Lesson 3: Avoiding Enroute Winter Hazards

Welcome to lesson three of our four part course. In this lesson we will learn about and review some of the tips and techniques to get weather information about what lies ahead on your route of flight.

Because winter hazards are such a dynamic force and a moving target, it is important to keep up with their locations along your route of flight once you know that they may play a role in your day of aviating. How can you do that? There are many ways. The first ways begin during the flight planning process.

The National Weather Service's Aviation Weather Center is the hub of activity for aviation specific planning and warning information. We should probably look at the organization and just what it does about keeping pilots aware of the hazards of winter weather. This portion of the lesson comes from one of the co-chairs of the National Weather Association's Aviation Committee, Ms. Carolyn Kloth. Many of you will recognize her name from the SIGMETS produced by the AWC.

Where do aviation weather forecasts come from?

Since the passage of the Air Commerce Act of 1926, the National Weather Service (NWS), and its forerunner the Weather Bureau, has been responsible for providing aviation weather forecasts in support of air commerce.
In the succeeding decades, there have been a number of administrative and bureaucratic changes within the U.S. government (see historical outline). However, two facts remain essentially unchanged to the present day: 1) the Federal Aviation Administration (FAA) is responsible for regulating air commerce and managing the flow of traffic within the National Airspace System (NAS), and 2) the National Weather Service (NWS) is responsible for providing weather forecasts in support of aviation and the mission of the FAA.

One of the main providers of this weather information is the Aviation Weather Center in Kansas City, MO.

Who or what is the Aviation Weather Center?

The Aviation Weather Center (AWC) is one of 9 National Centers for Environmental Prediction (NCEP) within the NWS.

NCEP and the AWC were established in October 1995 as part of the NWS Modernization and Reorganization.

The AWC operates 24 hours a day, 365 days a year issuing forecast products exclusively for the aviation community, both domestic and international. Of all the aviation weather forecasts issued by the NWS, the AWC accounts for approximately two-thirds of them.

The AWC staff consists of 54 full-time employees, 47 of which are degreed meteorologists, plus a number of contract personnel.

What does the AWC do for aviation?

AWC operations are divided into two parts, the Domestic Branch and the International Branch.
Products issued by the Domestic Branch include:

1. Convective SIGMETs (WSTs) for thunderstorms

2. Non-convective SIGMETs (WSs) for volcanic ash, severe icing, and severe or greater turbulence

3. AIRMETs (WAs) for moderate icing, moderate turbulence and Low Level Wind Shear (LLWS), IFR conditions and mountain obscuration

4. Area Forecasts (FAs)

5. Low Level Significant Weather Prognostic Chart (LoLvl SIGWX Prog)

6. Collaborative Convective Forecast Product (CCFP) — a planning tool for air traffic control which begins when meteorologists from the AWC, the National Weather Service Laboratories, Air Traffic Control facilities and the airlines produce the best forecast for thunderstorms in the continental United States and adjust the ATC routes accordingly.

Although thunderstorms are mentioned in the FA and are depicted on the LoLvl SIGWX Prog, the primary product containing information on winter hazards of concern to aviation is the Non-Convective SIGMET and AIRMET.

What is a Non-Convective SIGMET?

In the jargon of aviation weather, a SIGMET is a SIGnificant METeorological message that contains information about phenomena that are hazardous to aviation operations. A Non-Convective SIGMET is a message that contains information specifically about severe icing, turbulence, or volcanic ash that, in the judgment of the forecaster, are hazardous to aviation operations.

In-Flight Advisory Plotting Chart

What’s the difference between a Convective SIGMET and a non-convective SIGMET?

Convective SIGMETs are issued specifically for thunderstorms that will impact aviation operations. They are issued hourly and are valid for 2 hours. The word "convective" refers to thunderstorms.

Non-convective SIGMETs are issued for the non-convective hazards of severe icing, severe or greater turbulence, and volcanic ash. These SIGMETs are issued only as necessary, and are valid for 4 hours at a time. Each non-convective SIGMET is identified by a phonetic name (e.g. Alfa, Bravo, etc.) and number, beginning with 1 (one). If the hazard for which a non-convective SIGMET is issued lasts for more than the initial 4 hours, then the SIGMET is continued under the same phonetic name but with the number incremented by one. Also, a non-convective SIGMET must be formally canceled when the phenomenon ends or weakens to less-than-severe intensity.

