June 2001Flying Safe

The Weather Never Sleeps

Translate This: The Area Forecast


Cracking Those FA Codes

As a pilot gathers the information needed for a preflight weather briefing, the area forecast almost asks to be ignored. After all, you would really have to enjoy untangling coded weather reports to willingly tackle a forecast that starts off with something like: "SYNOPSIS... LOW PRES TROF 10Z NERN PA-SERN NY-SWRN CT FCST MOV EWD INTO CSTL WTRS AND WKN THRU 04Z. LARGE AREA HI PRES FCST BLD EWD OVR LE-OH THRU 04Z...SMITH ...NERN ME...OVC050. TOPS FL200. 15Z OVC030. AFT 18Z SCT-SHSN..."

If an area report was as short as the excerpt above, you might be willing to take a shot at it. But on an ordinary day, when meteorologists don't have a lot to say about the weather, the printout of an area forecast can run more than two pages long. As if page after page of code in capital letters - which any type designer will tell you are hard to read - isn't enough, area forecasts identify areas of dangerous weather by asking you to play connect-the-dots using the three-letter identifiers of VOR radio navigation stations and airports.

A pilot who has flown for a few years along the Appalachian Mountains might be able to look at something like, "FROM JHW TO PSB TO PSK TO HMV TO HNN TO JHW MTNS OCNL OBSC IN CLDS AND PCPN. CONDS ENDG 17-19Z" and draw a line on charts outlining the area where the "mountains (are expected to be) occasionally obscured in clouds and precipitation, conditions ending 1700 to 1900 Zulu time," as the forecast says. But most of us would have to search a list of three-letter identifiers to figure out that the mountains are going to be obscured inside the area enclosed by lines drawn from Jamestown, New York, to Phillipsburg, Pennsylvania, to Dublin, Virginia, to Houston Mountain, Tennessee, to Henderson, West Virginia, and back to Jamestown.

Yet the information in area forecasts is vital for any pilot who is planning to fly away from the airport where he or she launches. It's the only weather forecast that tells you what the ceilings and visibilities are anticipated to be over large areas. Area forecasts are also the only source that will give you an idea of what ceiling and visibility to expect if you are flying to one of the many airports for which no specific terminal aerodrome forecast is available.

Fortunately, if you call or visit an FAA flight service station for your weather briefing, the specialist you talk with will not only take care of translating the area forecast and other reports, but he or she will also pull out the information you need for your flight. This is only one of the reasons why a flight service station is the best way to obtain a preflight briefing, especially for a pilot who isn't thoroughly familiar with weather and the weather reporting and forecasting system.

If you want to use the Direct User Access Terminal System (DUATS) to call up a weather briefing on your computer, you can find ways to leap some of the hurdles offered by area forecasts, including finding the locations used to outline areas of dangerous weather. First, however, you need to learn some of the basics about area forecasts, including what they do and do not contain.

The National Weather Service's Aviation Weather Center in Kansas City, Missouri, issues area forecasts three times a day for six areas that cover the contiguous 48 states. Each forecast is named for a large city in the area covered. Figure 1 shows these areas with the city name used for each. Forecasts are issued in the predawn hours (local time), in the morning, and in the early afternoon. They cover a 12-hour period with a six-hour, generalized outlook. National Weather Service offices in Alaska and Hawaii issue area forecasts for their states four times a day. All of the times used in area forecasts are Coordinated Universal Time, which used to be called Greenwich Mean Time and is commonly called Zulu time by pilots and meteorologists.

The Aviation Weather Center describes the area forecast as "a forecast of visual flight rules (VFR) clouds and weather conditions over an area as large as the size of several states. It must be used in conjunction with the airmet Sierra (IFR) bulletin for the same area in order to get a complete picture of the weather."

