Today: Mostly cloudy with light rain...ending by afternoon. High 54F. Winds NNW at 10 to 15 mph. Chance of rain 70%.

Tonight: Mostly cloudy skies. Low 33F. Winds N at 10 to 15 mph.

Tomorrow: Mostly cloudy skies. High 47F. Winds N at 10 to 20 mph.

Tomorrow night: Partly to mostly cloudy. Low 31F. Winds NNE at 10 to 20 mph.

Wednesday: Mix of sun and clouds. Highs in the upper 40s and lows in the upper 20s.

Thursday: Times of sun and clouds. Highs in the upper 50s and lows in the low 30s.

Friday: Plenty of sun. Highs in the mid 60s and lows in the low 40s.  A nice day for a quiz.

- National Weather Service forecast for Charleston, IL 4/7/03

Weather forecasts, such as this one, provide critical information to many people, including farmers, construction workers, and those planning a trip to the beach.  In severe weather situations, short-term forecasts and warnings can help save lives and protect property.  But how does one take the wealth of weather information that is available and make a prediction from it?

There are several different methods that can be used to create a forecast.  The method a forecaster chooses depends upon the experience of the forecaster, the amount of information available to the forecaster, and the level of difficulty that the forecast situation presents.

The first of these is the Persistence Method - the simplest way of producing a forecast.  This method assumes that the conditions at the time of the forecast won't change.  For example, if it is sunny and 87 degrees today, the persistence method predicts that it will be sunny and 87 degrees tomorrow.  If two inches of rain fell today, the persistence method would predict two inches of rain for tomorrow.

The persistence method works well when weather patterns change very little, and features on the weather maps are moving very slowly.  It also works well in places like southern California, where summertime weather conditions vary little from day to day.  If, however, weather conditions are changing significantly from day to day, the persistence method usually breaks down and is not the best forecasting method to use.

The trends method involves determining the speed and direction of movement for fronts, high and low pressure centers, and areas of clouds and precipitation.  Using this information, the forecaster can predict where he or she expects those features to be at some future time.  For example, if a storm system is 1000 miles west of your location and moving to the east at 250 miles per day, using the trends method you would predict it to arrive in your area in 4 days.

Using the trends method to forecast only a few hours into the future is known as "Nowcasting".  This method is frequently used to forecast precipitation.  For example, if a line of thunderstorms were 60 miles to your northwest and moving southeast at 30 miles per hour, you would predict the storms to arrive in your area in 2 hours.  Here is an example of using the trends method to forecast the movement of a cold front.  Initially, the cold front moved 800 miles during the first 24 hours, from the central Plains to the Great Lakes.

Using the trends method, you would predict this weather system to move another 800 miles in the next 24 hours, and reach the East Coast of the United States.  The trends method works well when systems continue to move at the same speed in the same direction for a long period of time.  If they slow down, speed up, change intensity, or change direction, the trends forecast will probably not work as well.

Climatology:  The Climatology Method is another simple way of producing a forecast.  This method involves averaging weather statistics accumulated over many years to make the forecast.  For example, if you were using the climatology method to predict the weather for New York City on July 4th, you would go through all the weather data that has been recorded for every July 4th and take an average.  If you were making a forecast for temperature and precipitation, then you would use this recorded weather data to compute the averages for temperature and precipitation.

If these averages were 87 degrees with 0.18 inches of rain, then the weather forecast for New York City on July 4th, using the climatology method, would call for a high temperature of 87 degrees with 0.18 inches of rain.  The climatology method only works well when the weather pattern is similar to that expected for the chosen time of year.  If the pattern is quite unusual for the given time of year, the climatology method will often fail miserably.

Analog Method: The Analog Method is a slightly more complicated method of producing a forecast.  It involves examining today's forecast scenario and remembering a day in the past when the weather scenario looked very similar (an analogy).  The forecaster would predict that the weather in this forecast would behave the same as it did in the past.

For example, suppose that today is very warm, but a cold front is approaching your area.  You remember similar weather conditions on a warm day last week when a cold front was also approaching.

You also remember how heavy thunderstorms developed in the afternoon as the cold front pushed through the area.  Therefore, using the analog method, you would predict that this cold front would also produce thunderstorms in the afternoon.  The analog method is tough to use, because it is virtually impossible to find a perfect analog.  Various weather features rarely align themselves in the same locations they were in the previous time.  Even small differences between the current time and the analog can lead to very different results.

Numerical Weather Prediction:  Numerical Weather Prediction (NWP) uses the power of computers to make a forecast.  Complex computer programs, also known as forecast models, run on supercomputers and provide predictions on many atmospheric variables such as temperature, pressure, wind, and rainfall.  A forecaster examines how the features predicted by the computer will interact to produce the day's weather.

The NWP method is flawed in that the equations used by the models to simulate the atmosphere are not precise.  This leads to some error in the predictions.  In addition, there are many gaps in the initial data, as we don't receive many weather observations from areas in the mountains or in the ocean.  If the initial state isn't completely known, the computer's prediction of how that initial state will evolve won't be fully accurate.

Despite these flaws, the NWP method is probably the best of the 5 that are mentioned here.  Very few people, however, have access to the computer data.  In addition, the beginning forecaster does not have the knowledge to interpret the computer forecast, so the simpler forecasting methods (such as the trends or analog) are recommended for the beginner.

Modern forecasters are relatively accurate for the 6-12 hour range, but become increasing uncertain after that.  One of the reasons that we believe forecasters are so poor is that we have made increasing demands on them.  Not only do we want to know IF it will rain, but how much and when.

When a weather forecaster states that the high temperature tomorrow should reach 81°F and it only reaches 80°F - are they wrong?  The National Weather Service and the American Meteorological Service consider forecasts correct if they are within 4°F of the actual temperature.  Ranges have also been established for the other weather variables such as precipitation, wind, and cloud cover.  On average, NWS and AMS forecasters are correct more than 80 percent of the time.  Of course, short ranged forecasts are more accurate than extended forecasts.

Rather than just looking out the window, which is how I'm sure you think some weather forecasts are made, the NWS and AMW make use of various tools to help them make forecasts.

Automated Surface Observing Systems (ASOS)

Next Generation Weather Radar (NEXRAD)

Satellites in polar and geostationary orbits
    (Nimbus and GOES)

Other methods?