Wx Watch: Searching for Sigmets

Thunderstorm season is upon us, and with it comes the need to thoroughly check for convective advisories. These are available via many National Weather Service products: Convective Outlooks (abbreviated "AC" in text); Severe Weather Outlook Charts; Radar Summary Charts; Sigmets (WSs); Convective Sigmets (WSTs); Center Weather Advisories (CWAs); and Radar Reports (Rareps). In addition, several private weather service providers offer not just the above products, but also near-real time radar imagery derived from the nation's network of WSR-88D Doppler weather radars. Still more information is available over the Internet, or from weather providers that have fax services. Then there's The Weather Channel, a perennial favorite of pilots everywhere in the United States.

Let's not forget Terminal Aerodrome Forecasts (TAFs), Area Forecasts (FAs), and hourly surface observations (METARs). They'll also mention thunderstorm forecasts and reports. Oh, and for those so inclined, the Composite Moisture and Stability Chart can be checked for areas likely to experience thunderstorms. Then there are Severe Weather Watch Bulletins (WWs), which also define areas of possible severe thunderstorm and/or tornado activity-mets (WSTs); Center Weather Advisories (CWAs); and Radar Reports (Rareps). In addition, several private weather service providers offer not just the above products, but also near-real-time radar imagery derived from the nation's network of WSR-88D Doppler weather radars. Still more information is available over the Internet, or from weather providers that have fax services. Then there's The Weather Channel, a perennial favorite of pilots everywhere in the United States.

Let's not forget Terminal Aerodrome Forecasts (TAFs), Area Forecasts (FAs), and hourly surface observations (METARs). They'll also mention thunderstorm forecasts and reports. Oh, and for those so inclined, the Composite Moisture and Stability Chart can be checked for areas likely to experience thunderstorms. Then there are Severe Weather Watch Bulletins (WWs), which also define areas of possible severe thunderstorm and/or tornado activity — but these are plain-English reports aimed at the nonflying public. In any event, you get the idea. There is plenty of information about thunderstorms out there. It's just a matter of knowing where, when, and how the advisories are issued. That, and acting on them.

A good way to start your flying day in the warmer months is to check on the Convective Outlook and Severe Weather Outlook Chart. They're available over DUATs, or can be called up from the National Weather Service Storm Prediction Center's Web site ( www.spc.noaa.gov). The AC comes out twice a day — at 0700 and 1500Z — and is usually a few short paragraphs of text full of abbreviations, some of them jargon specific to meteorologists (they use these products too). The text uses the terms SLGT (slight), MDT (moderate), and HIGH to determine risk categories, and GNL (general) and SVR (severe) to determine storm severity. Slight risk means there's a chance of storms covering 2 to 5 percent of a delineated risk area; moderate risk means 6 percent to 10 percent coverage; and high risk means more than 10 percent coverage. Severe thunderstorms are defined as those with surface winds 50 knots or greater, hail three-quarter-inch or larger in diameter, and/or tornadoes. To define an area at risk of storms, the AC will use the phrase "to the right of a line..." and then list a string of location identifiers. Connect the dots formed by the identifiers, look to the right of the line, and there is the risk area.

The Severe Weather Outlook Chart is a graphic version of the AC, and it's available from weather fax providers and the Storm Prediction Center site mentioned earlier. Here, you can see those lines, as well as see the hatched-in areas that mark any severe weather outlook areas. This chart is a great way to pick up on the general situation, and is a great aid in pinpointing the worst trouble spots.

Radar summary charts are fine, as far as they go. And that's the problem. They come out as often as once an hour and show severe weather watch boxes and lines of thunderstorms, but radar summary charts are glimpses of the past, and can be hours old by the time you see one. It's always absolutely vital to check the issue and valid dates and times of any weather report, but with radar summary charts, it's especially critical. Never use them to plan a route, because it's almost certain that the situation will have changed by the time you take off. Another thing about radar summary charts: Because they use radar information, the tops reported in them won't correspond to the actual cloud tops. Radar sees precipitation and large-droplet conditions best. It can't identify plain old clouds, so the actual tops — as well as their horizontal ranges — will likely be several thousand feet higher than the "tops" labelled on the radar summary chart and will extend many miles farther from the edges of any depicted precipitation returns.

