After selecting visible or IR, you can view either the latest image or an animated loop of images from the past several hours. Then generate an image by clicking one of the sector codes on the map. Clicking on "Contiguous U.S." will provide a national view.
The time of the image will be at the top left in what's called Coordinated Universal Time, abbreviated UTC. In the summer, Illinois is 5 hours behind UTC; in the winter, 6 hours. Images are usually no more than 15-30 minutes old.
On a visible or black-and-white infrared image, clouds appear white, while land and water are dark. On a color-enhanced infrared image, greens and blues - representing colder temperatures - are usually clouds, while land and water appear in yellows and oranges.
Information on the other types of satellite images is available at the site under "Page Info" near the top right of the main page.
Tip: On a visible loop, snow cover looks like clouds that aren't
moving.
National
Doppler radar sites
This page features a map of the United States with links to all of the
NWS radar locations in the country. Choose your location to bring up
the current radar image from that site. What you'll see is the
short-range base reflectivity, which is just a basic radar image
depicting the intensity of precipitation - if there is any.
Light precipitation will show up in blue, moderate in green, and heavy in yellow and red. When precipitation is occurring, images should update every five to seven minutes. The time of the image is at the top and bottom in UTC and at the top right in local time.
To see which direction the precipitation is moving, choose the short-range loop under "Base Reflectivity."
When no precipitation is expected, the radar is usually set to the more sensitive "clear air" mode, which allows detection of small objects such as dust, insects, and birds. To verify which mode the radar is in, just run a loop and note the time between images - in clear air mode, images are updated only every 9 to 11 minutes.
More information on the various radar images is available by selecting "Radar Information" under "Additional Links."
Tip: Many local TV stations have their own radar and put current images and loops on their Web sites. Also, when wintry weather is in the forecast, check out radar images at
The images at these sites are updated less frequently but will distinguish between rain, snow, and ice.Computer models that predict the weather are the basis of every forecast. Current weather observations from all over the world are fed into the models, which then use that information to predict the weather. The predictions come out in the form of graphics and numbers that are available on the Web. They can be understood, even by the untrained meteorology enthusiast, with a little explanation.
One of the more user-friendly sites at which to view model graphics is http://weather.unisys.com
The four main models used to forecast weather in the United States are available from this page. They are NGM, Eta, AVN, and MRF. They're listed under "Forecasts" on the left side of the Unisys page. NGM and Eta are primarily for short-range forecasting (over the next couple of days), while AVN makes predictions out to 6 days and MRF out to 10 days.
Part of forecasting involves checking the various models to see how they differ. After a while, you'll notice that some are better than others, depending on the type of weather and location.
After choosing a model and selecting the link, you'll want to pull up the most basic and fundamental of the graphics available - the sea-level pressure and precipitation forecast chart. In all the models except MRF, you can generate this chart by clicking the main image on the model page. (On the MRF page, choose "SL Pressure/Precip Plots" under "Index" on the right navigation bar and then "9 panel (48 hr-10 day)" under "Plots" on the right.)
This chart shows the predicted location of low- and high-pressure systems and where and how much precipitation is expected to fall at various intervals from the time the model was run.
So what do all the lines and colors actually mean? The cyan lines are contours of pressure in millibars. Low-pressure areas marked with Ls are usually associated with cloudy and unsettled weather, while high-pressure areas marked with Hs often bring sunny and dry conditions.
The main reason to look at this chart, however, is to get an idea of where, when, and how much precipitation might occur. The colored areas represent how many inches of liquid precipitation are predicted to fall during the 12 hours prior to the valid forecast time at the top right of each panel (the color legend is at the bottom of each panel).
The yellow lines are contours of what's called "1000-500 millibar thickness," an indicator of temperature. Notice that in each panel, all but three of the lines are dashed. As a rule of thumb, the middle solid line, known as the "540 line," often represents the division between rain (to the south) and snow (to the north). Typically, 10 inches of snow contain about one inch of water. Therefore, to come up with a snow accumulation forecast, meteorologists usually multiply the amount of liquid precipitation predicted by about 10.
Tip: NGM, Eta, and AVN are updated every 12 hours, at around 10:15
a.m. and p.m., 11:15 a.m. and p.m., and 1:00 a.m. and p.m.,
respectively (all times EST). MRF is updated once a day at about 4:00
a.m. EST.
Model Output Statistics
Sometimes it's nice to have an idea what the weather will be like at a specific time, rather than just knowing the predicted high and low temperature for the day. That's why, in addition to the colorful graphics, the models also produce alphanumeric forecasts called Model Output Statistics, or MOS (pronounced "moss"). These are predictions of various parameters at three-hour intervals, including temperature, wind, and cloud cover.
The NWS Meteorological Development Lab Web site is one of the more convenient places to access MOS forecasts:
After choosing a state or territory on the left side of the screen, check off a specific location on the right and scroll all the way to the bottom. Pressing the "Submit Query" button will bring up the latest MOS forecast for that location from all four models.
All those numbers and letters might seem a bit confusing at first, but a few minutes glancing over our MOS guide above should help you read the Eta, AVN, and NGM MOS.
The MRF MOS varies slightly, but the codes used are mostly the same. For more information about the MRF MOS forecast, visit http://www.nws.noaa.gov/tdl/synop/mexcard.htm
Tip: The models tend to underestimate the effect of cloud cover on
temperature. When overcast skies are predicted, daytime temperatures
will often be a bit lower than forecast, and nighttime temperatures a
bit higher.
Area forecast discussions
By now, it's certainly evident that weather forecasting is not an exact science, although many forecasts sound as if they're a sure bet.
For those who want a behind-the-scenes look at how certain - or uncertain - the forecast really is, there's the Area Forecast Discussion, issued four times a day by every NWS forecast office. The most recent discussion from any office can be accessed using the map at http://wxweb.meteostar.com/disc
Originally intended for internal use among nearby forecast offices, the Area Forecast Discussion has become popular with weather enthusiasts everywhere. Prepared by the meteorologist on duty, the discussion usually starts with a brief analysis of current conditions, followed by an in-depth look at the various models and the reasoning behind his or her forecast.
The one obstacle to understanding the discussions, aside from the fact that they can get a bit technical, is that the forecasters sometimes use their own version of shorthand. For a list of commonly used abbreviations and acronyms, go to http://aviationweather.gov/info/domestic_contractions.html
Tip: The AVN and MRF models have recently been combined and renamed
the Global Forecast System (GFS). GFS and Eta are the models most
often referred to in Area Forecast Discussions.
Other resources