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NCAR Advanced Studies Program Summer Colloquium The Challenges of Convective Forecasting

CONVECTIVE FORECAST CHALLENGES FROM THE PERSPECTIVE OF THE STORM PREDICTION CENTER Steven Weiss steven.j.weiss@noaa.gov. NCAR Advanced Studies Program Summer Colloquium The Challenges of Convective Forecasting July 12, 2006 Boulder, CO. Where Americas Climate and Weather Services Begin.

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NCAR Advanced Studies Program Summer Colloquium The Challenges of Convective Forecasting

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  1. CONVECTIVE FORECAST CHALLENGES FROM THE PERSPECTIVE OF THE STORM PREDICTION CENTERSteven Weisssteven.j.weiss@noaa.gov NCAR Advanced Studies Program Summer Colloquium The Challenges of Convective Forecasting July 12, 2006 Boulder, CO Where Americas Climate and Weather Services Begin

  2. Instructions From Morris • “The colloquium is meant to be as much an open discourse as a class. Thus, it is just as important to present what we don’t know as what we do know.” • How do we distinguish between the known and the unknown?

  3. Morning Quiz - Who Said This?? • “As we know, there are known knowns. There are things we know we know. • We also know there are known unknowns. That is to say we know there are some things we do not know. • But there are also unknown unknowns, the ones we don't know we don’t know.”

  4. Morning Quiz - Who Said This?? • “As we know, there are known knowns. There are things we know we know. • We also know there are known unknowns. That is to say we know there are some things we do not know. • But there are also unknown unknowns, the ones we don't know we don’t know.” Secretary of Defense Donald Rumsfeld discussing events in Iraq?

  5. Morning Quiz - Who Said This?? • “As we know, there are known knowns. There are things we know we know. • We also know there are known unknowns. That is to say we know there are some things we do not know. • But there are also unknown unknowns, the ones we don't know we don’t know.” Morris gesturing to Lance about the large gaps in our convective knowledge?

  6. Outline • Brief Overview of Storm Prediction Center • Severe Thunderstorm Forecasting • Role of the environment in assessment of storm potential • Sampling and resolution issues • Use of models to supplement observational data • NAM and RUC Errors (especially PBL and instability) • Sensitivity of convection to environment details • Observed storms and convective mode • Modeled storms • WRF, cloud models, and high resolution ensembles • Verification of high resolution models • Summary of analysis and prediction limitations • Some focus areas that may result in improved short-term forecasting of severe thunderstorms

  7. STORM PREDICTION CENTER MISSION STATEMENT The Storm Prediction Center (SPC) exists solely to protect life and property of the American people through the issuance of timely, accurate watch and forecast products dealing with hazardous mesoscale weather phenomena. MISSION STATEMENT The Storm Prediction Center (SPC) exists solely to protect life and property of the American people through the issuance of timely, accurate watch and forecast products dealing with tornadoes, wildfires and other hazardous mesoscale weather phenomena.

  8. STORM PREDICTION CENTER HAZARDOUS PHENOMENA • Hail, Wind, Tornadoes • Excessive rainfall • Fire weather • Winter weather

  9. STORM PREDICTION CENTER HAZARDOUS PHENOMENA • Hail, Wind, Tornadoes • Excessive rainfall • Fire weather • Winter weather

  10. SPC Organization35 Total Staff 20 Full-Time Forecasters 3 Shifts per Day 4 Forecasters per Shift to Cover the Lower 48 States

  11. SPC Convective Forecast Suite Time - Weeks Days 4-8 Outlook (1*) Day 3 Outlook (1*) Day 2 Outlook (2*) - Days Day 1 Outlook (5*) Mesoscale Discussions# -Hours Watch# Status Rpt# -Minutes *Number of routine product issuances per day # Event driven product issued as needed

  12. SPC Forecast Products • TOR / SVR TSTM WATCHES (~1000 / yr) • WATCH STATUS REPORTS (~5000 / yr) • CONVECTIVE OUTLOOKS (~3200 / yr) • Day 1; Day 2; Day 3; Days 4-8 • MESOSCALE DISCUSSIONS (~2000 / yr) • Severe Thunderstorm Potential Preceding Watch Issuance • Analysis of Severe Weather in Active Watches • Hazardous Winter Weather • Heavy Rainfall • FIRE WEATHER OUTLOOK (~1200 / yr) • Day 1; Day 2; Days 3-8 • FORECASTS ARE BOTH DETERMINISTIC AND PROBABILISTIC 75% of all SPC products are valid for < 24h period

  13. Severe Weather Outlooks • Two Outlook Types • Categorical • Slight Risk • Moderate Risk • High Risk • Probabilistic • Tornadoes • Hail • Convective Winds

  14. Example of High Risk Outlook DayDay 1 Outlook 7 April 2006 Categorical Risk Tornado Probability Hail Probability Wind Probability

