Because of their rather frequent, inadvertent prevarications, meteorologists draw a lot of flack. You know how it goes. You turn on the local news show, and the weatherman issues tomorrow's forecast with the usual cop-outs. The weather will be "partly cloudy," for example, or "partly sunny," with a "50- percent chance of a passing shower." Great. Does "partly cloudy" mean that sunny skies will be the rule, with clouds interspersed here and there? Does "partly sunny" mean "mostly cloudy"? The weatherman is smiling because he's covered his bases. But if you're planning an outdoor event, you're apt to be miffed. You just want the truth, and the meteorologist can't deliver.
As we all know, it's the same with aviation forecasts. Area forecasts (FAs) and terminal forecasts (FTs) are liberally sprinkled with conditional phrases designed to cover all sorts of eventualities. But while the National Weather Service and flight service stations may be skilled practitioners of the art of posterior coverage, it's still important for pilots to understand what aviation forecasters really mean when they qualify their predictions.
First, some little-publicized news about area forecasts. In June 1991, the FAs underwent a major change. Prior to that time, the FAs were divided into five sections: hazards, synopsis, icing and freezing level, turbulence, and significant clouds and weather. All this made for a sometimes bewildering barrage of information. Now, the new, improved FA has just two segments — one is a summary of hazards and flight precautions, the other a synopsis and summary of clouds and weather by geographic regions.
One benefit of this rearrangement is that FAs became easier to read. But FAs can still carry bits of the same old-fashioned evasive phraseology. Here are a few examples.
- Occasional or occasionally. These terms are abbreviated OCNL and OCNLY, respectively. Translated, this means that there is a greater than 50 percent chance of the named weather occurring for less than half the forecast period. Because an area forecast's forecast period is 12 hours (the outlook — OTLK — covers the six hours following the forecast period), a mention of "occasional moderate to severe turbulence" is a signal that you have a better than fifty-fifty chance of encountering this type of turbulence over a duration of no more than six hours of the forecast period.
- Chance. Abbreviated CHC, this means there is a 30- to 50-percent probability of a certain phenomenon occurring during the forecast period.
- Slight chance. Abbreviated SLGT CHC, this indicates a 10- to 20-percent probability and demonstrates a bit more certainty on the part of the forecasters.
When mentioning precipitation or convective activity, FAs can waffle using the following terms:
- Isolated (ISOLD). A mention of isolated precipitation or thunderstorms is the lowest order of alert. It simply denotes that individual cells are expected.
- Widely scattered (WDLY SCT). This term is used when less than 25 percent of a forecast region is expected to receive precipitation and/or experience a thunderstorm.
- Scattered (SCT). Here, the odds increase to 25 to 54 percent of a forecast area being covered by precipitation or thunderstorms.
- Numerous (NMRS). This means that more than 54 percent of a forecast region could be covered by precipitation or thunderstorms.
It's important to remember that the percentages issued in an FA's conditional statements regarding precipitation and thunderstorms refer to expected areas of coverage, not the probability of a weather event. So a mention of "numerous" cells means that more than half of a certain area is expected to be covered — not that there is a greater than 54-percent chance of cells occurring at all.
On the other hand, an FT's conditional statements deal exclusively in terms of probabilities. So while some terms used in FTs appear the same as those in FAs, they can have completely different meanings.
For example, the term occasional in an FT means that forecasters predict a greater than 50-percent probability over less than one-half the forecast period. Chance means that there is a 50-percent probability or less.
Another term — vicinity (VCNTY) — is used when various weather conditions are expected to occur between 5 and 25 nm from the airport. Remember that FTs are only valid for an area within a 5-nm radius of an airport complex.
Whenever precipitation is expected, an FT may use the terms chance or slight chance. In these cases, chance means a 30- to 50-percent possibility of occurrence. Slight chance means a 10- to 20-percent possibility.
These conditional terms can have significant operational effects. For example, in choosing an alternate airport, it may be unsafe, illegal, or unwise to select an alternate airport carrying a prediction of an occasional ceiling or visibility below the prescribed minimums. For that matter, the same may hold true of situations where a chance of low ceilings and visibilities is mentioned.
For VFR operations, a waffling forecast may give enough pause to convince a non-instrument-rated pilot to cancel a flight. That's good, assuming the "chance" of adverse weather actually materializes. It's not so pleasant when the weather turns out better than originally forecast, and a perfectly safe VFR flight could have otherwise been conducted.
Is there reason to suspect that the NWS or FSSs abuse these conditional terms, applying them liberally so as to conveniently flip-flop when the weather doesn't turn out as expected? Perhaps. But let's not be too hard on aviation meteorologists. After all, they must often develop their forecasts with as little as a few previous surface observations, a computer analysis or two, some upper air soundings, and their own intuition. With this scarcity of information, it's surprising that FTs and FAs are as accurate as they are. Think of the pressure to come up — at least three times a day, in the case of FTs — with ceiling forecasts to within a few hundred feet or visibility predictions to within fractions of a mile. With so many atmospheric variables at work, so many geographic vagaries, and such a large number of stations to address, it's no wonder that forecasters tend to err on the safe side.
Strictly speaking, they're hardly ever wrong — the conditional terms help make sure of that. Consider that even when an FT is issued without conditional terms, the best the NWS can promise is a greater than 50-percent likelihood of an accurate forecast for better than half the forecast period. In other words, better than half-right for more than half the time. Apply the conditionals — OCNL, CHC, SLGT CHC, etc. — and the forecast reliability starts to slide gently away from the realm of the certain and toward the realm of the very vague.
It can be hard to develop trust in the people whom you know have issued bad forecasts in the past. But rely on FSSs we must. Just bear in mind that flight service's own confidence in FT reliability hovers around the 50-percent mark and that the appearance of any conditionals is a definite warning that things may not turn out as predicted.
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