The Weather Never Sleeps

Weather planning for a flight should begin hours, or even a couple of days, before takeoff. Such longer-range planning can help you to decide whether to put off the flight because the weather may be more than you or your aircraft can handle. If the forecast looks safe, tuning into the weather ahead of time will help you to identify trends and know what to look for when you obtain your detailed preflight weather briefing.

Television or Internet weather reports intended for the general public can get you started, but unless the forecast is completely hopeless for flying, you need additional information such as the outlook for ceilings, visibility, turbulence, and icing. The National Weather Service's Aviation Weather Center produces just the product you need: the low-level significant weather prog chart. Low-level means it covers weather from the surface to 24,000 feet. (There's a high-level chart for those who fly higher.) Significant means it depicts large weather features, and prog means it's a prognosis or forecast. Any weather map that's labeled prog is a snapshot of what forecasters expect the weather to be at a particular time in the future, called the valid time. If a map is labeled analysis, it's a snapshot of what the weather was actually doing at a particular time in the past.

The low-level significant weather prog chart has four panels and is issued four times a day. (Figure 1 is an example.) The two panels on the left show the forecast for roughly 12 hours after the observations on which the forecast is based. The two panels on the right are forecasts for 24 hours after the observations. The VT on the bottom of each map shows the time the forecast is valid, in Zulu time.

The top maps on both the 12- and 24-hour sides show weather conditions from right above the surface up to 24,000 feet, including ceilings or visibility that qualify as marginal VFR or IFR. The upper maps also show areas where moderate or stronger turbulence is forecast, freezing levels aloft, and where the freezing level should be at the surface. The bottom two maps show conditions at the surface, including the positions of fronts and high- and low-pressure areas. The lower maps also show where and what kind of precipitation is forecast.

Each set of maps is produced about seven hours before the valid time for the 12-hour forecasts because it takes time to collect global observations and apply them to the computer models that produce the forecasts the maps are based on. A forecast for seven hours ahead is a forecast for the distant future as far as most flights go. This is why you should use significant weather prog charts to get the general weather picture but rely on more frequently updated forecasts, like terminal aerodrome forecasts, for specific data. It's also why you should get observations of what the weather is actually doing as close to takeoff time as possible.

With the limitations of the prog charts in mind, let's see what they can tell us, starting with the top panels. Figure 2 is the 12-hour forecast for a day in early December. The areas outlined in red, like those over Tennessee and Kentucky, show where IFR conditions are forecast. The areas enclosed by scalloped blue lines indicate where pilots can expect marginal VFR conditions (ceilings from 1,000 to 3,000 feet and/or visibilities from three to five miles).

Moderate or stronger turbulence is expected in the area outlined by a dashed yellow line over parts of Washington, Idaho, and Montana. Note that the symbol for turbulence over Canada has a white arrow pointing into this area. The number 150 below the turbulence symbol shows that turbulence is expected from the surface to 15,000 feet. If turbulence is expected to begin above the surface, two figures will be used, one over the other with a line in between. (Figure 3 shows the most common symbols used on the prog chart and other weather charts.)

The solid blue line that runs south across the middle of Washington and Oregon and then runs to the East Coast shows where the freezing level is at the surface. The dashed green line marked 120 (from Florida to southern Louisiana), 80, and 40 shows where the freezing level is 12,000, 8,000, and 4,000 feet above the surface. The 4,000- and 8,000-foot freezing level lines turn north, cross the surface freezing line in Texas and New Mexico, and run north to the Canadian border. This shows that, while the air north of the blue line should be freezing at the surface, there will be above-freezing temperatures aloft. This situation means that freezing rain or drizzle, which can cause severe icing, is possible.

Figure 4 is a bottom panel from a significant weather prog chart that shows what's forecast to happen at the surface. A quick glance shows that the most active weather is expected in the southeastern United States, and this day did turn out to be a bad one, with rain, snow, and high winds. Panels like Figure 4 have a wealth of information, beginning with the low-pressure centers and fronts that account for much of the country's challenging weather, especially from the fall through winter and into late spring. The L off the Georgia coast is the center of a low-pressure area that formed over the warm Atlantic Ocean and drifted north and east. Fronts are shown by the conventional weather map symbols with added information given by the three-number codes on the map near each front. These codes are listed in Figure 5.

If you look off the South Carolina coast you'll see the number 257 south of the warm front in that area. Using Figure 5 to translate, you'll see that the front is a warm front of moderate intensity that's expected to change little and that it has waves on it.

Waves are kinks in a front that sometimes develop into separate low-pressure centers. The weather around a growing wave can become worse than forecast with thick clouds and precipitation. The stationary front over Mexico is also one to watch. To begin with, new storms can form on stationary fronts, and the code 037 with it tells us that the forecasters think this quasi-stationary front is weak but growing stronger and that it, too, has waves on it.

The four-digit numbers on the map give the forecast central pressure for the highs and lows. For instance the high where Wyoming, Colorado, and Utah meet is forecast to have a pressure of 1,034 millibars. The low over the Gulf of Mexico is expected to have a central pressure of 1,018 millibars. When you see that a low is forecast to have a pressure of less than 1,000 millibars, you know it's going to be the center of a reasonably strong storm.

The word trof, like that over North Dakota, shows where forecasters expect an elongated area of low pressure to form at the surface. Such troughs can bring clouds and precipitation to areas where the map does not show a front.

Areas where precipitation is expected are circled in green with weather symbols, like those shown in Figure 3, to indicate the kind of precipitation. If the area within a green circle is shaded, like the ones over central Tennessee, it indicates that the precipitation is going to cover most of the area. To see how such precipitation is likely to affect your flight, refer back to the top panels for their forecasts of ceiling and visibility.

In addition to showing you the general weather picture for particular times in the future, the low-level significant prog chart can help you to see how weather systems will be moving. The chart can help answer the question: "Will a front or storm center move into the area I plan to fly over?"

Low-level significant prog charts are one of the best tools for helping you to see the big weather picture as you prepare for a flight. But, like any other weather product, they are not the final word. You need to also look at observations of the weather that were made after the chart's observations were taken to see if the weather seems to be working out the way the forecasters expected. And always get the detailed forecasts for your route and your destination.