The Aviation Weather Center issues airmets (the term means airmen's meteorological information) every six hours for phenomena that can affect all aircraft, but which could be particularly hazardous for light aircraft and pilots with less experience. As with other weather forecasts, airmets are updated as necessary. Airmets Sierra for IFR conditions (See the "Cracking the FA Code" sidebar for definitions of terms such as IFR), airmets Tango for turbulence, and airmets Zulu for icing should be considered part of the area forecast.

The fragment from the beginning of this story describing where mountains in the East would be obscured by clouds and precipitation was from an airmet Sierra. Pilots no longer need to play the "connect-the-identifiers" game to locate the bad weather, however.

Figure 2 is a map from the Aviation Weather Center showing airmets Sierra for the East used earlier. In addition to the mountains from western New York to eastern Tennessee that will be obscured, it shows mountain obscuration occurring in northern New England as well as areas of IFR conditions over parts of Florida, Georgia, Iowa, and Minnesota.

Area forecasts begin with a header, which includes the area it covers, the time it was issued, and the times it covers. (The word valid refers to the times covered by a forecast.) The header also includes general information that applies to all area forecasts.

Immediately following the header is a synopsis, which provides a general description of the major weather patterns that are expected to create the conditions forecast.

Then the forecast moves into a state-by-state description of the predicted weather. Normally more than one small state or only part of a large state will be included in each discussion. The areas covered in each discussion are listed on a line by themselves, which makes it easier to scan the forecast to find the parts covering an area in which you are interested.

The area forecast sample given below, with translations, shows you what's in a typical forecast and how to understand it. You need to practice decoding area forecasts to become comfortable with them. Area forecasts can be found on the Center's Web site ( www.awc-KC.NOAA.gov/awc/awc-airmets.html ).

If you are obtaining a weather briefing for a flight, you should visit or call a flight service station or use an official service like DUATS in order for your briefing to meet legal requirements. The National Weather Service does not consider the Web to be an official source of its products because it can't guarantee the data on Web sites will always be the latest.

Still, the Aviation Weather Center's Web site is a good source of prebriefing information. For student pilots, visiting the site is a good way to practice obtaining weather briefings by using real material - the same material used by flight service weather briefers. Students should try to decode the materials on the site, then call a flight service station for a weather briefing. You can see how closely your interpretation of the material matches the flight service specialist's explanation.

Practice is the way to master making sense of area forecasts and relating this to a flight you are planning. Many student pilots like to dream about the flights they will make once they have earned their certificate. You can make practical use of your dreams by choosing some flying destinations and calling up the area forecasts and other weather information you'll need to give yourself a complete briefing. When you do begin taking off for your dream flights, you'll be ready to make sound weather decisions.

Cracking The FA Code

Below are some of the most common or most tricky codes used in area forecasts.

Locations

Ordinary two-letter U.S. postal state abbreviations are used for names of states.

LO, LE, LH, LM, LS are found in the Chicago and Boston area forecasts. They represent the five Great Lakes: Ontario, Erie, Huron, Michigan, and Superior.

Weather Phenomena

BR is mist, which is the same as fog or FG, but with visibility of more than five-eighths of a mile. Fog is the proper term when it's thicker and the visibility is forecast to be less than five-eighths of a mile.

CIG means ceiling, or the distance above the surface of the lowest level of broken or overcast clouds. While most reports of altitudes in FAs refer to height above mean sea level, reports of ceilings are always height above the surface.

CSTL WTRS stands for coastal waters. This, like many of the abbreviations, can be understood if you try to pronounce it, which means you will add the missing vowels or some vowels that will probably help you to make sense of the words.

FZ stands for freezing. It's usually used in conjunction with fog or precipitation, as in: FZDZ (freezing drizzle); FZFG (freezing fog); or FZRA (freezing rain).

SH indicates showers. Like freezing, this is normally combined with other weather codes such as: SHRN (rain showers) or SHSN (snow showers).