The Aviation Weather Center also lists international SIGMETS at their website.


More detailed information on icing conditions, especially in the low altitudes comes from AIRMETS.
An AIRMET (AIRman's METeorological Information) advises of weather that maybe hazardous, other than convective activity, to single engine, other light aircraft, and Visual Flight Rule (VFR) pilots. However, operators of large aircraft may also be concerned with these phenomena. The items covered are:

AIRMET Sierra (IFR):

1. Ceilings less than 1000 feet and/or visibility less than 3 miles affecting over 50% of the area at one time.

2. Extensive mountain obscuration

AIRMET Tango (Turbulence):

1. Moderate turbulence

2. Sustained surface winds of 30 knots or more at the surface

AIRMET Zulu (Icing):

3. Moderate icing

4. Freezing levels

These AIRMET items are considered to be widespread because they must be affecting or be forecast to affect an area of at least 3000 square miles at any one time. However, if the total area to be affected during the forecast period is very large, it could be that only a small portion of this total area would be affected at any one time.

AIRMETs are routinely issued for 6 hour periods beginning at 0145 UTC during Central Daylight Time and at 0245 UTC during Central Standard Time. AIRMETS are also amended as necessary due to changing weather conditions or issuance/cancellation of a SIGMET . (Note: Link to what are SIGMETs has been removed)

You can see current AWC AIRMETS for icing conditions at their website.

Pilots can rest assured that a trained forecaster who specializes in winter hazards is always on duty at the AWC, monitoring the skies for severe icing of concern to aviation operations.
Ground-based radar

Radar plays an important role in precipitation detection and avoidance. There are ground based radars at detect where moisture is located. Products that display that information are disseminated to you from a variety of distributors. Let's learn more about these radar systems and the products you use.

Radar theory is not easy. The University of Wisconsin at Madison has an internet course on aviation weather with background material on radars and how they work. Please review that basic material now. The Air Force Flight Standards Agency has more advanced material to help you understand the most advanced weather radar system in the world, the next-generation radar, or NEXRAD, composed of the network of WSR-88D Doppler weather radars.
If you don't have the link handy you can always see the radar product at the National Weather Service's website. The product you are looking at is called the Radar Composite based on the input from every WSR-88D in the United States. These products are updated every 15 minutes which is two complete scans of the atmosphere for this kind of radar.

The Aviation Weather Center also hosts the "experimental" ADDS (Aviation Digital Data Service) aviation weather website. There are new icing products that give much more detail on levels and extent of icing conditions along the route of flight.

Once you are airborne, there are other ways to keep you informed about winter weather. If you know that they will impact your flight, you must keep up with information that will ALWAYS give you options to avoid this hazardous weather condition. Let's look at some of the ways you can keep up with thunderstorms during flight.


There are also ways to get information about winter weather from your radio and NAVAIDS. These methods give you the bigger picture and help you see BEYOND your sensors and eyes.
Terry Lankford, a retired FSS specialist, author, and co-chair of the NWA Aviation Weather Committee, is a good source of information on the roles of the Flight Service Stations and how they can assist you in getting the 'big picture.'

by Terry Lankford

Flight Service Stations (FSS) are FAA air traffic facilities that provide preflight and inflight weather briefings, along with flight planning and other services. Selected facilities provide Enroute Flight Advisory Service—radio call "Flight Watch" and Hazardous Inflight Weather Advisory Service "HIWAS" broadcasts. (HIWAS is a continuous broadcast of weather advisories and urgent pilot weather reports (PIREPs) over selected navigational aids.)

The FSS provides pilots with access to aviation weather reports and forecasts. Many contain information on thunderstorms, among these are:


• NEXRAD weather radar (real-time)
• Weather Satellite images (real-time)
• Radar Summary Chart


• Weather Advisories, including Convective SIGMETs (WST), Center Weather Advisories (CWA), and Alert Weather Watches (WW)
• Area Forecasts (FA)
• Terminal Aerodrome Forecasts (TAF)

Significant Weather Prognostic Charts (PROGS)

Although pilots can obtain weather information through any FSS frequency, Flight Watch is dedicated to weather information. Flight Watch is specifically intended to update information previously received, and serve as a focal point for system feedback in the form of PIREPs.

Case Study

"A Bonanza pilot approached an area of thunderstorms in California's Central Valley. The pilot received the latest weather radar and satellite information, as well as PIREPs and surface observations from Flight Watch. The pilot safely traversed the area with minimum diversion or delay."

