Wx Watch: Sigmet Details

Sounding the severe-weather alarm

July 1, 2000

In last month's "WxWatch," we discussed airmets and touched on why so many pilots seem to ignore them. "The same routine warnings…you guys are ‘crying wolf,'" was a flight service briefer's impression of how pilots view airmets.

For this article, I tried to talk to another briefer about his opinions on pilot attitudes toward sigmets, and was told to call an FAA public relations office. Guess that's a sensitive topic.

There's no question that sigmets are the most serious kinds of weather warnings. That's why you'll hear about them first in a pilot briefing, and why briefers take extra care to identify the nature and extent of any areas affected by a sigmet.

There are two types of sigmets— sigmets (abbreviator: WS) and convective sigmets (WST).

Sigmets

Sigmets are issued for severe icing not associated with thunderstorms; severe or extreme turbulence or clear air turbulence (CAT) not associated with thunderstorms; dust storms, sandstorms; volcanic ash lowering surface or in-flight visibility to below three statute miles; or volcanic eruptions.

Text versions of sigmets—the kind you'll see in a Duats printout—will have headers that describe the area forecast zone issuing the sigmet, along with the date and time of the posting, the sigmet identifier (e.g., Papa 2—meaning the second time a particular sigmet has been issued), and the valid time for the weather events in question. Then there will be a list of VORs and other fixes that—going clockwise—define the boundaries of an affected area.

Identifiers change when a new sigmet (i.e., Quebec 1) comes out and when an existing adverse weather event travels, affecting new territory in another area forecast zone (in which case sigmets Papa 3 and Papa 4 would be issued, to stick with the example in the previous paragraph). Alphabetic designators November through Yankee—but not Sierra or Tango—are only used for sigmets. The designators Sierra, Tango, and Zulu are reserved for use with airmets, as explained in last month's "WxWatch."

Visualizing the boundaries of a sigmet can be a challenge. I mean, who can remember all the VOR identifiers and where they are located? That's why a weather advisory plotting chart is nice to use. They have all the identifiers printed right on the chart, and all you do is "connect the dots" to see the limits of a sigmet. The chart can be downloaded from the Aviation Weather Center's Web site. The same site will also let you call up preplotted sigmet boundaries that you can print out and take to the cockpit. This is a great aid, but remember that weather moves; by the time you're en route, the bad weather will have probably moved. That's why it's imperative to keep abreast of the weather as you fly, by calling the En route Flight Advisory Service (EFAS—better known as Flight Watch) on 122.0 MHz for the latest weather updates.

While any sigmet can mean dangerous weather, it's important to remember that they can cover huge areas (as much as 3,000 square miles) and the OCNL (occasional) or OCNLY (occasionally) modifiers will be used in a sigmet. To the Aviation Weather Center, occasional means that there's a better than 50-percent chance of a forecast event's happening during less than half of the forecast period. So if the valid time of a sigmet is measured in terms of, say, six hours, this means that the adverse weather may only occur within a three-hour time frame. For this reason, some pilots—on some days—may get away with flying through a sigmet area. The next day those pilots may not be so lucky.

Convective sigmets

Convective sigmets are no-fooling, near-real-time severe weather warnings. They only address thunderstorms. If you've got a convective sigmet along your route of flight, take action to avoid the affected area. Ideally, you've got lightning-detection equipment and/or onboard weather radar to help you evade convective sigmet weather. The best way to avoid convective weather, however, is to stay in visual conditions good enough to allow you to keep well away from any storm cells by either turning around, flying around, or flying above (for the jet pilots out there) the weather.

Meteorologists spend a lot of time in thunderstorm season analyzing radar echoes and their movements. When they spot radar returns that look like they might develop into something serious, they put out a convective sigmet.

Convective sigmets are issued for:

  • Severe thunderstorms. Severe thunderstorms are those that may create surface winds of 50 knots or more; hail at the surface greater than or equal to three-quarter-inch in diameter; or tornadoes.
  • Embedded thunderstorms. These are thunderstorms that exist within a greater mass of clouds. The danger here is stumbling into a cell while flying in instrument conditions. Without any storm-detection equipment you can't see trouble coming, although the Center Weather Service Units at air route traffic control centers can provide—via controllers—information useful for vectors away from areas experiencing heavy precipitation rates, or radar signatures associated with tornadic activity—hooks, scalloped edges, pendant shapes, and zones with very steep radar contours.
  • A line or lines of thunderstorms.
  • Thunderstorms greater than or equal to VIP (video integrator processor) Level 4, affecting 40 percent or more of an area of at least 3,000 square miles.

The text header of a convective sigmet will include the WST abbreviator, plus the date and time of its posting. The next entry will give a convective sigmet number, such as "4 E," meaning—in this example—that it is the fourth convective sigmet issuance in the Eastern region of the United States. Other WST regions include the Central (C) and Western (W). Then a valid time is listed, followed by coordinates defining the affected region, a description of the hazard or hazards, a forecast for the coming two hours, and an outlook for the following two to six hours.

The plotted outlines of a convective sigmet are also available on many aviation weather Web sites. Like those for airmets and sigmets, these charts spare you the effort of plotting convective sigmets boundaries VOR by VOR. It's already done for you.

Convective sigmets don't waffle much. Their language is full of vivid, useful detail, and terse explanations of the meteorological whys behind a hazardous weather phenomenon. You'll see the length and width of any lines of thunderstorms, along with cloud top information and direction of movement. If hail has been spotted, then that will be mentioned, too.

Every morning and before every shift change meteorologists brief their incoming replacements as to what has been going on, meteorologically speaking. By studying upper-atmosphere charts and consulting computer models, they know when conditions are ripe for convection. But where will that first cell pop up? How big will it be, how fast will it grow, and where and how fast will it move? These are questions to be answered with the help of the nation's network of ground-based Doppler weather radar—and pilots.

This is when the value of high-quality pireps can become invaluable. Who best to describe an area of towering cumulus than a pilot who just laid eyes on one? Realizing this, convective sigmets are often issued as soon as a pilot confirms that a storm cell has indeed popped up and become a factor in his flight. Think of us as flying guinea pigs if you want, but the fact is that we serve as high-quality verifiers of predicted, fast-breaking storms.

So next time you come across a growing cumulus—or worse—be sure to call flight watch and give a report detailing the outside air temperature (this helps give information relating to the atmosphere's stability or instability); the nature of any turbulence, the extent and type of any cloud masses; and the type of precipitation accompanying them, if any. You'll be doing forecasters and other pilots a great favor.

Now that we're smack in the middle of thunderstorm season, pay close attention to sigmets and convective sigmets. You may blow off airmets, but if you treat thunderstorm warnings the same way, you might just ruin your day—or the rest of your life.


E-mail the author at tom.horne@aopa.org.