With the recent directive to begin transfer of aviation forecast responsibility to our spin-up NWSOs, the information we'll cover in the next few editions of the METAR/TAF Training Notes will be of considerable importance. We will, of course, devote much of our time to actually encoding a TAF from an example FT. But, in addition, we will direct some of our attention toward such issues as forecast philosophy and impact on the aviation customer.

So, as before, we'll launch the process by considering a complex example FT:

	DFW FT 150909 C8 OVC 3F 1206 OCNL C4 OVC 1L-F.
            15Z C15 BKN 50 OVC 5F 1412 CHC RW-.
	    20Z 25 SCT C100 BKN 1615G25 CHC C5 X 1/2TRW+ G45.
            00Z 50 SCT 2515.
            06Z C15 BKN 1308..

We'll examine each parameter in the FT as we construct the corresponding TAF. The TAF utilizes the same coding methodology as that found in the METAR. However, there are some coding peculiarities in the TAF that may make the basic parameter-to-parameter comparison that was used in the SAO-to-METAR example a bit trickier to employ. Again, we want to avoid getting lost in the encoding process, so we'll break the discussion out over two editions of these Training Notes.

PARAMETER 1: Type of Report

	DFW FT 150909 C8 OVC 3F 1206 OCNL C4 OVC 1L-F.
            15Z C15 BKN 50 OVC 5F 1412 CHC RW-.
	    20Z 25 SCT C100 BKN 1615G25 CHC C5 X 1/2TRW+ G45.
            00Z 50 SCT 2515.
	    06Z C15 BKN 1308..

Some things are certain in the coding process, and the placement of the report type header will always appear as the first parameter in the TAF forecast. There are two types of TAF issuances; a routine forecast (TAF) and an amended forecast (TAF AMD). Regardless, the report type header will appear on a separate line preceding the actual forecast elements.

For example,

TAF

Forecast elements...

or

TAF AMD

Forecast elements...

It is important to note that corrected (COR) and routine-delayed (RTD) forecasts are identified only in the communications header and will not appear as part of the forecast text.

PARAMETER 2: Station Identifier

The coding policy for station identifier is the same as that of the METAR. In other words, the U.S. will use the ICAO four-letter identifiers as described in METAR/TAF Training Note #4. All forecast sites in the contiguous U.S. will maintain their current identifiers, but preceded by a "K" (e.g. KDFW).

PARAMETER 3: Issuance Date/Time

The next parameter to examine is the forecast issuance date and time. It will be a six-digit number immediately followed by a "Z" to indicate UTC. The first two digits will represent the date (day of the month), with the last four digits representing actual issuance time. TAFs will be scheduled for issuance four times daily, at 0000Z, 0600Z, 1200Z, and 1800Z.

Of course, FTs are issued only three times daily and not always at times that correspond to TAF issuance. In fact, unlike the TAF, the FT issuance times are adjusted for the various time zones. Further, the actual time of FT issuance is not shown (except for updates, corrections, and routine delayed forecasts).

Assuming that our example FT is actually issued 40 minutes prior to its valid time, the corresponding TAF would be encoded as:

TAF

KDFW 151120Z..

PARAMETER 4: Valid Period Date/Time

	DFW FT 150909 C8 OVC 3F 1206 OCNL C4 OVC 1L-F.
            15Z C15 BKN 50 OVC 5F 1412 CHC RW-.
	    20Z 25 SCT C100 BKN 1615G25 CHC C5 X 1/2TRW+ G45.
	    00Z 50 SCT 2515.
	    06Z C15 BKN 1308..

Following the issuance date and time is the valid period, usually 24 hours, of the forecast in UTC. Again, it is a six-digit group, with the first two digits representing the date (day of the month), the next two digits representing the beginning hour of the valid period, and the last two digits representing the ending hour of the valid period. This coding convention is no different than that used in an FT, including the use of "00" for a midnight beginning valid time and "24" for a midnight ending valid time. For an amended (updated), corrected, or delayed forecast, the valid period can be, and usually is, less than 24 hours.

Thus, our encoded TAF now reads:

TAF

KDFW 151120Z 151212..

PARAMETER 5: Forecast Meteorological Conditions

	DFW FT 150909 C8 OVC 3F 1206 OCNL C4 OVC 1L-F.
            15Z C15 BKN 50 OVC 5F 1412 CHC RW-.
	    20Z 25 SCT C100 BKN 1615G25 CHC C5 X 1/2TRW+ G45.
	    00Z 50 SCT 2515.
	    06Z C15 BKN 1308..

Here's where the rubber meets the road. The parameters that comprise the actual body of the forecast are reported in the following sequence:

WIND VISIBILITY WEATHER SKY CONDITION

We'll examine each of these in some detail, but let's first make some general comments regarding their inclusion in the forecast.

