March 2005 Cover Story:
Flying Weather in Arizona
Story by David N. Runyan, Warning Coordination Meteorologist, National Weather Service, Glider Pilot

Teams of National Weather Service aviation weather forecasters are working at this moment to provide pilots with the best possible aviation weather services. The forecasters are located in weather forecast offices in Phoenix, Tucson, Flagstaff, Las Vegas, the Albuquerque Air Traffic Control Center, and the Aviation Weather Center in Kansas City. They are gathering weather information from surface and airborne weather observation systems, Doppler weather radars, pilot reports, and state-of-the-art atmospheric models. The result: a safer flight, but it's up to the pilot to get the weather briefing and apply desert-mountain weather flying skills.

Simply put, flying weather in Arizona can challenge the most experienced pilot. Pilots who want to learn about Arizona weather and how terrain affects the weather should attend local mountain flying workshops, talk to experienced pilots, and take a few flights with a regional savvy CFI to hone mountain terrain aviation skills. Knowledge is the first step to acquiring the necessary skills.

The Weather Observation System
Weather observations reported at 30 Arizona airports are taken by automated equipment. The Automated Surface Observation System (ASOS) senses weather continuously at "B" class airports as well as a few "C" and "D" airports. This sophisticated system takes observations when it senses changes to/from IFR, VFR and field minimums. The ASOS observation may be augmented by a trained observer to ensure accuracy.

The Automated Weather Observation System (AWOS) is slightly different, in that it does less. AWOS takes a weather observation every 20 minutes. It is located at a few "C" and "D" class airports. The AWOS observation may also be augmented by an observer to add in weather phenomena such as rain showers or thunderstorms. When the system is not augmented, it will report precipitation or what may be reducing the visibility.

At a few part-time, tower controlled Arizona airports, weather observations are taken the old fashioned way -- manually -- by the tower air traffic controllers. These hourly weather observations are sent to the FAA centers to be relayed to the National Weather Service weather forecast offices.

Pilots may get the current ASOS and AWOS observations by a telephone call to the system, or to a service such as the commercially funded service 1-800-ANY-AWOS. The observations are also relayed on ATIS or by radio request to ATC. In pre-briefing, pilots can get the latest observation via the Internet web service called the Aviation Digital Data Service (ADDS) which is provided by the National Weather Service at www.aviationweather.gov.

Why is knowledge about the airport's weather observation system important to the pilot? ASOS and AWOS observations have time-averaged weather elements such as wind, prevailing visibility and cloud heights. The observation may be several minutes, if not tens-of-minutes old. The reported weather may not be what the pilot experiences.

The pilot experiences instantaneous and spatially unique weather events, such as an adverse wind gust, poor visibility in precipitation or a different cloud layer on approach to the airfield and not occurring directly over the automated weather observation system. The "difference" between averaged weather observations and what the pilot instantaneously experiences may be the pilot's challenge to safely complete the flight.

In the last decade, statistics of general aviation weather-related accidents point to the unexpected "difference" problem. Research by the AOPA Air Safety Foundation reveals about 75% of the weather-related accidents are wind-related, i.e. crosswinds, wind gusts, tailwinds, or unfavorable/variable winds. Even more sobering, about 75% of the weather-related accidents involve fatalities. This information leads to a situational awareness concept: "If the pilot doesn't know what to expect, the pilot will not recognize "it" when "it" first begins to happen." The next step, learn how to anticipate the change in weather.

The NWS Aviation Forecast
The National Weather Service airport weather forecast, called the Terminal Aerodrome Forecast (TAF), gives the pilot the variables between what is happening now and what will be happening in the next 24 hours. Arizona "B" class airports and many "C" and "D" airports have TAFs. The NWS Aviation Weather Center's forecasts for areas are available at www.aviationweather.gov. This web site provides weather forecast guidance for those airfields that do not have a TAF.

The TAF, like the coded weather observation, uses a METAR SPECI code format. While the TAF code format may be difficult to learn, the good news is that resources are available to teach the pilot how to read the TAF. As the pilot gains decoding familiarity, there is a TAF code translator available at the NWS ADDS web site to change the coded forecast into plain language.

While visiting the NWS ADDS web site, look around and see how much aviation weather information is available to help plan your next flight. Graphical illustrations of Airmets and Sigmets, wind aloft forecasts, significant weather forecasts for all flight levels, and access to the national and local Doppler weather radars make for a robust menu of aviation service and a well informed pilot.

The Pilot Report is Essential to Aviation Weather Services
The National Weather Service is a partner with the FAA and AOPA. Pilots may link to the AOPA Air Safety Foundation online course called the Sky Spotter® program from the NWS' Aviation Weather web site. Click on the Sky Spotter® icon located on the front page of the NWS Aviation Weather web site. Pilot reports assist fellow pilots, air traffic controllers, and NWS aviation weather forecasters. Learn what weather reports are important and be a frequent participant in the SkySpotter® program.