  15. What Happened on 7 April 2006?

  16. Mesoscale Convective Discussions (MD) • Goal is to issue pre-watch MDs 1 to 3 hours prior to a Severe Thunderstorm or Tornado watch issuance. - Define area(s) of concern - State expected watch type - Provide meteorological reasoning – most important • Also issued ~ every 2 hours for each active watch to provide diagnostic/short-term forecast information (when WFOs are busy with warning activities)

  17. Example of Pre-Watch MD ZCZC MKCSWOMCD ALL;334,0996 373,0979 353,0979 314,0996; ACUS3 KMKC 032023 >MKC MCD 032023 TXZ000_OKZ000_032300_ SPC MESOSCALE DISCUSSION #0345 FOR...SW OK/NW TX... CONCERNING...SEVERE THUNDERSTORM POTENTIAL... WATER VAPOR IMAGERY SHOWS A LEAD MID LEVEL SHORT WAVE TROUGH MOVING ENEWD OVER E/NE NM THIS AFTERNOON...AND THIS IS CONFIRMED BY PROFILER TIME SERIES FROM AZC/GDA/TCC/JTN. MID/UPPER 60 DEWPOINTS AND TEMPERATURES NEAR 80 ARE CONTRIBUTING TO SURFACE_BASED CAPE VALUES OF 3500_5000 J/KG OVER WRN OK AND NW TX TO THE E OF THE DRYLINE. CONVERGENCE ON THE DRYLINE IS NOT STRONG AND A CIRRUS SHIELD OVER THE TX PANHANDLE/NW TX/WRN OK SHOULD LIMIT ADDITIONAL SURFACE HEATING...BUT VISIBLE/RADAR IMAGERY HAS SHOWN THE FIRST ATTEMPTS AT TCU OVER FAR NW TX AS OF 20Z WITHIN A BREAK IN THE CIRRUS. MID LEVEL FLOW AND VERTICAL SHEAR WILL INCREASE OVER NW TX AND WRN OK THROUGH LATE AFTERNOON... WITH AN INCREASING THREAT OF SUPERCELLS NEAR THE DRYLINE FROM 00_03Z. THIS AREA IS BEING MONITORED FOR A POSSIBLE TORNADO WATCH LATER THIS AFTERNOON. ..THOMPSON.. 05/03/99 ...PLEASE SEE WWW.SPC.NOAA.GOV/ FOR GRAPHIC PRODUCT... NNNN

  18. Convective Watch Goals • WATCHES ATTEMPT TO CAPTURE: • ALL SIGNIFICANT SEVERE: • 2” OR GREATER HAIL • 65+ kt WIND • F2 OR GREATER TORNADOES • MULTIPLE SEVERE EVENTS FROM ORGANIZED CONVECTION. • SUPERCELLS • SQUALL LINES • MULTICELL COMPLEXES • Isolated and/or marginal severe storms may not occur in watches.

  19. Lead Time Goal • Watches should be issued prior to onset of severe weather • WATCH IN EFFECT: • 1 HOUR PRIOR TO FIRST SEVERE THUNDERSTORM. • 2 HOURS PRIOR TO FIRST TORNADO.

  20. Example of a Tornado Watch

  21. SPC WATCH VERIFICATION(1970-2005) I

  22. CombinedTornado and Severe ThunderstormReports (1970-2004)

  23. SPC WATCH VERIFICATION Significant Tornadoes (F2+)

  24. Severe Thunderstorm Forecasting

  25. Some Differences Between Severe Weather Forecasting and Warning • Detection (warning) of existing severe weather is not the same as prediction (forecasting) of future occurrence or evolution • Warnings have improved because of advances in: • Technology (NEXRAD & Workstation Analysis Tools) • Science (Understanding of storm structure/processes) • Forecaster training and education • Delivery systems to the public • But analogous technological advancement for severe weather prediction has not yet occurred • Considerable uncertainty can exist in both the prediction and detection phases

  26. Modified Forecast Funnel • SPC focuses on relationship between synoptic - mesoscale environment and subsequent thunderstorm development and evolution • Must maintain awareness of mesoscale - synoptic scale interactions • Severe weather events occur on scales smaller than standard observational data (and typical model data) • The real atmosphere is more important than a model atmosphere

  27. Severe Thunderstorm Forecasting • Assessment of convective potential is often limited by insufficient sampling on the mesoscale in time and space (especially 3D water vapor) • Radiosondes • High vertical resolution, poor time and space resolution • Surface METARS • High horizontal and time resolution, no vertical information • Wind profilers and VAD winds • High vertical and time resolution, moderate horizontal res. • No thermodynamic data • Satellite retrievals (winds and thermodynamic) • Mod./high horizontal and time resolution, poor vertical res. • GPS Integrated Water Vapor • High time res., mod./high horizontal res., poor vertical res.

