The Weather Never Sleeps

Understanding aviation forecasts

For most people in Evansville, Indiana, on a Sunday afternoon the local National Weather Service forecast for "scattered showers, cloudy, high near 42, northwest wind around 11 mph," was all they needed. Forget the backyard barbeque.

Evansville-area pilots needed more, because forecasts for the general public don't tell pilots all they need to know. That Sunday certainly doesn't sound like a day to send a student pilot on a solo cross-country flight--but an instructor and student might be perfectly fine staying in the airport traffic pattern practicing takeoff and landings.

Public forecasts don't answer an important question for pilots: What are the ceiling and visibility forecast to be? Ceiling is the height above the ground of the lowest level of broken or overcast clouds. Visibility is how far you can see horizontally at ground level. Even if pilots are staying in the airport traffic pattern, they need ceiling and visibility information. Before taking off, they should also learn whether any dangerous weather such as thunderstorms or freezing rain are expected.

Terminal aerodrome forecasts (TAFs), which are predictions for the weather at particular airports, are among the forecasts produced to meet the unique needs of pilots. Obtain the latest TAF for any airport you plan to use, including those along the route of cross-country flights. Since TAFs aren't produced for all airports, you might have to use those TAFs for nearby airports.

That Sunday afternoon's TAF for Evansville Regional Airport (EVV), like all NWS forecasts, is in code, which is translated here:

KEVV 131738Z 131818 31008KT P6SM
-RA SCT023 OVC045
TEMPO 1820 BKN022 OVC035
FM2300 33006KT P6SM VCSH BKN045 OVC060

The item-by-item translation is:

KEVV shows it's for the Evansville Regional Airport. TAFs are listed using the International Civil Aviation Organization airport code. For airports in the 48 contiguous U.S. states, add a K before an airport's three-letter identifier. (ICAO identifiers for Alaska and Hawaii begin with P.)

131738Z: The date and time the forecast was made, the thirteenth day of the month at 1738 Zulu.

131818: The TAF is valid for the 24 hours from 18Z on the thirteenth to 18Z on the fourteenth.

31008KT: By 18Z on the thirteenth, the wind will be from 310 degrees at eight knots.

P6SM: Visibility will be greater than six statute miles. P6 is used to show more than six miles visibility. Otherwise, only numerals are used, such as 2 for two miles, or 2 1/2 for two and one-half miles.

-RA: Light rain is forecast. The minus sign means light and a plus sign would mean heavy. The TAF decoding page has the complete listing codes for different weather phenomena.

SCT023: Scattered clouds (see table below) are forecast to be 2,300 feet above the surface. You add two zeros to cloud height figures, which are always shown as three digits.

OVC045: Clouds will cover the sky (overcast) 4,500 feet above the ground. This is a ceiling since the lowest level of broken or overcast clouds are defined as the ceiling.

TEMPO 1820 BKN022 OVC035: TEMPO means temporary. Between 18Z and 20Z a ceiling of broken clouds will be 2,200 feet above the ground with an overcast layer of clouds 3,500 feet for less than half the time between 18Z and 20Z. A TEMPO section includes only the conditions that will be changing.

FM2300 33006KT P6SM VCSH BKN045 OVC060: By 23Z conditions will change to winds from 330 degrees at six knots, visibility will remain more than six miles, showers are likely in the vicinity (VSCH), a broken ceiling 4,500 feet above the ground is forecast along with an overcast layer at 6,000 feet agl.

Today's weather forecasting begins with collecting as much data as possible from all over the world about the weather at ground level and aloft. In the United States, most surface data comes from automated instruments that measure temperature, humidity, air pressure, and wind speed and direction. Weather radar and satellites supply some information about what's going on aloft and in places far from weather stations. Weather balloons launched twice a day around the world measure temperature, air pressure, and wind speeds and directions from the ground to higher than 50,000 feet, plus humidity at lower altitudes. Automated reports from many passenger and freight airliners add more upper air wind and temperature data. Pilot reports (pireps) from general aviation and airline pilots help fill gaps in this data stream.

All of this data goes into computers at the National Weather Service's Centers for Environmental Prediction in the Washington, D.C., suburbs, and also to similar centers in other nations. Supercomputers at these centers run various models that apply the laws of physics to the data to create forecasts for times ranging from an hour in the future to more than 10 days out. Meteorologists at NWS offices, private forecasting firms, and television stations use them to make their own forecasts.

Although forecasts have improved, anyone trying to forecast the weather faces some formidable obstacles, the major one being the chaotic nature of the atmosphere. It is a complicated system with several variables affecting each other in different ways. One result is that any weather forecasting computer program will have what scientists call "a sensitive dependence on initial conditions." That is, little changes now can lead to big changes in the future. This has two important consequences:

• The older a forecast, the more likely it is to go wrong. That is, a forecast made this morning for this afternoon's weather is very likely to be more accurate than one made yesterday.
• All weather forecasts contain some amount of uncertainty.

While they aren't explicitly stated, TAFs have various indications of the uncertainty of any weather forecast. The TEMPO (temporary) group in the Evansville TAF above is an example. Instead of trying to say exactly when the ceiling will lower to 2,200 feet and when it will lift, the forecasters say only that this will happen sometime during the two-hour period beginning at 18Z. The state of today's forecasting does not allow an honest, exact statement of when the clouds will lower and lift.

What is known as a PROB40 (probability 40 percent) group in TAFs is a more explicit example of characterizing forecast uncertainty. Such a group, which is not used during the first six hours of a forecast, says that there is a 30 percent to 50 percent probability of thunderstorms or precipitation during the indicated time period. For example, such a statement might read "PROB40 2102 1/2SM +TSRA." This means that there is a 30-percent to 50-percent chance that, between 21Z and 02Z, thunderstorms with heavy rain could reduce visibility to one-half of a statute mile.

By the way, in any weather forecast, a 30- percent probability of a phenomena means that during the forecast period any particular spot in the area the forecast covers has a 30 percent chance of the phenomena occurring there.

What are called from groups in TAFs, such as the FM2300 33006KT P6SM VCSH BKN045 OVC06 in the Evansville forecast above, indicate that the forecaster expects the change to occur over a period of less than one hour. If you expect to arrive at an airport any time close to such a forecast change, you should be alert to the possibility that it will occur earlier or later than the time listed.

When an important weather change is expected to take more than an hour, the meteorologist writing a TAF will use a becoming group. An example: OVC012 BECMG 1416 BKN 020 translates as: "An overcast ceiling 1,200 feet above the ground is expected to lift to a ceiling of broken clouds 2,000 feet high between 14Z and 16Z."

Based on the indications of uncertainty that meteorologists include in their forecasts, weather-wise pilots should consider the possibilities of forecasts being wrong. As pilots, we must adapt to the uncertainty of weather forecasts.

Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.