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Snowmageddon

- 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!

Numerical Weather Models (NWP) and Ensemble Systems

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The Forecaster’s Dilemma

Initialization Forecast Possibility #1 Forecast Possibility #2

Two Days LaterToday

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An Ensemble of PossibilitiesEnsembles provide an envelope of solutions representing the possible storm tracks as well as storm intensity and precipitation amount/type

Storm Tracks Predicted Over a 2 Day Period

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3 daysprior

2 daysprior

1 day prior

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HPC Snowfall Probabilities for Feb 6, 2010

3 Days Prior 2 Days Prior

1 Day Prior

Probability Legend

SLGT: At Least 10% ProbMDT: At least 40% ProbHIGH: At least 70% Prob

>12”

>12”>12”

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Hurricane Sandy

Forecasters Use Ensemble Means for

Official Track Forecasts

Low Track Forecast – 1.5 days prior to landfall

* Note the ensemble spread at each forecast time.

Storm accelerated towards New Jersey

coast

• 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?

• 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

Major FY2012 Implementations (cont.)

NH Anomaly Correlation for 500hPa Height

Period: September 1st – November 30th 2011

Week 2: Adding 3-4 days of skill

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