Forecasting With Computer ModelsPrimitive computer models were first used by weather forecasters during the 1950's. These computers factor in the current surface and upper air data, including temperature, humidity, air pressure, and other weather dynamics to project the movement of surface and upper air weather features.
During the 1970's, the models became more complex and reliable forecasting tools. Today, there are a number of computer models available for forecasters to use.
Examples of computer models used as of the mid-1990's for weather forecasting are the NGM (Nested Grid Model), the AVN (Aviation Model), and the ECMWF (an European model).
The equations of upper air motion and the number of atmospheric levels that computer models use in their databases varies from model to model. Because of the differences in programming, one model's prediction of a weather event may vary greatly from a second model's.
If a computer model's forecast is initially wrong, its projection error will continue to be compounded as time goes on. Twenty years ago, computer models improved 24 to 48 hour weather forecasts. Today, computers can project general weather forecasts more accurately than before.
For example, suppose that one model's data projects that a winter storm will move over the Great Lakes, while a second model has the same storm system moving through the Ohio Valley to the middle Atlantic coast.
In this case, the forecaster will need to use his or her experience and intuition to decide which model is the best forecast for that situation and then closely track the storm as it evolves.
Forecasting With RadarRadar is another helpful tool in monitoring and predicting weather. It is particularly successful in tracking the motion of the precipitation, allowing forecasters to make accurate short-term forecasts on when and where winter storms will strike.
Radar, was originally used as a detection device in military aircraft during World War II. It was then modified after the war for meteorological use in the weather community.
The physical structure of snow makes it more difficult to track on conventional radar than other types of precipitation. Snow flakes have lower moisture and higher air content than other types of precipitation. This quality is physically apparent in the holes found in a snowflake's structure.
Extremely light snow can sometimes go undetected on radar. However, the newest generation of Doppler radar is significantly more sensitive than previous generations of radar, and even capable of picking up virga, a situation in which snow is falling from clouds but evaporating before reaching the ground.
Forecasters must be careful to combine both surface observations and information from Doppler radar to determine where and at what rate snow is falling.
Sleet shows up very well on all radar. Because it is a solid ball of ice, it possesses a high reflectivity level, which sometimes on radar can be mistaken for heavy snow. Forecasters must use both surface observations and radar to make accurate forecasts.
Forecasting With SatelliteA useful tool in determining cloud patterns and movement of winter storms is satellite imagery. The GOES-8 and GOES-9 satellites provide several pictures per hour and with greater clarity than ever before.
Satellite imagery can be very helpful when used by forecasters to monitor rapidly developing winter storms off the east and west coasts of the United States. By looping a series of satellite pictures, forecasters can monitor a storm's development and its movement with greater accuracy.
Forecasting With Upper Air ObservationsAn important aspect of the forecaster's job is to know what weather conditions are present at different levels of the atmosphere. For instance, the temperature at a cloud level of 6,000-7,000 feet can be an important factor in determining the type of precipitation a winter storm will produce.
One way that weather data is gathered is by weather observation balloons launched from selected National Weather Service (NWS) observation sites twice daily.
Weather balloons record air pressure, wind flow, moisture, dew point, and temperature at various levels within the atmosphere, from the ground to above the jet stream level.