Arizona Weather by the Seasons

Winter to Spring ... November to March
Low cloud ceilings, visibility deteriorating to a few yards in fog, mountains obscured by clouds, turbulent high winds, rain, freezing rain, snow, icing, thunderstorms and even tornados -- this is sunny Arizona? Oh yes, Arizona's winter to spring months can best be described as generally mild with short periods of unusually harsh, chaotic weather. This is the time of the year when weather studied in ground school actually applies to Arizona.

Riding on strong westerly winds, Pacific cold fronts move ashore and sweep inland, across the region with a cyclic regularity of about every four to seven days -- more so during an El Nino year. Most experienced pilots stay home when surface winds exceed 30 miles per hour -- which may last six hours or more, and are slow to subside. Higher winds aloft are a sure bet.

In moderate to high winds, crossing mountains and ridges can be treacherous. Pilots encounter upward vertical moving air well in advance of the mountain or knife-edged ridge. It's not unusual to encounter 1000 fpm or more. On the leeward side, downward moving air will be encountered -- count on it. The safe pilot never approaches a mountain or ridge at an altitude equal or slightly above the elevation of the obstacle, nor does he or she approach an obstacle such as a knife-edged ridge head-on straight.

Experienced pilots expect mountain waves and ridge-induced vertical winds, even in clear skies. With respect to the wind speed, they add a buffer of 2000 feet altitude or more, and approach the knife-edged ridges with a 45-degree or less angled approach to allow plenty of room to turn back towards lower terrain and a safe exit.

Summer to Fall ... April to October
The low deserts of Arizona -- basically the southern and western third of the state, have temperatures warming to the lower 90s in April, and rocket into the 100s by the end of May. In the highlands, above 4,000 ft., overnight low temperatures still dip below freezing in April and May. Simply said, it's the elevation that rules the relatively dry early summer months.

By the time the temperature nears 100, the pilot's interest in air density calculation increases as the airplane's performance decreases. Much to the dismay of the uninitiated in high air density, airplanes climb (vertical) more slowly and true air speed increases-- which translate to an increase in actual horizontal, ground, traveling speed. Terrain maps should be closely studied in the vicinity of airports to make sure that terrain and obstacles can be cleared with enough altitude.

July begins a transition of the prevailing westerly winds aloft. The weak westerly wind eventually flips over to an easterly "from" direction. This is the beginning of the great southwest monsoon. The word monsoon means a change in seasonal wind. In Arizona, weather during the monsoon is best described as tropical. Huge thunderstorms, with gusty winds and some rain, occur each day over the higher terrain of north central Arizona, nick-named the "Rim Country" after the more formal name of Mogollon (pronounced muggy-on) Rim.

These thunderstorms move slowly to the west or at times northwest, drifting in the weak easterly winds. If they move out over the low deserts, the base of the thunderstorms is about 8,000 ft. above the desert floor. The thunderstorms do not form along cold fronts; this is the tropics -- no cold fronts allowed. The thunderstorms are sun- and terrain-driven.

The lift that the thunderstorms need to grow is supplied by the heating of the earth's surface temperatures of 100 to 115 degrees and by mountainous terrain. The thunderstorms tend to be short-lived, going from mature to the dissipating stage in an hour or less, and since they are slow to move, they don't travel far. A deadly hazard to the pilot on final or take-off, the base of the thunderstorm may be over two miles above the airplane, yet the airplane is actually in the trajectory path of one the most violent thunderstorm induced phenomena, called the downburst.

The downburst, unlike the infrequent and usually weak Arizona tornado, is an all too common threat to aviation. It originates inside of the mature thunderstorm and exits the bottom of the thunderstorms. In some cases it actually accelerates as it falls toward the ground. It comes in all sizes, from micro (small) to macro (large), and can be bone dry or wet and heavy with precipitation.

Once the downburst hits the ground, resulting horizontal winds up to about 90 mph are possible. While the parent thunderstorm from which this errant offspring is produced barely moves, the downburst can travel great distances -- horizontally speaking -- up to 100 miles away from its place of origin. For instance, a common summer monsoon event is a large thunderstorm in the vicinity of Tucson that creates a downburst. This downburst will travel to the west and north, all the way to Phoenix, arriving over the east Phoenix valley airports two hours later!

A dust storm often develops on the advancing edge of the downburst, becoming a towering wall of dust a couple of thousand feet high. It's a formidable sight and it is not standing still. If the pilot sees a dust storm off in the distance in the low Arizona deserts, it's probably moving at a ground speed of 30 mph or faster, and as it moves across and airport, low level wind shear wind and visibility dropping to a few yards should be expected.

The National Weather Service's aviation weather forecast and weather information services stand ready to serve the pilot. For more information about services to your local aviation community contact the nearest NWS Weather Forecast Office.


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