Weather Forecasts: From Tragedy to TriumphCliff Mass

University of Washington

2012: Hurricane Sandy

125 dead, 60+ billion dollars damage

Well predicted over a week ahead of time

ECMWF Forecast of Sea Level Pressure

1938 Hurricane: Similar in Strength to Sandy

Nearly a thousand died

Not forecast the day before

1962 Columbus Day Storm

Not forecast the day before either

Seattle Times

Jan 1993: Inauguration Day Storm: Near Perfect Forecast

Something Has Changed

Before 1990 the National Weather Service got virtually every major storm wrong, even the

day before.

After 1990, they gave good warnings for nearly all.

Forecast Skill Improvement

ForecastError

Year

Better

National Weather Service

Skill Improvements (ECMWF)

Major improvements, mainly due to satellite data and improved models

The Revolution in Weather Prediction Technology

The Key Technology of Modern Weather Forecasting is

Numerical Weather Prediction

Numerical Weather Prediction

• The basic idea is that if you can determine the current state of the atmosphere (known as the initialization) , you can predict the future using the equations that describe the physics of the atmosphere.

• These equations can be solved on a three-dimensional grid.

The “Primitive” Equations

Numerical Weather Prediction• Numerical weather prediction is limited by the

available computer resources.

• As computer speed increases, the number of grid points can be increased.

• More (and thus) closer grid points means we can simulate (forecast) smaller scale features.

National WeatherService WeatherPrediction Computer

NGM, 80 km,1995

2007-2008

4-km MM5Real-time

1.33 km resolution available on the UW web site

But just as important has been the weather data revolution, with

satellites giving us three dimension data over the entire

planet

Example: The Pacific Data Void No Longer Exists

Cloud Track Winds

Better than Star Trek!

NOAA PolarOrbiter WeatherSatellite

Satellite Sensors Provide Thousands of High Quality Vertical Soundings Daily over the Pacific

Cosmic GPS Satellites Provide More Soundings!

We are now starting to see frequent examples of forecast

skill past one week:

Hurricane Sandy is only one example

Observed 180 hr (7.5 days)

Forecast Skill Will Continue to Extend Further in Time…with

limits (about 2 weeks)

• More satellite assets will provide a far better description of the atmosphere.

• Better models and higher resolution

• Better data assimilation: how we use the observations to produce an initialization for our models.

Increasing Resolution and Better Models Will Not Be Enough

The Next Major Revolution in Numerical Weather Prediction

Will Come Elsewhere

The Transition from Deterministic to Probabilistic

Prediction

A Fundamental Problem• The way we have been forecasting

has been essentially flawed.

• The atmosphere is a chaotic system, in which small differences in the initialization…well within observational error… can have large impacts on the forecasts, particularly for longer forecasts.

• Not unlike a pinball game….

A Fundamental Problem

• Similarly, uncertainty in our model physics (e.g., clouds and precipitation processes) also produces uncertainty in forecasts.

• Thus, all forecasts have some uncertainty.

• The uncertainty generally increases in time.

This is Ridiculous!

Forecast Probabilistically

• We should be using probabilities for all our forecasts or at least providing the range of possibilities.

• There is an approach to handling this issue that is being explored by the forecasting community…ensemble forecasts

Ensemble Prediction

• Instead of making one forecast…make many…each with a slightly different initialization or different model physics.

• Possible to do this now with the vastly greater computation resources that are available.

Ensemble Prediction

•Can use ensembles to give the probabilities that some weather feature will occur.

• Ensemble mean is more accurate than any individual member.

•Can also predict forecast skill!•When forecasts are similar, forecast skill is generally higher.•When forecasts differ greatly, forecast skill is less.

Prediction!

• The meteorological profession is rapidly gaining the ability to produce high-resolution probabilistic weather forecasts AND analyses.

• Probabilistic forecasts and analyses will be available for a wide range of weather parameters.

The Nowcasting Revolution

AMS Nowcasting Definition

A description of current weather and a short-term forecast varying from minutes to a few hours; typically shorter than most operational short-range forecasts.

American Meteorological Society’s Glossary of Weather and Climate

During the past decade or so the geographical and temporal detail

the weather profession can provide has greatly increased.

• High resolution forecasting, NWS forecasts on a 2.5 km grid, radar data, satellite imagery, huge numbers of surface stations, and now probabilistic prediction!

Example:The Pacific Northwest

Based on 72 different networks

3000-4000 observations per hour over WA and OR

Traditional Approaches of Weather Information Dissemination Are Incapable of Delivering the Specificity and Detail

Meteorologists Can Provide

Typical TV weathercasters have only 2.5 minutes!

Many of us worried about this problem in the 90’s but now the

solution is literally at hand

Smartphones are Ideal for Weather Data Delivery!

• Lots of bandwidth

• They know where they are, so forecast information can be tailored to the user

• Substantial computational capacity.

There are now thousands of weather apps for smartphones…and the best

are yet to come!

Some Tragedies Continue

Little Improvement in Forecasting the Intensity of Hurricanes

Tragedy: The U.S. is Now in Second or Third Place in Global

Weather Prediction

The End