Lesson 3: Avoiding Thunderstorms in the Terminal Area
|Welcome back to lesson three.
This lesson is about avoiding thunderstorms in the terminal or airport area.
While we will cover a lot, we won't cover it all. We will actually concentrate
on the wind shear and microburst hazards in the terminal area in lesson
five. For our international students, this lesson is about thunderstorm
detection, reporting, and forecasting in the National Airspace System (NAS)
of the United States. Several weather organizations from around the world
will join us later on in the course to supplement this material with the
weather services in other countries.
It is my privilege to introduce today's "guest speaker," Mr. Jack May, the Acting Director of the National Weather Service's Aviation Weather Center (AWC) in Kansas City, Missouri.
When I learned to fly nearly thirty years ago, my instructor gave me this maxim: It's better to be on the ground wishing you were in the air than in the air wishing you were on the ground. Your safe flight depends on your ability to discern the weather hazards that may lie on your route and at your destination. Your safety and the safety of your passengers depend on your ability to recognize the weather hazards related to your flying experience and the equipment on your aircraft.
There is one hazard, however, that is dangerous to all pilots and all types of aircraft -- thunderstorms, especially at the time of take off and landing. The violent updrafts and downdrafts of a thunderstorm are deadly to an object dependent on the smooth flow of air for controlled flight.
The National Weather Service WSR-88D weather radar network has improved our ability to observe thunderstorms, their development, and their movement. The research community has improved our ability to predict thunderstorms. But thunderstorms are just as dangerous as they were thirty years ago. In fact, several major accidents have helped us understand thunderstorm dangers that were unknown thirty years ago.
We at the Aviation Weather Center in Kansas City are pleased you have taken the time to learn more about the dangers of thunderstorms in the terminal area.
Jack May, Acting Director
National Weather Service Aviation Weather Center
And now on to the lesson...How do we as aviators and operations personnel know about thunderstorms in the terminal or airport area? In this lesson will cover the basic ways an aviator will be told about the weather either in written or verbal form.
All aviation weather begins at the airport.
The first step in the process of creating aviation weather is the observation. This report is also one of the first clues that the airport you are using has a thunderstorm impacting it. Before we even worry about the encoding or decoding language, we have to understand the things a observer looks for when an observation is taken. Only then can we understand exactly what the observer is trying to communicate when he or she mentions gusting winds, falling pressures, and the other thunderstorm clues. The following sections come directly from the Federal Meteorological Handbook Number 1, the Weather Observers ultimate guide.
|5.4.1 Wind Direction.
The wind direction shall be determined by averaging the direction over a
2-minute period. When the wind direction sensor(s) is out of service, at
designated stations, the direction may be estimated by observing the wind
cone or tee, movement of twigs, leaves, smoke, etc., or by facing into the
wind in an unsheltered area.
5.4.2 Variable Wind Direction. The wind direction may be considered variable if, during the 2-minute evaluation period, the wind speed is 6 knots or less. Also, the wind direction shall be considered variable if, during the 2-minute evaluation period, it varies by 60 degrees or more when the average wind speed is greater than 6 knots.
5.4.3 Wind Speed. The wind speed shall be determined by averaging the speed over a 2-minute period. At designated stations, Table 5-1 shall be used to estimate wind speeds when instruments are out of service or the wind speed is below the starting speed of the anemometer in use.
5.4.4 Wind Gust. The wind speed data for the most recent 10 minutes shall be examined to evaluate the occurrence of gusts. Gusts are indicated by rapid fluctuations in wind speed with a variation of 10 knots or more between peaks and lulls. The speed of a gust shall be the maximum instantaneous wind speed.
5.4.5 Peak Wind Speed. Peak wind data shall be determined with wind speed recorders. The peak wind speed shall be the maximum instantaneous speed measured since the last routine observation.
5.4.6 Wind Shifts. Wind data shall be examined to determine the occurrence of a wind shift. A wind shift is indicated by a change in wind direction of 45 degrees or more in less than 15 minutes with sustained winds of 10 knots or more throughout the wind shift.
