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Snowmageddon

Snowmageddon. Numerical Weather Models (NWP) and Ensemble Systems. Numerical weather models... All forecasts contain errors (either in physics or initial analysis) that increase with time Doubling time of small initial errors ~1 to 2 days

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Snowmageddon

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  1. Snowmageddon

  2. Numerical Weather Models (NWP) and Ensemble Systems • Numerical weather models... • All forecasts contain errors (either in physics or initial analysis) that increase with time • Doubling time of small initial errors ~1 to 2 days • Maximum large-scale (synoptic) predictability ~10 to 14 days • Ensembles… • A collection of models providing information on a range of plausible forecasts, statistical measures of confidence, and extend predictability • Ensemble Model runs provide a range or “envelope” of solutions • The spread of solutions can be used to provide probabilities or “confidence” limits for any forecasts Weather forecasting: It’s impossible to be certain all of the time!

  3. Initialization Forecast Possibility #1 Forecast Possibility #2 The Forecaster’s Dilemma Today Two Days Later

  4. Ensembles provide an envelope of solutions representing the possible storm tracks as well as storm intensity and precipitation amount/type An Ensemble of Possibilities Storm Tracks Predicted Over a 2 Day Period

  5. 2 days prior 3 days prior 1 day prior

  6. HPC Snowfall Probabilities for Feb 6, 2010 >12” >12” 3 Days Prior 2 Days Prior Probability Legend SLGT: At Least 10% Prob MDT: At least 40% Prob HIGH: At least 70% Prob >12” 1 Day Prior

  7. Hurricane Sandy

  8. Forecasters Use Ensemble Means for Official Track Forecasts

  9. Low Track Forecast – 1.5 days prior to landfall * Note the ensemble spread at each forecast time. • Acceleration of storm - only model that hinted at this within 24 hours of landfall – SREF • Is acceleration a function of the interaction of Sandy with the extratropical trough? The blocking ridge? Both? Storm accelerated towards New Jersey coast

  10. Major FY2012 Implementations (cont.) • GEFS – February 14, 2012 • Latest GFS, improved ensemble initialization • Resolution upgrade • Horizontal: T190(~70km)  T254 (~50km) out to 8 days • Vertical: 28 to 42 levels out to 16 days Week 2: Adding 3-4 days of skill NH Anomaly Correlation for 500hPa HeightPeriod: September 1st – November 30th 2011

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