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Wx Watch: Convection CountdownWx Watch: Convection Countdown

Avoiding thunderstorms via daily Web surfingAvoiding thunderstorms via daily Web surfing

When it comes to a pilot's primal fears, an inadvertent thunderstorm penetration must certainly rank at the top of the list. That's why we need to develop a comprehensive thunderstorm forecasting and tracking strategy.

When it comes to a pilot's primal fears, an inadvertent thunderstorm penetration must certainly rank at the top of the list. That's why we need to develop a comprehensive thunderstorm forecasting and tracking strategy. Of course, everybody has his own way of preparing for cross-country flights in the convective season, and you're legal as long as you obtain a thorough preflight weather briefing. Legal, but not necessarily systematic or wise. It may not be good enough to wake up on the day of your long-awaited flight to pick up a DUATS or flight service briefing, then make your go/no-go decision. Why? On trips where schedule changes need to be made with as much advance notice as possible, you need to develop a feel for how the weather is trending in the days before your flight. Besides, watching the weather in the days before your flight is a great educational experience.

A good strategy is to use the Internet to watch the weather. There are a number of excellent Web sites that let you self-brief in the days leading up to your flight, and in many cases they can provide you with more information than flight service can. Here's a suggested procedure for a day-by-day count-down before a planned flight. The terms Day 2 and Day 3 are used, meaning "tomorrow" and the "day after tomorrow," respectively. Day 1 is today.

Two days ahead

Begin your Internet surfing two days prior to your planned flight. I start by checking the National Weather Service Storm Prediction Center (SPC) Web site. The day's current (Day 1) weather watches will be featured in graphical form at the top of the home page, and each will be accompanied by a mesoscale discussion. Mesoscale discussions include helpful graphical portrayals of adverse weather dynamics, and these are easily understood. More challenging are the text portions of the mesoscale discussions. Anyone with a background in meteorology will derive a great deal from these discussions, and so will pilots who have taken their weather self-study beyond the bare bones required to pass the FAA knowledge exams.

Click on All SPC Forecasts at the left of the home page, then scroll down to Current Convective Outlooks. Click on the Day 3 Outlook map, and there you'll see any zones where severe thunderstorms are expected. You'll see an amber line with an arrow at one end; the area to the right of the line is the anticipated area of convection. Below the map will be a forecast discussion.

You won't see any highs, lows, or fronts on this map. That's because it's intended to show where the risks of the worst convection are predicted — not the synoptic conditions. For that, you should check other online sources of forecast weather information — such as the prognosis charts issued by Meteorlogix on AOPA Online.

And remember: Convective outlooks are concerned with severe thunderstorms (those with 50-knot surface winds, three-quarter-inch hail, or tornadoes). Plenty of garden-variety — yet still very dangerous — storms can exist outside the lines delineating SPC convective outlooks.

One day ahead

This is the time to check out the Day 2 convective outlook on the SPC Web site. Forecasters will have fine-tuned their predictions, so forecast accuracy gets a boost. Once again, you'll see the worst anticipated convection to the right of the arrow-headed lines on the chart. Is the area along your planned route of flight? If so, check the 48-hour prognosis (Prog) chart to see where any fronts or areas of precipitation are forecast. The Aviation Digital Data Service (ADDS) Web site is a good source, although Meteorlogix and other providers also provide essentially the same graphics.

Terminal aerodrome forecasts (TAFs) also begin to include forecast information at this point. TAFs cover a 24-hour period, and are issued and updated four times per day. They will give you your first inkling of the wind, ceiling, and visibility conditions for your time of departure. Forecast thunderstorm and/or rain shower activity also will be mentioned.

Day 1

On "D-day" — the day of your planned flight — the number and accuracy of weather forecast and reporting products increase dramatically. By this time, there should be no surprises if thunderstorms are predicted. After all, you've been watching the situation for two days!