5.5.5 Peak Wind Data. The peak wind shall be reported in the remarks section whenever the maximum instantaneous speed in knots (since the last observation) is greater than 25 knots
The visibility parameters are:
Prevailing visibility. The visibility that is considered representative of visibility conditions at the station; the greatest distance that can be seen throughout at least half the horizon circle, not necessarily continuous.
Sector visibility. The visibility in a specified direction that represents at least a 45 degree arc of the horizon circle.
Surface visibility. The prevailing visibility determined from the usual point of observation.
Tower visibility. The prevailing visibility determined
from the airport traffic control tower (ATCT) at stations that also report
either in the form of drops larger than 0.02 inch (0.5 mm), or smaller drops
which, in contrast to drizzle, are widely separated.
Hail. Precipitation in the form of small balls or other pieces of ice falling separately or frozen together in irregular lumps.
Small Hail and/or Snow Pellets. Precipitation of white, opaque grains of ice. The grains are round or sometimes conical. Diameters range from about 0.08 to 0.2 inch (2 to 5 mm).
Squall. A strong wind characterized by a sudden onset in which the wind speed increases at least 16 knots and is sustained at 22 knots or more for at least one minute
Funnel Cloud (Tornadic Activity) (1) Tornado. A violent, rotating column of air touching the ground. (2) Funnel Cloud. A violent, rotating column of air which does not touch the surface. (3) Waterspout. A violent, rotating column of air that forms over a body of water, and touches the water surface.
Present weather qualifiers fall into two categories: intensity or proximity and descriptors. Qualifiers may be used in various combinations to describe weather phenomena.
Intensity/Proximity. The intensity qualifiers are: light, moderate, and heavy. The proximity qualifier is vicinity.
|Intensity of Precipitation.|
|When more than one form of precipitation is occurring at a time or precipitation is occurring with an obscuration, the intensities determined shall be no greater than that which would be determined if any forms were occurring alone.|
| The intensity of precipitation
shall be identified as light, moderate, or heavy in accordance with one
of the following:
Intensity of Rain or Ice Pellets. The intensity of rain
and ice pellets shall be based on the criteria given in Table 8-1, Table
Descriptors are qualifiers which further amplify weather phenomena and are
used with certain types of precipitation and obscurations. The descriptor
qualifiers are: shallow, partial, patches, low drifting, blowing, shower(s),
thunderstorm, and freezing.
Shower(s). Precipitation characterized by the suddenness with which they start and stop, by the rapid changes of intensity, and usually by rapid changes in the appearance of the sky.
Thunderstorm. A local storm produced by a cumulonimbus cloud that is accompanied by lightning and/or thunder.
Present weather is reported when it is occurring at, or in the vicinity of, the station and at the time of observation. Unless directed elsewhere in the Handbook, the location of weather phenomena shall be reported as:
• "occurring at the station" when within 5 statute miles of the point(s) of observation.
• "in the vicinity of the station" when between 5 and 10 statute miles of the point(s) of observation.
• "distant from the station" when beyond 10 statute miles of the point(s) of observation.
Thunderstorm. A thunderstorm occurring with or without accompanying precipitation shall be reported when observed to begin, to be in progress, or to end. In addition to reporting a thunderstorm in the body of the observation, remarks may be added to report the time, location, and movement of the storm
Beginning of Thunderstorm. The beginning of a thunderstorm shall be reported as the earliest time:
(1) thunder is heard;
(2) lightning is observed at the station when the local noise level is sufficient to prevent hearing thunder; or
(3) lightning is detected by an automated sensor.
Ending of Thunderstorm. The ending of a thunderstorm shall be reported as 15 minutes after the last occurrence of any of the above criteria.
Beginning/Ending Times of Precipitation, Tornadic Activity, and
b. Tornadic Activity. At designated stations, the time tornadic activity begins or ends shall be reported to the nearest minute. The beginning and ending times shall be reported in a SPECI and the next METAR after the event
c. Thunderstorm. At designated stations, the time thunderstorm(s) begins or ends shall be reported to the nearest minute. The beginning and ending times shall be reported in a SPECI and the next METAR after the event. Beginning and ending times of separate thunderstorm(s) shall be reported only in a METAR if the intervening time exceeds 15 minutes
The FAA and NWS have several different types of AOS. There are AWOS and
there are ASOS.
