Of all the aviation products of the National Weather Service, the most perplexing one to pilots is the area forecast (FA); six FAs cover the entire United States. This is where, if you use a computer, pages spew out of your printer, describing weather by state as well as by specific potential hazards. If you get telephone briefings, it is where the briefer finds the "flight precautions." The latter are for IFR conditions, mountain obscuration, moderate turbulence, moderate icing, and sustained surface winds of 30 knots or more. These items are covered in airmets if they are not adequately addressed in FAs; though in practice, they are usually offered in both forms. More severe items are covered in sigmets and convective sigmets.
The reason the FA is such a challenge to wade through — especially if the computer prints two or three different FAs for your flight — is because it paints with such a broad brush. The forecaster is trying to give the best possible word description of what the weather might be over the area, leaving nothing to chance. The flight precautions alert you to things that might occur in the area described during the forecast period.
The time interval is an important part of these forecasts, which are issued three times a day. For our purposes, consider the Boston FA issued on March 23 at 0945Z. The synopsis was valid until 0400 the next day, and the clouds and weather portion was valid until 2200 on March 23, with the outlook valid to the twenty-fourth at 0400.
Within FA text portions, times are given for changes in the weather. Pilots using computers and telephone briefers have to evaluate the times in the forecast and the conditions that actually exist and make a judgment on whether or not the forecast conditions are pertinent to the flight at hand. I have had telephone briefers tell of an airmet for icing, for example, but add that "we know it's not there now because they are way off on the freezing level today." On the day that quote came from, the forecast freezing level was 5,000 feet, and the actual freezing level was 11,000 feet.
Despite the fact that FAs are often filled with cries of "wolf," there is good stuff in there. A pilot who wants to know as much as possible about the atmosphere before wrapping the airplane with air and launching is well advised to consider everything in the FA. It is a best shot by a professional meteorologist at describing a cross section of the atmosphere all over the area, perhaps influenced a bit by a desire to cover everything. Terminal forecasts (FTs), on the other hand, cover only the weather in the vicinity of the airport.
For an example, let's look at that flight on March 23, a day when the weather conditions were difficult to predict. There was a closed low aloft over the eastern United States, a condition that offers the forecasters the least clues. The flight was from northern Virginia to the Cincinnati area, and the Boston FA was issued 2 hours 45 minutes before takeoff. It contained flight precautions on IFR conditions, icing from 5,000 feet to 18,000 feet, and mountain obscuration but no mention of turbulence. Tops were forecast to be layered to 18,000 feet. Part of the Chicago FA was also included because it covered Kentucky, a state adjoining the route.
The applicable weather feature on the synopsis was a low in Indiana with a warm front through a Cincinnati-to-London, Kentucky, line. My airport at Winchester, Virginia, does not have an FT, and the Boston FA, which covers Virginia, called for 3,000 broken, variable scattered, with 5,000 overcast. Along the route, light rain was called for, even though the television weather was showing what was described as a wide area of moderate to heavy rain along the way. The destination airport in the Cincinnati area does not have an FT either, and the FA for there showed about the same as for the departure airport. However, the FT for an airport near the destination — a different product that comes from a different place — suggested somewhat worse weather than the FA. The weather over the entire area was expected to deteriorate as the day wore on.
All this suggested a rather benign IFR ride to Ohio. The wind aloft was not forecast to be strong, and with the cloud tops forecast to be at 18,000 feet, I decided to file for Flight Level 180 and spend a couple of hours in the sunshine — all with a grain of salt. I learned long ago that, because closed lows aloft do weird things to the weather, one had best go prepared. That is why I embarked on a 2.5-hour flight with almost seven hours of fuel on board.
The wind at the airport was out of the southeast, and though the weather was forecast to be good, it looked scuzzy. The pilot of a Bonanza who didn't see the airport until the last moment on his approach would probably agree. When I was cleared to fly, I found some light turbulence as well as solid clouds from 1,000 feet on up — much worse than forecast.
A forecast that is not correct is almost as valuable as one that is correct because it also tells you something about actual, as opposed to expected, weather conditions. In this case, it told me that the synopsis the forecaster thought he was dealing with was not quite correct, so I had to be ready to deal with a changing situation. One of the first things I did was ask the center controller if he could find out about tops. Airplanes as high as FL230 reported that they were still in the soup, so I scratched the plan to fly in the sunshine at FL180. Perhaps the forecaster was deluded by radar reports that put precipitation tops as uniform and below FL180. That simply does not always equate with cloud tops. The air at 8,000 feet was with some light jiggles, and the rain was heavy enough that the radar was attenuating at 5 miles. The temperature was well above freezing, as it was forecast to be on the winds aloft forecast, but there was another substantial error. The wind at 9,000 was forecast from 210 degrees at 33 knots. The direction was correct, but the velocity was almost twice the forecast, at 60 knots.
The bumps in the clouds continued, and some pilots reported it as moderate turbulence, though it met only the definition of light turbulence. Then, when about halfway across West Virginia, I perceived that I was flying close to the bases of clouds. I requested and received 6,000 feet as an assigned altitude, and this made a world of difference. It was below the bases of most clouds (there was some low scud), and the air was smoother, though the rain persisted.
The wind aloft was quite interesting. It reached a peak velocity of 60, then waned to about the forecast value of 33, then increased back to 60. The surface winds were quite light despite the strong winds at 6,000 feet. My assumption was that I passed through the slope of a warm front not long after takeoff. That explained the strong wind aloft. Then I flew away from the slope of the front, explaining the lighter wind for a while. Finally, I was getting close to the surface low pressure system, which would explain why the wind increased again.
A couple of hours into the flight, I looked back at the FA and found some accuracy and some inaccuracy. It had painted a picture of basically the same weather all along the route of flight where the weather was quite different along the way. Soon after takeoff, I flew through what had all the characteristics of a weak warm front. Then I was in an area of rain and relatively high ceilings, which was what the forecaster thought it would be all along the way. As I neared the Cincinnati area, lower solid clouds showed up, and instead of an easy approach in relatively good weather, it appeared that it would be one to minimums, or at least close to minimums. So there were three different weather conditions along the route.
The low-level wind was strong enough as I maneuvered for the approach that the controller had to revise the vector to final approach two times. The surface wind was still light, so I expected the wind-shear turbulence from about 3,000 feet on down. The ceiling was just above the minimum descent altitude for the approach, but later in the day, instead of the forecast IFR conditions in the area, it cleared, and the sun actually came out for a while.
The FA for this day was obviously not right on the money. The FTs were quite a bit closer to actual conditions.
Despite the FAs' problems, I was glad that I studied them, and the information that I gained from them was extremely useful as I flew along. Once I deduced that the weather systems were a bit farther east than indicated on the synopsis, it was possible to rationalize everything else.
When we do encounter forecasts that are not right on the money, we have to ask if this can affect the safety of a flight. The answer had best be an unqualified "no" because forecasting is not an exact science. While forecasters can often predict a major weather event — they were precisely correct on the big East Coast snowstorm in March — in aviation, we are concerned about whether or not a thunderstorm will be on Victor 128, whether the cloud tops are at 18,000 feet or 25,000 feet, whether the wind will be 33 knots or 60 knots at 8,000 feet, and whether the ceiling and visibility will be 2,000 and 3 or 900 and 1. In other words, when we fly, we have to deal with both major weather events plus what really amounts to weather trivia. The little details don't mean a lot to the ground- pounders, but they do mean a lot to us, and this is where the FAs are useful. If we use them properly, they become an alert mechanism to make us probe the system for information. When they are incorrect, that is also an alert and might even suggest a landing and new quest for information.
Related Articles