Rareps can be handy in locating precipitation and storm cells in relation to individual ground-based radar sites. The extent of any precipitation echoes are referenced by ranges and bearings (true bearings, that is) from the radar site. A list of range and bearing coordinates is published, and you "connect the dots" to outline the precip or cells. For example, 060/075 means 060 degrees and 75 nautical miles. A sequence of fixes like these make up a zone of detected precipitation. Abbreviations such as NC (no change), MT (maximum tops), +/+ (strong storms, increasing in intensity), LN (line), A (area), C (cell), and others help give more information about the character of the conditions at hand.

Convective sigmets (significant meteorological information) are issued whenever severe, embedded, and lines of thunderstorms are detected or forecast. Also, thunderstorms of Level 4 intensity covering 40 percent or more of a 3,000-square-mile area will trigger a convective sigmet. Sigmets imply severe or greater turbulence, severe icing, and low-level wind shear. They are issued at the first evidence of a problem, and then again at 55 minutes past the hour. In Alaska and Hawaii — where convective sigmets aren't used — sigmets are issued for thunderstorms in lines or embedded in cloud masses, as well as for hail and tornadoes.

TAFs, which are airport-oriented forecasts, come out every eight hours and will also mention the likelihood of thunderstorms and other adverse weather. METARs are hourly surface reports from selected airports with weather observers, and they also mention any storm activity.

Consulting the lifted and K indices on the composite moisture/stability chart is another good way to scope out the kind of instability that creates thunderstorms. The lifted index is a measure of temperature at altitude. The K index is a measure of the moisture content and saturation at altitude. The lifted and K indices are presented together as a fraction, with the lifted index on top and the K index below. There's a chance of convection if the lifted index is between minus two and minus four, and an even better chance if the index drops to the minus five to minus eight level. If there's a low (say, minus six) lifted index and a high (35-40) K index, then expect severe thunderstorms.

Briefing for storms

Convective sigmets, TAFs, and METARs will be given as part of any standard or abbreviated weather briefing. Pireps should also be provided; ask for them if the briefer doesn't volunteer these in-flight reports from pilots. They can be very helpful in proving or disproving forecast conditions, and pointing the way toward better weather should problems crop up in flight. You should make sure to check their issue dates and times, because many times pireps from the previous day are left in the system. You can also ask for rareps and descriptions of some of the graphic materials from a briefer, but you're at a disadvantage when it comes to envisioning trouble spots on a chart that you can't see. A big part of that disadvantage is that products such as sigmets and ACs use the connect-the-dots method of describing areas of storm coverage. Without a good knowledge of the location identifiers' three-letter abbreviations-as well as their positions — it's impossible to concoct a mental image you can take to the cockpit. Today's flight planning software, which depicts sigmets and airmets graphically, can help in this department. Coated charts with all the weather location identifiers printed on them are also available. With these, you can use a felt-tip pen to mark the outlines of any convective sigmet or convective outlook areas.

To make matters worse, here's a bulletin for you: The location identifiers have just been changed — effective March 1. So if you did have the identifiers memorized, take a look at the new ones to see the changes. As before, many of the identifiers remain those of high-altitude VORs.

Apart from AOPA's internet weather products (available in the members-only section of AOPA Online), there is another site that provides text as well as graphic products. It's run by the National Weather Service's Aviation Weather Center in Kansas City, Missouri ( http://aviationweather.gov), and its sigmet charts connect the dots for you on a dedicated chart — although a text message is provided as well. By the way, consulting the AWC's Web site now counts as a legal weather briefing, according to a recent FAA policy statement. Moreover, it has a chart with the new location identifiers.

In flight

Even though you might be armed with scads of preflight information backing up your "go" decision, it's vital to keep checking the weather en route. This goes for flying at any time of the year, not just thunderstorm season. As we all know, weather can change quickly.

One painless way to check on the weather as you fly is to simply tune in certain frequencies and listen. VORs with HIWAS (hazardous in-flight weather advisory service) capability broadcast weather information such as convective sigmets, airmets, urgent pireps, and widespread adverse conditions continuously. Look on your sectional, WAC, low-altitude enroute, or high-altitude navigation charts to find VORs with HIWAS. They're marked with a small blue or black square within the navaid identification box.

ATIS (automatic terminal information service), AWOS (automated weather observation system), and ASOS (automated surface observation system) frequencies broadcast continuous reports of surface conditions. Only ATIS, however, will mention any storm activity on or near the airport it serves. AWOS and ASOS stations of the future may be able to make reports on thunderstorm activity, location, and intensity, but so far this technology is not operational.