This is not a very exciting story, but that's the purpose of Flight Watch, to assist pilots in conducting uneventful flights.
ARTCC controllers are potentially helpful relaying reports of turbulence and icing, and providing advice on the location of convective activity, but the information is limited by equipment, and usually to immediate and surrounding sectors. Their primary responsibility is the separation of aircraft. On the other hand, Flight Watch has only one responsibility, weather. Flight Watch—with real-time—National Weather Service weather radar displays, satellite pictures, and the latest weather and pilot reports—provides specific real-time conditions, as well as the big picture. Additionally, Flight Watch controllers have direct communications with Center Weather Service Unit personnel and NWS aviation forecasters.

Continually updating the weather picture is the key to managing a flight, especially at high altitude in aircraft without ice protection and storm avoidance equipment, and with relatively limited range. A revised flight plan might be required. Flight Watch can provide needed additional information on current weather, PIREPs, and updated forecasts upon which to base an intelligent decision.

Many aircraft are equipped with airborne weather radar and lightning detection equipment. However, these systems are plagued by low power, attenuation, and limited range. A pilot might pick his or her way through a convective area only to find additional activity beyond. Flight Watch has the latest NWS weather radar information. Well before engaging any convective activity, a pilot should consult Flight Watch to determine the extent of the system, its movement, intensity, and intensity trend. Armed with this information, the pilot can determine whether to attempt to penetrate the system or select a suitable alternate. ATC prefers issuing alternate clearances compared to handling emergencies in congested airspace and severe weather.

So how do you get in touch with your FSS? Typically, pilots access the FSS for preflight briefings by telephone. The universal number is 1-800-WX-BRIEF (1-800-992-7433).

Inflight, pilots can obtain weather information through the FSS "Inflight Position" or "Flight Watch." FSS communication frequencies are published on aeronautical charts and in the Airport/Facility Directory. Flight Watch uses the common low altitude frequency of 122.00 MHz. For high altitude flights, each Air Route Traffic Control Center (ARTCC) has been assigned its own discrete frequency.

When contacting an FSS facility use the aircraft's complete identification, approximate location, and the frequency you expect a response (except Flight Watch).

For example:

"Rancho Radio, Cessna One One One Five Romeo, Fresno, listening One Two Two point Five Five, Over."
"Oakland Flight Watch (or Flight Watch), Piper Two Eight Six Two Tango, Monterey, Over."

Its really important that you know that you are part of the aviation weather system, too!

PIREPs are the most important ingredient for Weather Advisories and forecast amendments. Certain phenomena can only be observed by the pilot. An urgent need exists for information on weather conditions at flight altitudes, along routes between weather reporting stations—especially in mountainous areas—and at airports without weather reporting service. In many cases the pilot is the best and only source of actual weather conditions. The need for accurate pilot reports cannot be overemphasized.

ARTCC and tower controllers do accept PIREPs, but weather is a secondary duty and, unfortunately, PIREPs aren't always passed along. If at all possible, PIREPs should be reported directly to an FSS or Flight Watch.

The ABQ Flight Service Station has some winter aviation tips you might want to read now.

You can take a tour of the ABQ Flight Service Station if you want.

Let's now explore the local FSS that you deal with. Go to the FAA's FSS links page and find out which FSS serves your area. (Note: Link to FAA FSS has been removed)

Hazardous Inflight Weather Advisory Service

HIWAS is a significant way to find out about the big weather picture. Recorded by FSS, they are broadcast on the frequencies of selected NAVAIDS. The station and frequencies are usually on the enroute charts you fly with or check the FSS websites in the previous paragraph of this course.

 Center Weather Service Unit (CWSU) at ATC facilities and thunderstorms
The following lesson on the role of the meteorologist at the CWSU's came from Mr. S. Douglas Boyette who is a former dispatcher who has worked with the National Weather Service since 1986. He now serves as an Aviation Forecaster at the CWSU in Memphis, Tennessee.

Role of the ARTCC Meteorologist Pertaining to Thunderstorms

The FAA presently maintains Center Weather Service Units (CWSUs) within each of their 21 Air Route Traffic Control Centers across the lower 48 states and Alaska. Each of these CWSUs are staffed by National Weather Service meteorologists, generally operating during the peak traffic hours between 5:30 am and 10:00 pm seven days a week. The CWSUs came into being in 1980 due to a recommendation by the NTSB.

