1 / 38

Severe Thunderstorm Forecasting An Overview

Severe Thunderstorm Forecasting An Overview. Jeffry S. Evans Storm Prediction Center. Where America’s Weather and Climate Services Begin. Forecasting severe thunderstorms Two parts:. Anticipating favorable environments Recognition of severe storms once they develop.

odetta
Download Presentation

Severe Thunderstorm Forecasting An Overview

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Severe Thunderstorm ForecastingAn Overview Jeffry S. Evans Storm Prediction Center Where America’s Weather and Climate Services Begin

  2. Forecasting severe thunderstormsTwo parts: • Anticipating favorable environments • Recognition of severe storms once they develop

  3. Physical/Theoretical Understanding of Convective Processes Ingredients for deep moist convection (Doswell, 1987): 1) Moisture 2) Instability 3) Lift

  4. Physical/Theoretical Understanding of Convective Processes • Moisture • Typically we think about low level moisture (dew point or mixing ratio) residing in boundary layer • But sources of moisture above the PBL can be associated with elevated convection, and can “mix-down”/moisten PBL with diurnal heating • Moisture increases via advection, mixing or evapotranspiration • Instability • Temperature, lapse rates, boundary layer moisture are major contributors (juxtaposition of steeper lapse rates above moist layer) • Lift • A physical mechanism that allows lifted parcel to reach LFC and become positively buoyant (upward vertical motion often on the mesoscale) • The hardest to quantify!

  5. LFC LCL Essential issue is whether or not LFC can be reached. • Moisture, instability and lift all necessary, but each affect convective potential differently. • This determines “sufficiency” of moisture, instability, lift for any given day. • Not a numbers game!

  6. I. Physical/Theoretical Understanding of Convective Processes • Instability • Observational data – use basic temp/moisture data to compute instability thus incorporates strengths and weaknesses of observing network • Magnitude of potential instability is related to updraft strength – more is better! (Warm / moist in low levels and cool aloft) • Lapse rate • Model data – heavily utilized in forecast process, but physics in models have large impact on accuracy of instability forecasts • CAPE as an integrated quantity is preferred over LI • But not all CAPE are created equal (“tall/skinny” vs “short/fat”) • Choice of lifted parcel can make big difference on magnitude

  7. All CAPE Not Created Equal Same CAPE for both soundings

  8. All CAPE Not Created Equal Same CAPE for both soundings CAPE alone may not give complete picture!

  9. CAPE definitions(http://www.spc.noaa.gov/misc/acronyms.html) • MUCAPE: Most unstable CAPE • Lifts most unstable parcel in lowest 300 mb. • Useful in non-surface based (elevated) CAPE. • SBCAPE: Surface-based CAPE • Lift surface parcel. • Usually identical to MUCAPE in the afternoon. • MLCAPE: Mean-mixed CAPE • Parcel lifted using mean temp/moisture (lowest 100mb). • Most representative for diurnal development when boundary layer is well mixed.

  10. Importance of parcel choice

  11. Importance of parcel choice (cont.) Choice of parcel depends on time of day or expectation of lifted parcel. -MUCAPE overnight/early morning. -MLCAPE afternoon/early evening.

  12. Physical/Theoretical Understanding of Convective Processes • Lift • Observational data – unable to directly “observe” lift as mathematical quantity (but clouds reflect sufficient combo of moisture/lift) • Therefore, the assessment of vertical motion on the synoptic and mesoscale has long been a focused activity for forecasters • Synoptic scale • Continuity equation – upper divergence / lower convergence • Jet streaks, short wave troughs/PVA, thermal advection, CFA • Mesoscale • Convergence along fronts and other low level boundaries • Differential heating, terrain features, convective outflow, etc. • Sfc pressure fall/rise couplets Big Question – Where, when, and how much lift is needed to release instability?

  13. Can we overcome?

  14. Pattern RecognitionWhy is it important? • Enables forecaster to focus on areas where favorable “ingredients” come together. • Increasing low level moisture, drying at mid levels = Increasing cond. instability. • Favorable lift (plus likelihood of initiating mechanism. Dry line, trough, upslope, etc…)

  15. Manual Chart Analysis • Subjective analysis facilitates our ability to synthesize data from a variety of observational sources • We are actively engaged with the real atmosphere • We gain a better perspective of actual environmental conditions • Our human brain is better than computer algorithms at combining different data sets and forming logical patterns

  16. Some important parameters • CAPE (already discussed to some point) • Lapse Rates • Mid level reflection of ptnl updraft strength/EML • Low levels point to cap breaking/strong downdraft ptnl • Bulk Shear • Deep layer (sfc-6 km) impacts organization • Low level (sfc-1 km) useful for tornado fcsts • Effective shear • Helicity • Perhaps most crucial, but most difficult to fcst • Supercells and tornado fcsts • Effective helicity • Composite indicies • Most robust; illustrate relationships of different fields

  17. “Keys to the Game” • Parameters should always relate back to the ingredients-based approach (not a magic number game!) • Need to understand their strengths / limitations • Atmosphere is not static - otherwise persistence is all we need

  18. Parameters (cont.) • Parameters (Ingredients) used to not only assess convective potential, but can aid in severe type. • Instability, shear and combination of two.

