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
- 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.
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.
Finally look at the current weather indices in your area, the following link ios for BWI airport. 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
The forecast models discussed below give information at several levels in the atmosphere. Each level has a special significance.
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
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
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
250mb 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
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 primary model that we will use is the NAM model.
It is called a short-range model, meaning it only forecasts the next 42
temperature, humidity and wind Profiles (Skew-T diagrams). Sometimes
weather forecasters are interested in what type of
atmospheric changes are occurring, with altitude, over a particular point
on a weather map. To do this Skew-T diagrams are used. Data for
these plots is obtained from weather balloons; in fact, the images above were
created using a composite of data from all the weather balloons (as well as
satellite, wind profiler and numerical model data). View the skew-T profile over
Dulles international airport here.
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 primary model that we will use is the NAM model. It is called a short-range model, meaning it only forecasts the next 42 hours. This model not only forecasts conditions at the surface, but also forecast conditions for the upper levels of the atmosphere that were discussed above. For more details on this model, the UCAR COMET program provides a series of tutorials. If you wish to forecast beyond 42 hours then you will have to use another model, the GFS model (GlobalForecast System). This is a medium-range forecast model, forecasting for up to 10 days. The link below is to the College of DuPage 'Next Generation Weather Lab'. To reach their forecast page pull down 'Weather Analysis Tools' and click on 'Numerical Models'. Along the top of the page will appear a list of models, including NAM and GFS. Click on the model of choice. A menu will appear at the left of the page giving the pressure levels discussed above. Click on the level you want and a list will appear. Now click on the parametetr you want displayed. Precipitation is listed under 'SURFACE'. Under the line of the models will appear the times available for display, and a loop link. DUPAGE.
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 º (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).
By now, hopefully you have a general idea of how the weather over your area might change in the next day. In the future, you can visit this checklist again, at the following address:
For further information, here are some references:
Meteorology Today (Ahrens, C.D)
Meteorology – Understanding the Atmosphere (Ackerman, S.A, Knox, J.A)