where americas climate and weather services begin

Download Where Americas Climate and Weather Services Begin

Post on 01-Feb-2016




0 download

Embed Size (px)


A Historical Perspective on the Role of NWP Models in the Prediction of Severe Local Storms Steven J. Weiss and Joseph T. Schaefer steven.j.weiss@noaa.gov. Where Americas Climate and Weather Services Begin. Severe Weather Forecasting. - PowerPoint PPT Presentation


  • A Historical Perspective on the Role of NWP Models in the Prediction of Severe Local Storms

    Steven J. Weiss and Joseph T. Schaefer

    steven.j.weiss@noaa.govWhere Americas Climate and Weather Services Begin

  • Severe weather is defined as severe local storms (thunderstorms) producing: Tornadoes Large hail (> 3/4 inch diameter) Convective wind gusts > 50 kt or wind damage In the U.S. heavy rain/flash floods not includedFocus will be on prediction of severe storms (forecasts) rather than detection of existing severe storms (warnings) The presentation will be from the perspective of national center forecasting (SPC/NSSFC)

    Severe Weather Forecasting

  • The Beginning of Modern Severe Weather ForecastingNWP and tornado forecasting activities both began in the middle of the 20th centuryNWP focused on synoptic scale predictionTornado prediction focused on mesoscale and storm scale phenomenaDespite having similar origins in time, it would take several decades before the paths of NWP and severe weather forecasting would become closely aligned

  • The Beginning of Modern Severe Weather ForecastingOn March 25, 1948 Ernest Fawbush and Robert Miller (USAF) issued the first modern tornado forecast

  • The Beginning of Modern Severe Weather Forecasting

    20 Mar 1948

    25 Mar 1948Gen. Borum: Are you going to issue a tornado forecast?

    Maj. Fawbush: Sir, it sure looks like the last one, doesnt it Bob?

    Capt. Miller: Yes, E.J.,it is very similar to last week.

    Fawbush & Miller: But no one has ever issued an operational tornado forecast

    Gen. Borum: Youre about to set a precedent

  • US Weather Bureau Tornado ForecastsBased on their success March 25, 1948, Fawbush and Miller were assigned to issue USAF tornado forecastsBy early 1952 complaints from public and media, and Congressional pressure forced Weather Bureau to issue tornado forecastsA temporary group of research forecasters and supervising analysts at the Washington DC Analysis Center formed initial Severe Weather Unit (SWU)The first public tornado forecast was issued March 17 1952 (it did not verify)

  • The First Successful USWB Tornado Forecasts and Tornado ReportsIssued March 21-22, 195228 tornadoes killed 204 people

    (From Corfidi 1999)

  • US Weather Bureau Tornado ForecastsThe first tornado forecasts were similar to current tornado and severe thunderstorm watches in time/spaceShort-term forecasts (0-8 hours) covering parts of statesBased entirely on observational data (hourly surface data, 4 times daily raobs and PIBAL data) and detailed manual analysesDynamic and thermodynamic considerations guided the forecast processMoisture, instability, and lift sufficient for parcels to reach LFCUpper level jet dynamics, interaction of low and upper jetsApplication of basic concepts: climatology, pattern recognition, parameter evaluation Forecasters focused on understanding and predicting the synoptic scale / mesoscale environment and its relationship to thunderstorms

  • SELS Kansas City Operations in 1950s

  • Important Severe Weather Papers (1956)

  • Mesoscale Analysis of Surface DataResearch (Fujita et al. 1956) Transferred to Operations (Magor 1959)

  • Introduction of Digital Computers 1960sIBM 1620 computer installed at SELS in 1963System upgraded to CDC 3100 computer in 1965Used to automate plotting of surface and upper air charts (analysis still done by hand)Allowed computation of fields such as vorticity and divergence from upper air dataAutomated sounding plots and calculation of stability indices Local computers used to increase forecaster efficiency, and provide new diagnostic information

  • Examples of Early Dynamic Computations at SELS 500 mb Vorticity 300-200 mb Mean Divergence

  • Examples of Computer Plotted Surface Charts 18z 3 April 1974MSL Pressure / Wind Temperature / Dew Point

  • Examples of Computer Plotted Surface Charts 2 hr Changes 18z 3 April 1974 Altimeter Changes Temperature / Dew Point Changes

  • Actual Tornado Watch Formulation 21z 3 April 1974 Superoutbreak

  • Early Use of NWP at SELSBy the late 1960s NMC was running an operational 6-layer Primitive Equation modelBasic large scale fields (e.g.,500 mb height/vorticity, MSL pressure/thickness) available via facsimileUsed by SELS forecasters to provide guidance on synoptic scale features - upper troughs, surface lows, frontsFirst time that NWP model data was used as part of Convective Outlook preparation processNo direct impact on formulation of Tornado and Severe Thunderstorm WatchesVery coarse model resolution and limited output fields (North American domain at 12 hr intervals)

