Essentially all modern weather prediction is performed by numerical weather prediction models that use high speed computers. These computer models are based upon extrapolating the movement and development of atmospheric systems through the repetitive evaluation of mathematical equations for Newton's laws of motion, the gas laws and other relationships of mass and energy conservation.
A series of forecast charts is available on the Online Weather Homepage. These displays are forecast data from the Nested Grid Model (NGM), one of the types of short range, numerical weather prediction models that the National Centers for Environmental Prediction use to predict the weather across much of the North American continent for up to 48 hours in advance.
Many of the numerical weather prediction models are run twice daily using the 00Z and 12Z surface and upper air weather observations to provide a three-dimensional model of the atmosphere. Surface weather conditions for selected cities are then generated statistically from the numerical output and are plotted on Online Weather Forecast charts at 12 hour intervals.
The Online Weather Forecast charts are for 12, 24, 36 and 48 hours following the time that the computer model was run. These Online Weather forecast plots are updated once every 12 hours at approximately 0515Z and 1715 Z (12:15 AM and 12:15 PM EST). Make sure that you check to see for what time the chart is "valid". For example, if the model used observed weather data from 12 Z on Monday, then a 36 hour forecast would be valid at 00Z on Wednesday.
You can use the chart to determine the forecast weather for your area. Locate your city (or nearest neighbor) and use the forecast station model found on the Online Weather Homepage or page 24 of the Homepage User's Guide to determine the forecast. The plotted data represents the forecasted surface weather conditions for the valid time arranged around the forecast station model.
You should be familiar with most of the symbols and codes used on this chart. The Probability of Precipitation (also known as PoP) is an estimate of the likelihood of measurable precipitation (liquid or the liquid equivalent of snow or other frozen precipitation) occurring during the specified time interval at any given point in the forecast area. Measurable precipitation represents an event where 0.01 inch (2 mm) or more of liquid is expected. This forecast is expressed as a percentage and the time interval is usually 12 hours. Typically, a PoP of 20 percent is usually termed a "slight chance" of precipitation for the 12 hour interval, a PoP between 30 and 50 percent would indicate a "chance" of precipitation, while 60 to 70 percent would suggest a "likely" probability that precipitation would fall.
You can also use the forecast chart to locate the expected position of weather systems. While you will not be able to draw isobars, you should be able to judge where a surface low or high pressure system is from the wind arrows. Recall from the hand-twist model that the near-surface winds tend to spiral in a counterclockwise direction in toward the center of a low. So if you scan the chart and find a region where the wind arrows tend to converge, that would be a highly probable region where a low would be located. Similarly, a high would be located where the wind arrows diverge in a clockwise sense. Since the winds near the center of the high may be light and variable, you may find locating the center of the high to be more difficult.
In addition, the forecasted cloud cover would also aid in your identification. A region where most stations would have overcast or broken cloud cover often is near a low pressure cell, while regions where clear or scattered cloud conditions would signify proximity to a high pressure cell. The included isotherms, help you to locate fronts. Fronts are along the regions where the strongest horizontal temperature contrasts occur, or equivalently, where the closest packing in the isotherms would be found. Usually, the fronts emanate from a low, with a warm front to the east or northeast and a cold front to the south or southwest.
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological Society.