat of tornadoes with supercells.

  22. 0-3 km SR Helicity • 0-3km SR Helicity of 250 m2/s2 favors and increased threat of tornadoes with supercells.

  23. Energy Helicity Index (EHI) • Combines CAPE and shear. (SFC up to 1km and 3km) • Maximized when CAPE and shear is large. • Values larger than 1-2 have been associated with significant tornadoes.

  24. EHI (5/4/03)

  25. More advanced information www.spc.noaa.gov/exper/mesoanalysis • One of my favorite sites for analysis of severe weather data. • Free of charge. • Comprehensive look at severe weather potential.

  26. supercell from SPC mesoanalysis page EHI analysis Environment suggested strong potential for storm rotation over C and NE Kansas

  27. supercell from SPC mesoanalysis page LFC heights were near 3000 m along the track of the supercell, suggesting minimal threat for a tornado.

  28. Other Mesoscale Analysis Parameters available on the SPC page • VGP (3 KM) • EFFECTIVE STORM RELATIVE HELICITY (ESRH) • SUPERCELL COMPOSITE • SIGNIFICANT TORNADO • CRAVEN SIGNIFICANT SEVERE • HAIL PARAMETERS

  29. VGP • Vorticity Generation Parameter • Estimates the rate of tilting and stretching of horizontal vorticity by a thunderstorm updraft. • Values greater than 0.2 m/s2 indicate an increasing risk of tornadic storms.

  30. EFFECTIVE STORM RELATIVEY HELICITY (ESRH) • ESRH • Layer restricted to Sfc-3KM • Searches for the first parcel (Effective height) (starting at the sfc) which has 100 J/Kg and greater than -250 J/KG of CIN. • Restricts the available streamwise vorticity to the layer from the “Effective Height” to 3km. • ESRH graphics plots the SRH and Effective Height.

  31. 0-3 KM SRH versus 0-3km Effective SRH (5/4/03)

  32. Significant Tornado • Another multi-parameter index- • Includes sfc based effective bulk shear magnitude • 0-3km effective storm-relative helicity • The 100 mb mean parcel CAPE normalization value has increased to 1500 J/KG. • MLLCL normalization value of 1500 meters remains the same. • Values greater than 1 associated with significant tornadoes. • Most non-tornadic supercells associated with values less than 1.

  33. Craven Significant Severe • The simple product of 100mb MLCAPE and 0-6km magnitude of the vector difference (“deep layer shear”) • Uses a database of about 60,000 soundings • Majority of severe weather events occur when the product exceeds 20,000 m3/s3.

  34. Supercell Composite • Multi-parameter index that includes 0-3km effective SRH, CAPE, and effective bulk shear. • Each parameter is normalized to supercell “threshold” values. • Values greater than 1 suggest increase potential for supercells and tornadoes. SCP = (muCAPE/1000 J/kg) * (Effective SRH3/50 m**2/s**2) * (effective shear /20 m**2/s**2)

  35. Hail Parameters • This image depicts three forecasting parameters used to predict hail • CAPE in the layer from -20C to -40 C • 0-6km shear vector in excess of 30-40 knots support persistent updrafts. • Freezing level height

  36. Bow echoes and Derechoes • Favored Environment • Sfc-2.5 km shear – 15-20 m/s shear or greater • CAPE – 2000 J/kg or greater • Little or no extended shear above 2.5 km

  37. Other Tools From the SPC… • Composite Map • http://www.spc.noaa.gov/compmap/ • Other Forecast Maps • http://www.spc.noaa.gov/exper/sref

  38. Questions?

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