Now lets give you a chance to review or learn the written weather code
that comes from an observation, and is used to describe the forecasted
weather for an airport.
|Okay, you get to choose. Please
read one or more of the following presentations of the METAR/TAF codes and
look at the ways that are used to describe the phenomena we learned about
earlier in this lesson. When it comes to reading the TAF forecasts, PLEASE
use the study questions on the left for this lesson to make sure you know
some of the finer details of the code.
You can read about it in the Pilot's Handbook of Aeronautical Knowledge. Look in Chapter 11.
You can look in the Federal Meteorological Handbook.
You can review the quick reference on METAR/TAF codes.
Allright, can you decipher these codes? If not, go back to the quick
reference card above.
By the way, what criteria will change an observation? In the US there
INTERESTING FACT: The ASOS CANNOT make a change to the observation (make a SPECIAL OBSERVATION) for any reason between 46 and 53 minutes past the hour. This is the time it is doing a regular observation. Even if you had a tornado heading straight for the airport or on the airport, there is no way for anyone to override this process to update the observation or broadcast the fact over the ATIS.
Come from National Weather Service (NWS) forecast offices located in
the fifty states. Most are NOT on the airport.
Can you read a TAF?
aviation weather come from?
USAF and US Army personnel taking this course will want to link to a lesson that teaches where their weather information comes from.
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 thunderstorms. 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:
Convective SIGMETs (WSTs) for thunderstorms
Non-convective SIGMETs (WSs) for volcanic ash, severe icing, and severe or greater turbulence
AIRMETs (WAs) for moderate icing, moderate turbulence and Low Level Wind Shear (LLWS), IFR conditions and mountain obscuration
Area Forecasts (FAs)
Low Level Significant Weather Prognostic Chart (LoLvl SIGWX Prog)
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 thunderstorms of concern to aviation is the Convective SIGMET.
What is a 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 Convective SIGMET is a message that contains information specifically about thunderstorms that, in the judgement of the forecaster, are hazardous to aviation operations.
The Convective SIGMET product consists of three parts, or bulletins, each containing one or more individual Convective SIGMET advisories for a particular part of the country. The East bulletin (WSTE) covers that part of the U.S. east of 87 degrees West longitude. The West bulletin (WSTW) covers the U.S. west of 107 degrees West longitude. And the Central bulletin (WSTC) covers the middle part of the country between 87 degrees West longitude and 107 degrees West longitude (see Inflight Advisory Plotting Chart).
In-Flight Advisory Plotting Chart
The three Convective SIGMET bulletins are transmitted every hour at h + 55. The individual Convective SIGMET advisories are valid for 2 hours. Each Convective SIGMET advisory is identified by a discreet number and letter, the letters corresponding to the bulletin in which the thunderstorm is occurring (i.e., E for the East bulletin, C for the Central, etc.). The numbers are incremented sequentially, and are reset to 1 (one) every day at 00Z in each bulletin.
Each individual Convective SIGMET advisory is defined using the points contained on the Inflight Advisory Plotting Chart. These points are mainly high-altitude VORs that are evenly distributed across the continental U.S., and should be found on both high-altitude (IFR) and low-altitude (sectional) navigational charts.
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.
Most importantly, a Convective SIGMET for thunderstorms implies the presence of the associated hazardous phenomena of severe icing, severe or greater turbulence, and Low Level Wind Shear (LLWS), as well as possible IFR conditions.
What determines when a Convective SIGMET is needed?
A Convective SIGMET is issued when any of the following minimum criteria are met:
Severe thunderstorms (SEV). A severe thunderstorm is defined as one containing hail ³ 3/4 inch in diameter, winds ³ 50 knots, and/or a tornado.
Embedded thunderstorms (EMBD). A thunderstorm is defined as embedded when it occurs within a larger area of rain or rain showers, or is hidden in multi-layered clouds and/or IFR conditions, or is otherwise obscured such that its presence is not visually apparent.