Flight watch (on 122.0 MHz) is a service intended for in-flight weather updates. As your flight progresses, it's always a good idea to stay in touch with flight watch so that you can learn of any changes in forecast weather and, should it come to it, find out where the best alternate landing sites may be. Simply dial in 122.0, call up the nearest flight watch facility (it can be found on the inside back page of the Airport/Facility Directory), and don't forget to mention a prominent nearby VOR or other fix in your call-up. This will help flight watch specialists to locate and use the communications outlet nearest your position.

Flight watch specialists should tell you about any newly issued convective sigmets and other weather advisories and can tell how the weather is faring both along your route and at your destination. If the weather is heading downhill, then it's time to come up with an alternate plan: land soon, turn around, or divert to an alternate.

Flight service stations can also be contacted on their discrete frequencies. Check your charts for the proper frequencies in the area you're flying in; these are published above navaid boxes. Again, mention a fix near you as well as the frequency that you're calling on. Briefers monitor many frequencies, and you'll get a faster response if you tell them right up front which one you're using. Once in contact with an FSS specialist you can obtain a wide range of services: full route briefings, flight plan activation and cancellation, more detailed weather information, and maybe even direction finding (DF) steers for those times when you're temporarily in doubt as to your location — or flat out lost.

Center Weather Advisories (CWAs)

Pilots flying on instrument flight plans have one more method of obtaining advance warning of convective and other adverse weather, such as icing, strong surface winds, or severe turbulence. Center Weather Advisories are broadcast over ATC frequencies when sigmets are first issued, then rebroadcast as necessary as conditions change. Pilots are advised to tune in HIWAS (which also broadcasts CWAs) or call flight watch for any amplification they might need concerning a CWA.

Pilots flying under VFR can also hear CWAs — as long as they're either tuned in to an air route traffic control center frequency, or receiving flight following services from ATC. For this reason, the VFR flight-following tactic is one of the best ways to keep abreast of changing weather conditions. Just remember that flight following is granted on a workload-permitting basis. If controllers are up to their ears in IFR flights, you probably won't be given the luxury of flight following — but you can still listen in on center frequencies.

Sure, summer poses challenges when it comes to avoiding thunderstorms. And yes, it's still possible to become entangled in a sudden, unpredicted storm. But with a review of the reports and forecasts such as those we've reviewed here, you'll have gone as far as you can to fly storm-free in the months to come. All that's left in the equation is to follow good in-flight storm avoidance measures. That's a subject we'll address in the next few issues.

Wx Briefs

• In " Wx Watch: Icing Rules of Thumb" (January Pilot), I mentioned the age-old advice concerning the inflation of de-ice boots. This recommendation says to allow one-quarter to three-eighths-inch of ice to form on the wing leading edges before inflating the boots. Why? Because if you inflate the boots continuously, according to this theory, a bridge of ice will form over the leading edges and will be impossible to shed. But a recently convened FAA conference on ice bridging seems to indicate that there's no such thing as ice bridging. Go ahead and inflate the boots as often as you want, a report says. Ice will shed no matter how much you let it build. The myth got its beginning, some experts think, in the 1930s, when boot inflation channels of the day were smaller, didn't inflate as much, and were fewer in number than the channels in today's boots.
• Trade-A-Plane's new weather site ( www.weathertap.com) now has data from the national lightning detection network. Lightning flashes are mapped and can be animated to watch storm movement over the past 90 minutes. A subscription to Trade-A-Plane's weather service goes for $63 per year, and includes other aviation weather products, such as satellite and radar imagery, as well as METARs and TAFs.
• Honeywell's new Primus 700/701 weather radar is being used in tests aimed at approval for use by helicopters flying to offshore oil rigs in Canada. The goal is to achieve lower approach minima by using the radar to "paint" the rig, and allow pilots to fly closer than otherwise permitted. The radar's resolution allows helicopters to safely approach within 1,000 feet of a rig experiencing low-visibility weather. At its lowest scale, the 700/701 radar screen is marked in increments of one-tenth of a mile. Tests are currently under way using a Eurocopter AS 332 Super Puma and an oil rig situated 200 nm from St. John's, Newfoundland — one of the foggiest spots in the world.

E-mail the author at [email protected].