Before you call flight service or print out a DUATS briefing, check the following sources for thorough graphical presentations of the weather at hand:

  • Day 1 convective outlooks, on the SPC Web site. The areas of the most violent weather will have been pinpointed by now, and the mesoscale discussions accompanying them will be at their most insightful. Again, thunderstorms can exist outside the severe zones on the convective outlook.
  • Weather depiction charts, to locate areas of instrument meteorological conditions.
  • The Collaborative Convective Forecast Product (CCFP, posted under the Convection link on the ADDS Web site). Although not an operational product intended for use by general aviation, the CCFP can be used for supplemental information. It shows areas of convective activity for the next two-, four-, and six-hour time frames. The CCFP, while designed for use by the airlines and air traffic control to help flow control, can help in visualizing the locations of the biggest storm cells.
  • The National Convective Weather Forecast (NCWF) Java tool and loop. These are also under the Convection link on the ADDS Web site. Here you'll see animated radar imagery, and can call up blue polygons that describe the predicted positions of thunderstorm cells for the next hour. METARs and TAFs also can be superimposed on the NCWF Java tool page. Although the NCWF does not take the place of sigmets or convective sigmets, it is another helpful supplemental method of visualizing storm progress and dynamics.
  • Convective sigmets. These official warnings tell of the location, intensity, and direction of movement of active thunderstorms. They are the gold standard of official thunderstorm warnings.
  • Center Weather Service Unit Advisories (CWAs). Located under the ADDS' Advisories link, CWAs are like convective sigmets in that they give the coordinates, movements, and cloud-top heights of storm cells. An associated product, the Meteorological Impact Statement (MIS), goes into more detail, and can include information on freezing levels, icing conditions, lowering ceilings and visibilities, and turbulence levels.
  • Single-site and national radar views. These are available on most aviation weather Web sites, and should be consulted just prior to takeoff for the latest radar information.

With all this as background, you're ready to formalize the preflight briefing process with a call to flight service. Now's the time to collect all the relevant METAR and TAF updates, check the winds aloft, and gather the latest pireps. Nothing you're told about the nature of any thunderstorm activity should come as any surprise by this time. By starting your briefing way back on Day 3, you'll have already formed an educated opinion about the situation. Depending on the trends you've been watching, your experience level, your airplane's capabilities, and your personal minimums, you may well have decided to cancel the trip on Day 2 — and used the Day 1 information to confirm your good judgment.

E-mail the author at [email protected].

Links to additional information about convective weather may be found on AOPA Online.

Wx Briefs

Tip of the month: Bad neighborhoods

You can use constant pressure charts to help predict likely areas of dangerous fronts and thunderstorm complexes. Just look for troughs on the 500-millibar (this roughly corresponds to 18,000 feet msl) constant pressure chart. You're looking for pronounced troughs, which are evidenced by U-shape, southward-projecting height contours. The southeast corner of the trough's U is the trouble spot, just to the east of the trough's apex, or southernmost point. This is where temperature contrasts are greatest at altitude, where cores of the highest jet stream winds are strongest, and where divergence aloft is at a maximum. When air diverges — spreads out at altitude — low-level warm air rushes in to replace it. If this converging air is warm enough, and saturated enough, and the air it ascends through is cold enough, it will rise and condense its moisture into convective clouds. In addition to severe thunderstorms, research also has shown that an upper-level trough's southeast corner can also be the place where tornadoes and squall lines break out.


A specially instrumented Gulfstream V operated by the National Center for Atmospheric Research (NCAR) is roaming over California's Sierra Nevada, looking for dangerous rotors and mountain waves. The data it collects will help forecasters tweak upper-level computer models so that severe turbulence can be identified more accurately. In acronym-happy meteorology-speak, the Gulfstream V is called HIAPER (for High-Performance Instrumented Airborne Platform for Environmental Research) and the research project is called T-REX (for Terrain-Induced Rotor eXperiment; see " Pilot Briefing: A T-REX is Born," page 58). The Gulfstream V will make flights of up to 10 hours each, and airborne data will combine with information from radiosonde balloons, ASOS sites, and wind profilers to produce a three-dimensional representation of rotor structure and behavior. For more information, see the T-REX Web site. — TAH

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