  19. Deep layer shear • Strong support in science and observations for deep layer shear dictating storm scale organization. Some base values: • > 20 kt (sfc-6 km) favor multicellular/linear • > 40 kt (sfc-6 km) favor supercells • Less in high CAPE environments or large low level hodographs • 6 km used as an average mid point in mid-latitude deep moist convection. Normally okay. • Effective Shear developed to be more robust:

  20. Effective Inflow/Shear • What if 0-6 km or 0-3 km doesn’t represent ‘deep layer’ shear or the inflow layer? • What if storm is elevated, very shallow, extremely tall? • Shouldn’t SRH/shear calculations be tied to other quantities such as LFC or LCL height?

  21. Effective Layer parcel constraints:100 j/kg MUCAPE and -250 j/kg CINH • Lift parcels starting at sfc. 1st parcel meet constraints will set bottom of inflow layer. • 2. Keep lifting parcels. Once fail to meet constraints, then set top.

  22. EL~200 mb 50% storm depth Mu parcel ~825 mb Effective Shear Illustration • Instead of using sfc-6 km, use halfway between bottom of ‘inflow layer’ and EL.

  23. Effective shear ~ 40 kt Effective Shear versus 0-6 km Shear

  24. Effective SRH ~145 m2 s-2 Effective SRH versus 0-3 km SRH

  25. Why should height of cloud base matter? • Markowski in late 90s postulated that if air within RFD remains buoyant (some CAPE), tornado threat/intensity increases. • Contradicts earlier belief that baroclinicity (cool, dry RFD interacting with warm, moist inflow) under mesocyclone “enhances” tornado potential. • Appears if environment favors low LCLs, better chance of ingesting high theta-e air into low level mesocyclone.

  26. SPC Composite Parameters • Based on ‘Proxy’ sounding studies (both observed and RUC2), and the following principles: • 0-6 km shear/sfc-3 km SRH are good discriminators between supercells and non-supercells (effective shear/SRH now used) • MLCAPE shows only some discrimination ability between classes of supercells and discrete non-supercells • 0-1 km SRH and MLLCL showed best discrimination between supercells producing significant tornadoes other event classes • Combined fields show greatest skill.

  27. Supercell Composite Parameter (SCP) SCP = ( MUCAPE / 1000 J kg-1 ) X ( BRN shear / 40 m2 s-2 ) X ( 0-3 km SRH / 150 m2 s-2 ) SCP = 1 when MUCAPE = 1000 J kg-1, BRN shear = 40 m2 s-2, and SRH = 150 m2 s-2 Designed to favor right moving supercells

  28. Significant Tornado Parameter (STP) STP = ( MLCAPE / 1000 J kg-1 ) X ( 0-6 km shear / 20 m s-1 ) X ( 0-1 km SRH / 100 m2 s-2 ) X ( (2000 – MLLCL) / 1500 m) STP = 1 when MLCAPE = 1000 J kg-1, 0-6 km shear = 20 m s-1, 0-1 km SRH = 100 m2 s-2, and MLLCL = 500 m Also highlights right movers

  29. Other SPC composite parameters • EHI (Energy Helicity Index) • Derecho Composite Parameter • Sig. Hail Composite Parameter • Also utilize proximity sounding database to develop. • All composite parameters become more favorable for the event with values >1.

  30. “Not so fast my friend!” You need to know the Convective mode! Tornado Forecasting Summary • So, if deep layer shear and CAPE sufficient for supercells (40+ kt and 1500+ J/kg). And, • low LCLs and strong 0-1 km shear support threat of significant tornadoes, then, • expect main threat of F2 or greater tornadoes?

  31. Discrete supercell, F4 tornado proxy sounding BMX soundings Discrete supercell, F4 tornado proxy sounding

  32. Application of SCP and STP is predicated on an accurate forecast of convective mode! 18 UTC BMX 16 Dec 2000 18 UTC BMX 16 Feb 2001 Tornadic supercell (F4 at TCL) Bow Echo/ Derecho

  33. Note similarities

  34. What affects mode? • Initiation and subsequent movement relative to boundary. • ‘Boundary’ may become a rapidly surging, common outflow boundary and drive evolution into MCS or QLCS

  35. Parameter Cheat sheet • Supercell environments are characterized by: • 0-6 km shear > 30-40 kt • BRN shear > 25-40 m2 s-2 • MLCAPE > 500-1000 J kg-1 • F2-F5 supercell tornado environments are characterized by: • 0-1 km SRH > 100 m2 s-2 • MLLCL heights < 1000 m • Combinations of CAPE/shear/moisture (SCP and STP) provide best statistical measures of supercell and tornado potential, given discrete storms.

  36. Simplistic tornado forecast • No magic parameter or value! • 1st question: Thunderstorms? • CAPE, CIN, lifting • Supercells? • 0-6 km shear > 40 kt (can be >30 kt in extreme CAPE), 0-3 km SRH, EHI, SCP • Tornado? • 0-1 km shear/SRH > 100 m2 s-2 • Significant Tornadoes? • Low LCLs • STP • Pressure falls/backed low level flow MUST KNOW MODE!

  37. The End

  38. Low level (sfc-3km) lapse rates Deepest boundary layer mixing

More Related