  • Example of PE Model Lifted Index Forecast for 3 April 197400 hr

    24 hr12 hr

    36 hr

  • The LFM Model Plays a Larger Role in Severe Weather ForecastingLimited-area Fine Mesh (LFM) model introduced in 1971First regional model at NMCSimilar to PE with half the grid spacing (190.5 km) and run over smaller domainMore output fields and increased resolution provided improved guidance for Convective Outlook preparationFirst time SELS forecasters produced forecast composite charts based on model dataprovided more structured basis for Convective OutlookAgain, relatively coarse resolution and inability to resolve convective systems provided little short-term (watch) guidance

  • Examples of LFM Facsimile Chart Output500 mb Height / Vorticity MSL Pressure / Thickness

  • Examples of LFM Facsimile Chart Output700 mb Height / Mean RH 700 mb Vert. Vel / 12 hr Precip.

  • NWP / Technology Advances in the 1980sTechnologyAutomation of Field Operations and Services (AFOS - 1980)Designed to enhance display of NWP model data via electronic monitorsIncreased communications bandwidth allowed dissemination of more model fields (compared to FAX)Used at SELS primarily for Convective Outlook preparationFrequency and forecast intervals of NWP output similar to FAX (12 hour intervals), although smaller geographic domain could be specified

  • NWP / Technology Advances in the 1980sTechnologyCentralized Storm Information System (CSIS - 1982)McIDAS-based interactive workstations specifically designed for severe weather forecasting at SELSUnique advantage was integrated display of observational data sets Satellite, radar, surface and upper air Objective analysis of hourly derived fields (lifted index, mstr convg.)Facilitated better application of traditional forecasting procedures developed in 1950s-1970s Revolutionized short-term severe weather forecasting through access to real-time dataResulted in more accurate timing and placement of severe weather watchesHad limited utility for NWP output display => less impact on forecasts beyond ~6 hours (such as Convective Outlooks)

  • CSIS Workstation 1980sIR Satellite and Mstr Flux Convg. Radar and Surface Data

  • NWP / Technology Advances in the 1980sNWPNested Grid Model (NGM - 1985)Triple nested model with 80 km grid spacing inner domain and 16 vertical levels and forecasts to 48 hrsIncreased resolution resulted in overall improved forecast skill compared to LFMAllowed extension of SELS Convective Outlook product into the Day 2 time period (1986)Instability (lifted index) forecasts introduced concept of Best lifted index to account for elevated instability (good news), butNGM LIs were often less accurate than LFM, particularly when extreme instability consideredCaused by systematic errors in temperature / dew point profiles SELS forecasters still focused on synoptic scale environment

  • Links Between Observable Scales and Storm Scale Remained Largely UnknownThanks to Sidney HarrisSynoptic scaleStorm scale

  • NWP / Technology Advances in the 1980sResearch Cloud-Scale ModelsProvided physical insights about links between observable scales and storm scale phenomenaHelped explain dynamics of supercell thunderstormsEmphasized role of CAPE and vertical shear on storm evolution Relationship of CAPE / Shear ratio (BRN) and modeled storm type (from Weisman and Klemp 1986)

  • NWP / Technology Advances in the 1990sNWP and Computer WorkstationsEta and RUC ModelsRapid increases in model resolution, physics complexity, model frequency, amount of output, and skill levelsFor the first time, models became useful sources of information for short-term convective forecasting (severe storm watches)Output complemented real-time observational dataModel forecasts of key convective parameters such as CAPE, vertical shear, and helicityForecasters could more directly identify synoptic / mesoscale environments favorable for supercell developmentAdvanced Operational WorkstationsNational-center AWIPS (NAWIPS)Designed to display gridded model data plus observational data

  • Eta Model 24 hr Forecast of CAPE / Shear / Helicity CAPE 0-3 km Helicity contours 0-6 km Shear vectors

  • NWP / Science &Technology Advances in the 1990sImprovements in scientific understanding of severe storms through field programs such as VORTEX, coupled with better NWP guidance, resulted in improvements to SPC product suiteEta ModelIssuance of Probabilistic Severe Storm Outlooks to complement traditional Categorical (Slight, Moderate, High Risk) OutlooksExtension of Convective Outlooks to Day 3 Time PeriodRUC Model Real-time 3D hourly analyses by combining surface data and RUC dataObjective analyses of hundreds of convective parametersExtensively used at SPC for short-term severe weather forecasting and shared with WFOs via SPC web page

  • Example of RUC - SfcOA O


View more >