The morning surprise. That's what I call the shock that pilots get when they call flight service and hear of a sudden change — for the worse, of course! — in the day's forecast. This can be especially frustrating during the warmer months of the year. Yesterday, the briefers' forecasts were for a walk in the meteorological park, and featured mentions of severe-clear, light winds, and minimal turbulence. This morning the world changed, and now potentially severe air mass thunderstorms are heavily advertised. How could things have gone downhill so quickly?
Meteorology is not an exact science. Forecasters will eagerly tell you that, reconciled as they are to making prediction errors great and small through their careers. Sure, our understanding of meteorological science is getting better each year, as reflected in the escalating accuracy of computerized forecast models. These models crunch tons of weather data — wind speed and direction, moisture transport, convergent and divergent vertical flows, upper-air dynamics, temperature profiles, recent synoptic features, you name it — then apply thermodynamic and other physical laws to come up with dozens of predictions a day. The speed and frequency of the predictions are functions of the banks of high-speed computers that the National Weather Service operates around the clock and every day. This aspect of forecasting is called numerical weather prediction.
Numerical prediction has proven seductive. Look at most any broadcast television weather report. There's Guy Smiley, confidently gesturing at an animated weather map, and explaining why the rain will arrive two days from now, around noon. The animation is the product of a computer model. So is the forecast. For all we know, Guy was an English major. To forecast, he relies solely on the models. And they're accurate — most of the time.
Model site
There are many Internet sites that feature forecast models. To see a few models for yourself, visit www.weather.unisys.com/eta/ and click on the models listed in the menus at the left of your display screen. Although the symbology can be vexing, the graphics should make the forecasts clear. — TAH
This kind of blind trust won't cut it in aviation forecasting. However, temptation always lurks. Take the amended thunderstorm forecast mentioned earlier. Models for the past two days may not have identified convective threats, but when first-string meteorologists show up for the morning shift they — like we pilots — must take another look at what's happening right now.
Jeff Johnson, a meteorologist and a senior aviation forecaster for Meteorlogix, says that predicting the times that scattered thunderstorms will form can be the trickiest forecast challenge. "You use the models, sure, but what do you do when you have conflicting information — or no information — about what's happening in a given area?" Johnson mused. "That's when you have to think on your feet."
Johnson says that when new meteorologists are hired to join Meteorlogix's 28-member aviation unit in Burnsville, Minnesota (Meteorlogix has 50 other meteorologists in Boston serving agricultural, utility, and other interests), the neophytes "find a whole new world in operational meteorology. This isn't like the theory they learned in college or graduate school, and it takes more than two months to get comfortable with calling your own shots," he said, referring to a new-hire's training period.
Johnson begins his forecasting by using the models to look ahead. "The ECMWF [European Center for Medium Range Weather Forecasts] model is a good predictor for three to five days in the future. The AVN [Aviation] model is good for two to four days ahead but I think the Eta model is the best bet for a 24-hour heads-up," Johnson said. "That's because this model has a higher resolution." By resolution Johnson means more information per data grid. For updates every three hours, the RUC [Rapid Update Cycle] model can be consulted.
Meteorlogix doesn't restrict its forecasters to using the six most popular models. The company, which provides customized weather reports and forecasts for AOPA's Web site, and custom aviation weekend forecasts for AOPA ePilot, USAirways, and numerous other private aviation-related customers, also relies on five of its own proprietary forecast models. This helps the company provide forecasts tailored to the specific needs of each customer. Sometimes those needs really put the pressure on forecasters. Take airline customers, for example. Dispatchers ask, "Will visibilities stay below one mile?" If forecasters say so, then extra fuel must be taken on, in case flights must hold. "Will visibilities remain below one-quarter mile?" If forecasters say yes, then the airlines must consider delaying or canceling flights.
Johnson augments his thunderstorm forecasts by delving into his own storehouse of experience. "After a while you see the same patterns over and over," he said. "And this helps you look at the models and say, 'No, that can't be right,' and after a while you develop a better sense of what's most likely to happen."
How do you know when thunderstorms are likely to reach severe levels (surface winds 50 knots or more, three-quarter-inch hail, or tornadic)? Johnson's rules have relevance for all who fly.
"I look at the vertical wind profile. If the winds aloft are turning with altitude, then you know that well-organized storms, and maybe tornadoes, are possible. So if you see that the winds at 1,000 feet are out of the south, and at 3,000 or 4,000 feet they're stronger and blowing out of the southwest, and at 10,000 feet they're blowing hard out of the west, you know that there's going to be a lot of shear and rotation in any cloud masses.
"Then there's tilting with altitude," he continued. "If strong winds aloft blow the upper parts of a storm cell downwind, and away from the rest of the cell, that's a sign that the thunderstorm will last longer and be self-sustaining.
"What happens is that rain falls in a different area than the storm's updraft area — maybe a mile away. The upwind part of the storm has all the rainfall, and the downwind part of the storm has all the updrafts." Normally, a thunderstorm cell has both updrafts and downdrafts, and eventually the storm loses energy and dies out after these forces fight it out. But with this two-part type of storm structure, the cell feeds itself and strengthens over a longer time," Johnson said. "Of course, after thunderstorms begin, we look at Doppler weather radar and satellite imagery to forecast their behavior."
Pilots often become frustrated when briefers and other weather briefing sources hedge their bets on the timing and likelihood of thunderstorms. But pilots aren't alone. All you have to do is imagine the pressure surrounding Johnson and his ilk on a day promising convection. For all concerned, it's almost a relief when storms finally crop up, because now there's something tangible to focus on. The switch is from prediction to observation and analysis.
For pilots, the job becomes one of avoidance. That's great as long as you maintain a healthy visual separation from any buildups, and use cockpit weather detection devices — if you're lucky enough to have lightning detection instruments, airborne weather radar, or, luxury of luxuries, datalinked Nexrad imagery.
The cardinal rule is to avoid flying into clouds near thunderstorm cells. Once in this embedded-in-cloud situation, you're blind to any dangers ahead and in the worst of worlds: You know thunderstorms are out there, but you can't pinpoint their locations. Best to turn around and fly to better weather. Hopefully, the meteorologists who have fed your preflight briefers will give you an idea of where to steer if things turn sour. You can always call flight watch (122.0 MHz) for the latest information on adverse weather, escape paths to better conditions, and late-breaking METARs and TAFs.
Pilots and meteorologists alike have access to more weather data than ever before, so an absence of information is not the problem it was, say, 10 short years ago. Instead, the challenge is in sorting through this avalanche of data. Meteorologists benefit by providing more accurate assessments and forecasts. But pilots must confront weather face-to-face, and make what can be fateful decisions. All alone. As Johnson said, it takes time to be comfortable calling your own shots. There's no substitute for on-the-job training.
E-mail the author at [email protected].
AOPA members can ask questions of Meteorlogix forecasters by visiting the weather page on AOPA Online ( www.aopa.org/members/wx/).
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