In order to make a good weather forecast, you must first understand the current weather conditions. You can accomplish this by looking at satellite images, reviewing the surface analysis, and studying several important levels of the atmosphere. Once you know the current weather, you can make a forecast for the future. Some forecasting tools that will be useful for your prediction are the images and charts you just reviewed as well as several different weather models. With all the information these provide, you will be able to create an accurate weather forecast.

I. Understanding the current weather

A. Satellites

- The first satellite image one should examine is the visible. This is essentially a “photograph” of how the atmosphere looks from space. On this image, one can see cloud features that can help identify storm systems, fronts, and areas of high and low pressure - visible image loop.

- The next image is of water vapor. This shows areas of high and low concentrations of water vapor in the atmosphere. You should notice areas of high water vapor concentration generally correspond to cloudiness on the visible satellite image – water vapor loop.

- The last image is the infrared. This image displays the cloud top temperatures. The colder the temperature of the cloud top, the higher the cloud. Therefore, when viewing the infrared over time, one can see whether a system is growing or weakening based on the height of the system – IR loop.

B. Surface

After looking at the satellite imagery, you probably have an idea of where any major storm systems are located. By looking at a radar reflectivity image, it is possible to estimate the intensity of each system. The legend for this image is in “dBz”, the higher the number: the stronger the system. Generally, dBz values above 30 indicate some type of precipitation is occurring - image.

In addition to radar, a surface analysis plot is another way to view the current weather. The most important aspects of this plot are the locations of any fronts or pressure systems. First, locate any of these features associated with your area. Ask yourself what general weather conditions would you expect to see with the systems you have identified. For example, if a front were approaching your area, you would expect to see increasing clouds, stronger winds, and maybe some precipitation - image.

As you move on to the upper levels, look to see if there are any similarities between the surface features and those in the upper levels. In general, strong surface systems are reflected high into the atmosphere. Try to find the point where the surface system is no longer reflected.

C. Upper Levels of the Atmosphere

850mb level (~1,500ft) – This image shows temperature (in color), and the flow pattern. One can see what kinds of temperatures are headed into your area. In general, changes in temperature at this level are good indicators of changes in temperature at the surface – 850mb.

700mb level (~3,000ft) – The most important feature of this image is the flow pattern. In general, the winds at this level are useful for determining where individual systems at the surface may go (i.e. the steering flow). In addition to horizontal motion, this level also shows vertical motion. The strength of upward and downward motion is displayed in color. Upward motion is indicative of storm potential, while downward motion hinders storm growth – 700mb.

500mb level (~5,500ft) – From this and other images, the wave-like flow of the atmosphere can be seen. This level is particularly useful in locating areas of high and low pressure and their associated flow patterns. A trough is seen when the flow takes on a “U” shape, whereas the “upside-down U” part of the flow is called a ridge. The region ahead of a trough is susceptible to storm development. You might have heard your local TV weatherperson say, “there is an upper level trough approaching the area”. This would indicate an increased chance of storm activity. In addition, one can also see areas of rotation on this image (shown in color). Regions of positive rotation are favorable for storm development, while negative rotation inhibits development – 500mb.

300mb level (~30,000ft) – At this level, one can see the jet stream’s orientation and speed. The orientation of the jet stream determines what type of weather will be able to affect your area. North of the jet stream, the air is colder and dryer, while south of the jet stream the air is warmer and moister. The colors on the image indicate wind speed (areas of densely packed color change indicate the jet core). The white arrows show the direction of the wind – 300mb.

II. Forecasting

You have now examined the current weather conditions. From this analysis, you know what weather features are currently affecting your location. In addition, you have seen the possible weather systems or features that could become important to your forecast. You have also identified how these potential systems are reflected through the atmosphere. This information is important to determine the strength(s) of the approaching system(s). With this knowledge, you can now begin to forecast how the weather might change over your area.

To aid you in the forecasting process, several weather forecast models are available for your use. These models predict how the atmosphere will move and change the strengths of the systems you took note of in your initial analysis. The first model is called the eta model. This is what is called a short-range model, meaning it only forecasts the next 84 hours. The next model is called the GFS model (Global Forecast System) . This is a medium-range forecast model. It forecasts the next 14 days. Both models not only forecast conditions at the surface, but also forecast conditions for the upper levels of the atmosphere that you have previously examined.

The first screen you come to will be a table of each atmospheric level available. After clicking on a particular level, it will display forecasts in six-hour increments for the eta model, and 12-hour increments for the GFS. The eta model finishes with a 48-hour forecast, whereas the GFS forecasts out to 180 hours. It is good to begin by comparing the models up until 48 hours. If both models agree well, you can be confident in their forecast, and even look further using the GFS. If they disagree, then generally meteorologists rely on their expertise, and past experience to choose which model is producing the more accurate forecast.

The models provide a tremendous amount of data for each forecast period. The information is spread over the individual images, making it difficult to put together a cohesive picture of the atmosphere. A meteogram helps by summarizing important surface characteristics for the entire forecast period. It is a simple diagram based on the model results that displays the temperature and precipitation forecast, among other quantities. Meteograms are produced for specific points around the country. For this area, for example, meteograms are available for Dulles International Airport and Baltimore Washington International Airport.

On each meteogram, you will see five graphs. For the purposes of this forecast primer, we will focus on the first two. The first shows the surface temperature (in red), and the surface dewpoint (in green) for the entire forecast period. The next displays precipitation amount (in solid blue, inches), and weather conditions (in purple). Some common weather symbols are · (rain), * (snow), and s (fog). The second graph also displays the surface wind direction and speed. It is important to remember that the wind barb points in the direction the wind is blowing. The long ticks on the barbs represent 10 knots (11.5mph), and the short ticks are 5 knots (~6mph).