  28. The Link Between Observable Scales and Stormscale is not Necessarily Clear Observable scales Stormscale Courtesy, Sydney Harris

  29. Severe Weather Forecasting • Key premise - We must use our (incomplete) knowledge of the environment and convective processes to determine the spectrum of storms that are possible, where and when they may occur, and how they may evolve over time

  30. Severe Thunderstorm Forecasting • We utilize NWP model output to supplement the limited sampling of real atmosphere (e.g., NAM and RUC output) • Model output forms the foundation for most SPC outlooks, and it also impacts watch decisions • But accounting for uncertainties in IC’s (inadequate sampling) and model physics errors is not easy • Example: Eta forecast soundings exhibit characteristic errors caused by: • Early / late activation of deep convection • Shallow convective scheme in BMJ

  31. BMJ Convective Parameterization in the NAM Model • Both deep and shallow convective processes alter the sounding structure leaving identifiable “footprints” when active • Deep Convection - nudges temperature toward reference profiles where profile is slightly unstable, with high RH through column • Shallow Convection – distributes moisture upward and heat downward through cloud layer • Warming/drying near LCL • Cooling/moistening near cloud top • These processes impact evolution of the model environment (e.g. CAPE/CIN fields forecasters look at)

  32. Impact of Eta Model Deep Convection on Forecasts of CAPE Eta 24 hr forecast valid 12z 8 Nov 2000 3 hr Conv Pcpn CAPE

  33. Impact of Eta Model Deep Convection on Forecasts of CAPE Verifying Data 12z 8 Nov 2000 Radar Reflectivity CAPE

  34. Impact of Eta Model Deep Convection on Forecast Soundings Observed LCH Sounding 12z 8 Nov 24 hr Eta Fcst Valid 12z 8 Nov MUCAPE 929 J/kg Mean RH 75% MUCAPE 2634 J/kg Mean RH 28%

  35. 12 hr Loop of Eta Forecast Sounding Showing Impact of BMJ Shallow Convection

  36. Impact of Eta BMJ Shallow Convection Observed Verifying Sounding(Red/Green)and 12 hr Eta Fcst(Purple)

  37. Short-Term Severe Thunderstorm Environmental Parameter Guidance • Hourly Update Information on 3D Convective Parameters is routinely available • SPC “sfcoa” in N-AWIPS (Mesoscale Analysis Web Page) • LAPS in AWIPS • MSAS/RSAS in AWIPS • All utilize observational data blended with model data for atmosphere above the ground

  38. Diagnosis of Instability • Measures of instability such as CAPE or LI can vary depending on choice of lifted parcel • Surface-Based (SB) • Allows use of high resolution hourly METAR obs • Assumes surface conditions representative of well-mixed PBL • Can overestimate instability • Mean Layer (ML) • More representative of actual convective cloud processes • Requires accurate information about PBL profile • Default PBL depth is 100 mb in NSHARP • Most Unstable (MU) • Uses level of maximum theta-e as lifted parcel level • Most useful in identifying elevated instability above PBL • May be identical to SB parcel (when max theta-e is at surface) • Overestimates instability if theta-e “spike” exists at one level

  39. Diagnosis of Instability – ML vs SB • Craven, Brooks, and Jewell (2002) examined more than 400 warm season 00z soundings • They estimated convective cloud base height using 100 mb ML and SB parcels and compared with observed ASOS cloud base heights ML parcels-little hgt bias SB parcels – low hgt bias SB parcels tended to underestimate convective cloud bases (SB parcel too warm/moist), whereas ML parcels better represented convective processes

  40. Craven et al. Findings (cont’d) • SBCAPE almost always larger than MLCAPE • Suggests surface conditions do not typically represent late afternoon PBL structure • Implies MLCAPE more representative of convective processes and potential (SBCAPE less useful) Dilemma – high resolution surface data allows hourly updates to environment But using lifted surface parcel may overestimate actual CAPE

  41. April 20, 2004 Challenges in Sfc Data Assim. and Fcstg PBL Evolution 34 Tornadoes Including One F3 8 Deaths, 21 Inj., $19 Million in Damage Jim Krancic

  42. 12z Eta Model Guidance

  43. 12 hr Eta Model 500 mb ForecastsValid 00z 21 Apr 04 Height and Vorticity Height, Temperature, Wind

  44. 12 hr Eta Model 850 mb and Sfc ForecastsValid 00z 21 Apr 04 850 mb Height,Temperature, Wind MSLP Isobars, 2m Dewpoint

  45. 12 hr Eta CAPE/Shear/SRH ForecastsValid 00z 21 Apr 04 MLCAPE/SHR6/SRH3 MUCAPE/SHR6/SRH3

  46. 12 hr Eta 3h Accum. Pcpn/VV ForecastValid 00z 21 Apr 04

  47. 15 hr Eta 3h Accum. Pcpn/VV ForecastValid 03z 21 Apr 04

  48. 18 hr Eta 3h Accum. Pcpn/VV ForecastValid 06z 21 Apr 04

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