Lines of thunderstorms greater than 60 nm long with greater than 40% coverage of significant radar echoes (i.e., those ³ VIP 4 or _ 40 dBz).
Areas of thunderstorms greater than 3,000 square nm in size with greater than 40% coverage of significant echoes (see above).
In the judgement of the forecaster, the SIGMET-ed thunderstorms pose a threat to aviation operations.
If no thunderstorms meet the above criteria, the bulletin is still transmitted at h+55, but it will contain a negative message
It should be noted that a negative message does not indicate the absence of thunderstorms. It indicates only that any thunderstorms that may be occurring do not meet the above minimum criteria.
Who determines whether a Convective SIGMET should be issued?
The Convective SIGMET Unit at the AWC is responsible for monitoring the weather across the continental U.S. (CONUS) plus coastal waters for thunderstorms that may impact aviation operations. The five meteorologists who staff the unit all have a minimum of a Bachelor of Science degree in meteorology, and are trained specifically to forecast thunderstorms. The unit operates around the clock, with one meteorologist on duty at any given time.
The forecaster uses a wide variety of data sources to monitor the atmosphere and assess whether a Convective SIGMET is needed or not. Satellite, radar, and lightning data are all essential for monitoring the current state of the atmosphere. Other data sources include hourly surface observations, twice-daily rawinsonde data, and a variety of computer-generated forecasts that mathematically mimic the atmosphere. All of this information must be synthesized by the forecaster on duty in order to determine if a) conditions are right for thunderstorm development, and b) a given thunderstorm or group of thunderstorms meets the minimum Convective SIGMET criteria.
Pilots can rest assured that a trained forecaster who specializes in thunderstorms is always on duty at the AWC, monitoring the skies for thunderstorms of concern to aviation operations.
Link to an example of a Convective SIGMET product
|This lesson we've covered some
important aspects of knowing that thunderstorms are impacting an airport.
In lesson 4 we will cover radar as we discuss enroute problems. In lesson
5 we will look at microburst and wind shear detection systems which give
near-real time warnings while we are flying in the terminal areas.
Finally, thunderstorms pose hazards at the airport before we are even airborne. Last year, a ramp worker was killed when he was standing beside an aircraft during a thunderstorm. Please go to the 45th Weather Squadron's lightning safety page for tips on protecting yourself while on the ground.
Thank you for continuing to participate in the National Weather Association's "Thunderstorm and Flying" internet course. We're halfway through right now and have three more lessons to go.
1. What is the name of the observer's handbook?
2. How is the observation wind defined?
3. What are the requirements for a variable wind?
4. When did gusty winds have to take place to be reported?
5. What is a wind shift?
6. How fast must the wind speed be to be reported as a "peak"?
7. How long does an automated observing platform have to work to create the surface visibility?
8. What is a squall?
9. When a thunderstorm has an intensity what is it refering to?
10. What is a thunderstorm?
11. What is the definition of "vicinity" and "distant" thunderstorms?
12. Can a thunderstorm be reported without precipitation?
13. When does a thunderstorm begin?
14. When does a thunderstorm officially end?
15. How many minutes must seperate thunderstorms in order to have multiple events at the airport?
16. What is the definition of "pressure falling rapidly"?
17. Which of the automated observing systems can report a thunderstorm?
18. What is the difference between augmentation levels A, B, C, and D for an ASOS?
19. What is a CB?
20. What is +TSRA refer to?
21. What are the codes for lightning?
22. What are the codes for pressure rapidly falling?
23. What does AUTO mean?
24. What conditions force a new observation to be made and hense change the ATIS?
25. What does TEMPO refer to?
26. What does PROB 40 mean?
27. What are some of the products of the domestic AWC division?
28. What is a severe thunderstorm according to the NWS?
29. How long are SIGMETs valid?
30. What hazards does a convective SIGMET imply?
31. What is an embedded thunderstorm?
32. If a convective SIGMET message is negative, does that mean there are NO thunderstorms