After examining a crash involving a Southern Airways DC-9 that flew into the hail shaft of a thunderstorm, the NTSB felt that having a meteorologist in each ARTCC would enhance safety. The ability to quickly pass hazardous weather information to the FAA personnel in direct communication with the pilot was the driving force.

Although low ceilings/visibilities, turbulence and icing can play havoc with en-route air traffic, thunderstorms cause the most trouble. Organized convection, such as squall lines or large clusters of thunderstorms (referred to as Mesoscale Convective Systems--MCS), produce most of the weather-related problems for both the airlines and controllers. Airlines generally do not prefer flying their passengers into hazardous weather activity, therefore en-route deviations or other "traffic management unit" (TMU) initiatives must be undertaken by the FAA. The end result? Obviously delays, but delays are a trade-off many people will accept if it means a safe flight.
n today’s world, there are rush hours in aviation just like on the ground. Since most of the major airlines operate under a hub and spoke concept, this requires large numbers of aircraft to depart and arrive into a handful of airports in distinct "rushes" each day. These rushes translate into large numbers of airplanes flying along the same airways at the same time. Throw large areas of convection into the mix and you have trouble, especially if the cloud tops are higher than FL410. For example, in the spring (and sometimes fall), long squall lines commonly form ahead of advancing cold fronts. A solid line of thunderstorms may develop from Chicago to Dallas and block off several major airways, requiring TMU decision-makers both locally and at the Central Flow Command Center in Washington, DC to orchestrate "programs" to effectively steer aircraft around the weather. Coordination is essential since there is no desire to re-route traffic unnecessarily. Meteorologists at each center are involved in these coordination events by providing two scheduled stand-up briefings each day for facility supervisors, supplementing those with various as-needed short-term forecasts for their respective TMUs. If CWSU meteorologists can provide effective input about expected hazardous conditions several hours prior to an event, dollars can potentially be saved by avoiding unneeded delays and re-routes. Conversely, if action plans call for air traffic to be routed into areas that have a high probability of becoming blocked by thunderstorms, input from center meteorologists could perhaps save lives, and most certainly time and money. Yes, it is sometimes quite difficult to be accurate when forecasting thunderstorms, but therein lies the ultimate goal of the CWSU meteorologist: the safe and efficient flow of air traffic through their airspace.

CWSU meteorologists also issue short-term hazardous weather advisories in the form of a Center Weather Advisory, or CWAs. The CWA is similar to an AIRMET or SIGMET, but is usually issued for small-scale events, or in rapidly developing situations not yet covered by AIRMETs or SIGMETs. An example of a CWA pertaining to thunderstorms might be a rapidly developing, but isolated supercell. The storm may contain a tornado, but since it is isolated, most aircraft will have no trouble deviating around it. This information is mostly useful to smaller, lower flying aircraft, especially those without on-board radar, however if the storm is heading for an airport the impact would obviously affect larger commercial aircraft as well. This could include aircraft hundreds of miles away that are heading to that particular airport, who may need to be placed into en-route holding to allow for spacing should the weather temporarily close the field.
Want to interact with ATC? See where there are real-time CWAs happening now.
Pilot to Metro (USAF)

For members of the United States military, there is usually a weatherman only a radio call away while airborne. Usually, your needs are one of the highest priorities of the base weather stations.

The staff of the Directorate of Weather at Headquarters USAF prepared a lesson on the Pilot-to-Metro Service (PMSV) and the important information available on thunderstorms for this course.

Finally, contact your Dispatcher (if you are flying commercially and have one) for the big picture as time allows.

Before we end this lesson, we want to make you aware of some research that taking place to help you and those involved with aircraft operations know where thunderstorms are located.

The National Center for Atmospheric Research is working on several icing products. You can see their research at their website. Pilots will want to see the experimental real-time icing products for commuter airlines and overlays of precipitation types on radar data.
Study Questions:
1. What are the two primary products produced by the AWC that warn about icing conditions?
2. How long is a non-convective SIGMET valid?
3. What does the number stand for in the Alfa3 name?
4. What intensity level of icing is covered by SIGMETs?
5. What intensity level of icing is covered by AIRMETs?
6. How long are AIRMETs usually valid?
7. What is ADDS?
8. Where is the best location to give an icing PIREP if you have the time?
9. What is a CWSU?
10. What weather products are produced by CWSU?
11. What are some research efforts that will improve icing warnings to pilots?