Forecasting

ATS 113

Forecasts made by PEOPLE

• Folklore:– Groundhog Day– Fuzzy caterpillars– Walnut shells– Farmers Almanac

Forecasts made by PEOPLE

• Persistence:– Assumes that the current weather is going to

continue.

Forecasts made by PEOPLE

• Climatology– Assumes that the weather in the near future

will be a lot like the mean weather of the last 30 years.

Forecasts made by PEOPLE

• Analog forecasts:– Find times in the past in which the weather

was just like the current weather– Assumes that the weather in the near future

will be just like the weather was after these analogous times

Forecasts made by COMPUTERS

• A.K.A. “Numerical Weather Prediction”, NWP

• A.K.A. “Modeling”

Forecasts made by COMPUTERS

• Starts with good OBSERVATIONS OF THE CURRENT WEATHER

• Weather prediction is an “initial value problem”

Forecasts made by COMPUTERS

• Assimilation of observations

Forecasts made by COMPUTERS

• Seven “Primitive Equations”:– Three equations about forces: gravity,

pressure gradient force, coriolis force, etc.

Forecasts made by COMPUTERS

• Seven “Primitive Equations”:– The Ideal Gas Law

Forecasts made by COMPUTERS

• Seven “Primitive Equations”:– The First Law of Thermodynamics: about

moving heat around (radiation, conduction, convection, etc.)

Forecasts made by COMPUTERS

• Seven “Primitive Equations”:– The Continuity Equation (about how

convergence and divergence lead to rising and sinking motion)

Forecasts made by COMPUTERS

• Seven “Primitive Equations”:– Conservation of Water – about how

evaporation and condensation affect the amount of water vapor in an air parcel

Forecasts made by COMPUTERS

• The Seven Primitive Equations are prognostic—they make a forecast of the temperature, pressure, winds, humidity, etc. for some time in the future.

• About ONE SECOND in the future.

Forecasts made by COMPUTERS

1. Take the current OBSERVATIONS.

2. ASSIMILATE the observations into the model grid.

3. Apply the 7 equations to every gridpoint to get a forecast for about 1 second later.

4. Repeat step 3 until you have a forecast for 12, 24, 36, 72, 120 hours into the future!

Forecasts made by COMPUTERS

• Resolution versus domain:

Current ETA forecast: 12 hours

Current ETA forecast: 24 hours

Current ETA forecast: 36 hours

Current ETA forecast: 48 hours

What’s going on in the weather?

48 hr ETA forecast: Pressure

48 hr ETA forecast: Temp.

48 hr ETA forecast: Precip

48 hr ETA forecast: Snow?

What do the other models say?

NGM: No snow

AVN: Lots of snow in MO!

Why the difference?

• Different resolutions

• Different approximations to the 7 primitive equations

• Different errors in the model

Ensemble Forecasting

• Using MANY models of the weather to reach a consensus forecast

• Typically at least a dozen models!