A Brief History of Weather Forecasting

The Beginning: Weather Sayings• "Red Sky at night, sailor's delight. Red sky in the morning,

sailor take warning."

• "Mare's tails and mackerel scales make tall ships take in their sails."

• "Clear moon, frost soon."• .• "Halo around the sun or moon, rain or snow soon."

• "Rainbow in the morning gives you fair warning."

• "When the stars begin to huddle, the earth will soon become a puddle."

By the late 1700s, reasonable weather instruments became

available

More and more people took observations….and even some

early networks were started

The First Weather Forecaster?

The problem: no way to rapidly communicate weather

observations• This changed around 1845 with the

invention of the telegraph

First Real-Time Weather Maps

Weather Prediction Began• The main approach to weather prediction

during the late 1800s and early 1900s…simple extrapolation using simple schematic “models”

L L L

“Ol Probs”

Professor Cleveland Abbe, who issued the first public“Weather Synopsis and Probabilities” on February 19, 1871

Cleveland Abbe (“Ol’ Probabilities”), who led the establishment of a weather forecasting division within the U.S. Army Signal Corps,

Produced the first known communication of a weather probability to users and the public.

On May 7, 1869, Abbe proposed to the Cincinnati Chamber of Commerce "to inaugurate such a system, by publishing in the daily papers, a weather bulletin, which shall give the probable state of the weather and river for Cincinnati and vicinity one or two days in advance”.

Cleveland Abbe released the first public weather forecast on September 1, 1869.

Following the signing by President Ulysses S. Grant of an authorization to establish a system of weather observations and warnings of approaching storms, on February 19, 1871, Abbe issued the first “official” public Weather Synopsis and Probabilities based on observations taken at 7:35 a.m.

An early example of a report:

"Synopsis for past twenty-four hours; the barometric pressure had diminished in the southern and Gulf states this morning; it has remained nearly stationary on the Lakes. A decided diminution has appeared unannounced in Missouri accompanied with a rapid rise in the thermometer which is felt as far east as Cincinnati; the barometer in Missouri is about four-tenths of an inch lower than on Erie and on the Gulf. Fresh north and west winds are prevailing in the north; southerly winds in the south. Probabilities [emphasis added]; it is probable that the low pressure in Missouri will make itself felt decidedly tomorrow with northerly winds and clouds on the Lakes, and brisk southerly winds on the Gulf."

The Next Major Advance• The Norwegian Cyclone Model around

1920

Next Major Advance: Upper Air Observations Became Available from

Radiosonde in the 1920’s, 30s, and 40s

1940s: Upper Air Charts Using the New Data

• Gave a 3D picture of what was happening

• Learned how the upper flow steered storms, and thus provided a tool for forecasting cyclone movement.

UpperLevelChart

The Development of Numerical Weather Prediction (NWP)

• Vilhelm Bjerknes in his landmark paper of 1904 suggested that NWP was possible.– A set of equations existed that

could predict the future atmosphere (primitive equations)

– But it wasn’t practical then because there was no reasonable way to do the computations and sufficient data for initialization did not exist.

Numerical Weather Prediction• The advent of digital computers in the late

1940s and early 1950’s made possible the simulation of atmospheric evolution numerically.

• The basic idea is if you understand the current state of the atmosphere, you can predict the future using the basic physical equations that describe the atmosphere.

The Eniac

Numerical Weather PredictionOne such equation is Newton’s Second Law:

F = maForce = mass x acceleration

Mass is the amount of matter

Acceleration is how velocity changes with time

Force is a push or pull on some object (e.g., gravitational force, pressure forces, friction)

This equation is a time machine!

Using a wide range of weather observations we can create a three-dimensional description of the atmosphere… known as the initialization

Numerical Weather Prediction

•Observations give the distribution of mass and allows us to calculate the various forces.

•Then… we can solve for the acceleration using F=ma

•This gives us the future since the acceleration can be used to calculate the velocities in the future.

•Similar idea with temperature and humidity.

Numerical Weather Prediction

Numerical Weather Prediction• These equations can be solved on a three-

dimensional grid.

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

• More (and thus) closer grid points means we can simulate (forecast) smaller and smaller scale features. We call this improved resolution.

A Steady Improvement

• Faster computers and better understanding of the atmosphere, allowed a better representation of important physical processes in the models

• More and more data became available for initialization

• As a result there has been a steady increase in forecast skill from 1960 to now.

Forecast Skill Improvement

ForecastError

Year

Better

National Weather Service

Satellite and Weather Radars Give Us a More

Comprehensive View of the Atmosphere

Before Satellites We Had a VERY Poor Knowledge of What Was Happening Over the Oceans!

• As a result, forecasts were often very poor, particularly in coastal locations.

The 1938 Hurricane was basically unforecast

Better than Star Trek!

Weather Satellites Give Us Much More than Pretty Pictures

• We start with imagery in several wavelengths:– Visible– Infrared– Water vapor (wavelengths where we see the

water vapor distribution)

• Plus many new capabilities

Each wavelengthgives us information

Cloud andWater VaporTrack WindsBased on Geostationary Weather Satellites

CamanoIslandWeatherRadar

1995-2003+The computers models become

capable of simulating/forecasting local weather.

As the grid spacing decreased to 15 km and below… it became apparent that many of the local weather features could often be simulated and forecast by the models.

Forecaster at the Seattle National Weather Service Office

The National Weather Service

But even with all this improving technology, some forecasts fail or

are inadequate. Why?

Problems with the Models

• Some forecasts fail due to inadequacies in model physics…. How the model handles precipitation, friction, and other processes.

Example: too much precipitation on mountain slopes

• Intensive work at the UW to address this problems.

Some forecasts fail due to poor initialization, i.e., a poor starting description of the atmosphere.

This is particularly a problem for the Pacific Northwest, because we are downstream of a relatively data poor region…the Pacific Ocean.

Pacific AnalysisAt 4 PM18 November2003

Bad Observation

Eta 48 hr SLP Forecast valid 00 UTC 3 March 1999

3 March 1999: Forecast a snowstorm … got a windstorm instead

The problem of initialization should lessen as new observation

technologies come on line and mature.

New ways of using or assimilating the data are also

being developed.

A More Fundamental Problem• In a real sense, the way we have

been forecasting is 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 More Fundamental Problem• Thus, there is fundamental uncertainty in

weather forecasts that can not be ignored.• Similarly, uncertainty in our model physics

also produces uncertainty in the forecasts.• 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

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

The Thanksgiving Forecast 200142h forecast (valid Thu 10AM)

13: avn*

11: ngps*

12: cmcg*

10: tcwb*

9: ukmo*

8: eta*

Verification

1: cent

7: avn

5: ngps

6: cmcg

4: tcwb

3: ukmo

2: eta

- Reveals high uncertainty in storm track and intensity- Indicates low probability of Puget Sound wind event

SLP and winds

Ensemble Prediction

•Can use ensembles to provide a new generation of products that give the probabilities that some weather feature will occur.

•Can also predict forecast skill!

•It appears that when forecasts are similar, forecast skill is higher.

•When forecasts differ greatly, forecast skill is less.

Ensemble-Based Probabilistic Products

The End