In the initial forecast group (as highlighted in our example FT), wind, visibility and sky condition is always reported, with weather included only if deemed significant to aviation concerns. Expected changes of significance from the initial period are given in a new forecast time group. It is important to note that only the parameters that are forecast to actually change or, in the case of weather, become significant will be given in the new group, and not those that are forecast to remain essentially the same.

Wind. The forecast wind group is a five-digit group that includes expected wind direction (first three digits) in tens of degrees from true north and speed (last two digits). In the rare case of a wind speed greater than 99 knots, three digits will be used. Immediately following the actual wind group, if there are no gusts expected, the contraction "KT" is given to indicate the speed in knots. If gustiness is forecast, a "G" followed by the highest expected gust (again, to two or three digits) will be given just before the "KT" contraction. Wind speeds less than 3 knots are encoded as "00000KT", indicating a calm wind.

With the forecast wind included, let's look at our developing TAF:

TAF

KDFW 151120Z 151212 12006KT..

The other wind groups included in the example FT would be coded as 14012KT, 16015G25KT, 25015KT, and 13008KT, respectively. The expected convective- induced gust in the 2000 UTC time group would be encoded using a variable wind direction, since many times it is extremely difficult, if not impossible, to forecast the direction from which a thunderstorm gust might blow across an airport. It would be encoded as VRB45KT.

There is one final issue to discuss regarding the wind before we move on. There is a proposal being considered that addresses the forecasting of non-convective wind shear in the TAF. Essentially, the coding would consist of an identifier contraction, "WS", a three-digit height of the wind shear zone in hundreds of feet, a "/" delimiter, and a wind direction and speed above the shear zone. For example, if there is a forecast of a wind shear zone at 1500 feet in which the wind will change to 240 degrees at 20 knots, then it would be encoded in a TAF as: WS015/24020KT. The wind shear group would immediately follow the surface wind group, thus providing a clear indication of how dramatic the wind change (shear) is expected to be. If it is uncertain as to what the wind direction and speed might be above the shear zone, or the height itself is in question, then the group may only include WS015 or WS, respectively.

Visibility. The TAF will use statute miles and fractions of statute miles in forecasts of prevailing visibility. Fractions, used for expected visibilities less than 3 miles, will, when appropriate, be separated from whole miles by one space, e.g. 2 1/2SM. When the prevailing visibility is expected to be above 6 miles, it will be encoded as "P6SM".

Significant Weather. The forecast of significant present weather follows the same guidelines as that of METAR. This parameter is omitted if no significant weather is forecast to occur during a specified time period. There is one exception, though. Following a period of significant weather, if the weather becomes insignificant, the contraction "NSW" (No Significant Weather) is included to indicate the change in conditions.

While it is understood that in most cases, only one significant weather code will be included in a TAF, it certainly is possible to include more than one, e.g. -RA FG. Notice that multiple codes are separated by one space. Also, the intensity designator always immediately precedes the associated weather code, with "-" indicating light, "+" indicating heavy, and no designator for phenomena of moderate intensity. Approved weather codes and their qualifiers are given in Appendix B of WSOM Chapter D-37, and will not be listed here.

Sky Condition. The final parameter we will examine in this edition of the Training Notes is sky condition. In a nutshell, forecast cloud information in a TAF, which can be for more than one layer, is given in the following sequence: amount, height, and type. The amount or coverage is presented in terms of qualitative designators based on the number of oktas of sky covered, i.e. FEW (1/8 to 2/8 coverage), SCT (3/8 to 4/8 coverage), BKN (5/8 to 7/8 coverage), and OVC (8/8 coverage).

The associated cloud height that is forecast is in hundreds of feet. For example, a cloud based at 2000 feet would be encoded as "020". For expected obscurations, the height becomes a vertical visibility into the obscuration and is preceded by the contraction "VV", e.g. "VV005" indicates an obstruction with a vertical visibility of 500 feet.

Significant cloud types will be forecast in a TAF. These include cumulonimbus (CB) and Towering Cumulus (TCU), with the contraction immediately following the cloud height, e.g. "BKN020CB".

To close out our discussion, let's consider our partially constructed TAF:

TAF

KDFW 151120Z 151212 12006KT 3SM BR OVC008..

We'll focus on change indicators and the other forecast time groups in our next edition of METAR/TAF Training Notes.

Sources:

FAA, 1995: New Aviation Weather Formats: METAR/TAF.

FAA, 1994: Contractions, Order 7340.1N.

NWS, April 1995: METAR/TAF Training Aids.

NWS, August 1993: Weather Service Operations Manual, Chapter D-37.