TS means thunderstorms. TS alone means thunder but no precipitation is falling at the weather station. TS alone will not be seen in an FA, only in reports of weather conditions. In forecasts, such as area forecasts, TS will be followed by other letters, including: TSRN (thunder with rain, or an ordinary thunderstorm); TSSN (thunder with snow); TSPE (thunder with ice pellets - sleet to you and me); TSGR (thunder with hail); TSRAGR (thunder with rain and hail); and TSRASN (thunder with rain and snow).

General Flight Weather Descriptions

IFR means instrument flight rules conditions. IFR includes ceilings less than 1,000 feet and/or visibility less than three statute miles.

MVFR stands for marginal visual flight rules conditions, including ceilings from 1,000 to 3,000 feet and/or visibility between three and five statute miles.

VFR is visual flight rules conditions when ceilings are higher than 3,000 feet and visibility is greater than five statute miles.

Terms describing amount of sky covered by clouds

Clear means there are no clouds.

Few means that clouds cover one-eighth to two-eighths of the sky.

Scattered means that clouds cover three-eighths to four-eighths of the sky.

Broken means clouds cover five-eighths to seven-eighths of the sky.

Overcast means clouds cover more than seven-eighths of the sky.

Terms describing shower or thunderstorm coverage

ISOL represents isolated weather, such as single thunderstorm cells.

WDLY SCT means widely scattered. In other words, less than 25 percent of a given area is affected.

SCT or AREAS means that the showers or thunderstorms are scattered and 25 to 54 percent of a given area is affected.

NMRS or WDSPRD means that the showers or thunderstorm cells are numerous or widespread, affecting 55 percent or more of a given area.

More Information

A complete list of codes and a list of the VOR stations used in FAs and other weather products is on the National Weather Service's Aviation Weather Center Web site ( SYNOPSIS...LOW PRES TROF 10Z NERN PA-SERN NY-SWRN CT FCST MOV EWD INTO CSTL WTRS AND WKN THRU 04Z. LARGE AREA HI PRES FCST BLD EWD OVR LE-OH THRU 04Z...SMITH.

[Synopsis. A low-pressure trough at 10 Zulu extending from northern Pennsylvania to southern New York to southwestern Connecticut is forecast to move eastward into coastal waters and weaken through 04 Zulu. A large area of high pressure is forecast to build eastward over Lake Erie and Ohio through 04 Zulu. The forecaster's last name is Smith.]

ME

NERN ME...OVC050. TOPS FL200. 15Z OVC030. AFT 18Z SCT -SHSN. OTLK...MVFR CIG -SHSN.

[Maine. Northeastern Maine. Overcast clouds with bottoms at 5,000 feet and tops at 20,000 feet above mean sea level. The figure 050 is read as 5,000 feet because two zeros are added to the right of all altitude figures in aviation forecasts. We know that the height refers to the bottoms of the clouds because the word "tops" is used to designate cloud-top heights, as here. At 1500 Zulu expect overcast clouds with bottoms 3,000 feet msl. After 1800 Zulu, expect scattered clouds with light snow showers. The minus sign in front of any weather condition is read as light. No sign means moderate, and a plus sign means heavy.]

NH VT

SRN HLF...OVC030 VIS 3-5SM -SN BR. TOPS FL200. 12-15Z BECMG BKN050 BKN100. TOPS 150. 17Z SCT050 SCT100. OTLK...VFR.

[New Hampshire, Vermont. Southern half. Overcast clouds with bottoms at 3,000 feet msl. Visibility three to five statute miles in light snow and mist. Cloud tops at flight level 200-20,000 feet. From 1200 to 1500 Zulu, clouds becoming broken with bottoms at 5,000 feet msl. Another layer of broken clouds with bottoms at 10,000 feet msl and tops at 15,000 feet msl at 1700 Zulu, scattered clouds with bottoms at 5,000 feet msl and another layer of scattered clouds with bottoms at 10,000 feet msl.


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