You will recall from the previous edition of these Training Notes that we had begun the construction of a TAF from an example domestic FT. The complex FT of choice was as follows:

	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..

Our discussion had taken us through the coding of report type, station identifier, issuance date and time, valid period date and time, and into forecast meteorological conditions. Under the forecast conditions, we encoded wind, visibility, weather, and sky condition. Our partially-constructed TAF had taken the form:

TAF

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

In this edition, we'll focus our attention on forecast change indicators. Specifically, we will address forecasts of significant changes, temporary changes, and those based on probability of occurrence. The actual conversion of these change groups from the FT code to TAF code will be deferred until completing the discussion of each indicator group.

We'll round out our discussion by acquainting readers with the various optional groups that may show up in the TAF. Be aware, however, that there is no agreement, at present, with the international community requiring the coding of these groups in NWS-prepared TAFs.

Significant Changes:

Denoting long-lasting changes of significance to existing conditions within a TAF are indicated by use of the change groups FroM or BECoMinG.

FroM. This group is included in a TAF whenever a set of prevailing weather conditions is expected to change rapidly, usually in less than one hour, such as when a front passes a forecast location. It takes the form of FMGGgg, where GGgg represents the hour and minutes (UTC) at which the change is expected to occur. This beginning time is always four digits in length, even though the forecaster may not be able to predict precisely (i.e. to the exact minute) when the expected change will occur. In those situations, he/she would code "00" for the minutes, indicating a change to the nearest hour.

It should be understood that the weather conditions denoted by the change group are expected to persist until the end of the 24-hour forecast or until the time of the next FM group. Each successive FM group should be placed on a new line of the forecast and indented five (5) spaces for clarity. The group should contain all the conditions of change and thus provide a complete description of the weather. Elements given in the change group supersede those of a previous group.

BECoMinG. This change indicator is used in a TAF whenever expected changes are forecast to occur gradually over a longer period of time. The group is of the form BECMG GGGeGe. The "GG" represents the time of onset of the change, whereas the "GeGe" represents the time that the change is expected to end or be complete. The BECMG group would be the indicator of choice when wind is gradually shifting as with a broad or diffuse front that is moving slowly through an area, or when cloud and/or visibility conditions are gradually lowering as is often the case with the approach of a warm front.

The duration of the time period over which the change is expected to occur should normally NOT exceed two (2) hours, but in any case should NEVER exceed four (4) hours. All parameters that are expected to change during the specified change period should be included in the BECMG group, and these changes are in effect until the end of the 24-hour forecast or until the next FM or BECMG group. It is not necessary to place a BECMG group on a new line in the TAF.

Temporary Changes:

Temporary changes are indicated in the TAF by use of the contraction TEMPO. It takes the form of TEMPO GGGeGe, where, again, "GG" represents the beginning time of the change and "GeGe" represents the end time of the change. This group is used to denote a change in wind, visibility, weather, or sky condition that is expected to persist for less than an hour at a time. As a whole, the fluctuations in conditions should cover less than half of the period between GG and GeGe. For example, if temporary fluctuations in wind are expected during the three-hour period from 1000 UTC to 1300 UTC, they must not persist for two of those hours. Otherwise, their duration would exceed one half of the forecast period and thus would need to be included as a prevailing condition.

The TEMPO group can be thought of as the equivalent to the OCNL (occasional) group in the FT, and as such, should be indicative of high confidence (50 percent or greater) that a change in a particular weather phenomenon will occur. In general, the period of change in a TEMPO group should NOT exceed four (4) hours.

Probability Forecast:

The probability forecast, denoted by the contraction PROB in the TAF, is equivalent to "CHC" in the domestic FT. The PROB statement is immediately followed by a two-digit percent (always 40 in NWS-prepared TAFs), a space, and the beginning and ending time (UTC) in which the probability of occurrence is valid. Thus, PROB40 1820 would indicate a chance probability of occurrence (30 to 49 percent) of a forecast condition between the hours of 1800 UTC and 2000 UTC.

It is important to note that the PROB group is to be used by NWS offices only to forecast the probability of occurrence of a thunderstorm and associated precipitation, or a precipitation event with associated changes in wind, visibility and sky condition. As with CHC in the FT, the PROB group is NOT allowed in the first six (6) hours of the TAF. Further, the group is NOT allowed as a direct modifier of another change group in NWS-prepared TAFs. For example, PROB40 TEMPO 1517 is not allowed in our TAFs.

So, with the above discussion in mind, let's complete construction of our example TAF:

        TAF
	KDFW 151120Z 151212 12006KT 3SM BR OVC008 TEMPO 1215 1SM-DZ
	     FM1500 14012KT 5SM BR BKN015 OVC050 PROB40 1720 -SHRA
	      BECMG 1920 16015G25KT P6SM NSW SCT025 PROB40 2100 
              VRB30G45KT 1/2SM +TSRA VV005CB
	     FM0000 25015KT P6SM SCT050
	     FM0600 13008KT P6SM BKN015=

Again, note that each "FroM" group begins on a new line indented five (5) spaces. Continuation lines for a particular forecast group are indented six (6) spaces. An equal (=) sign signals the end of a forecast.

Optional Groups:

Again, we emphasize that the following optional groups are not required groups for coding in NWS-prepared TAFs. We present them here only for the purpose of familiarization among our forecasters who may encounter TAFs from an international source.

Icing Group. One can recognize this group in a TAF by the leading "6". It follows the format: 6IchihihitL and is used to describe forecasted layers of icing. The "Ic" denotes the type of ice accretion expected (i.e. light, moderate, severe, in cloud, in precipitation, etc.) as given in the appropriate international code table (see Attachment). The "hihihi" group represents the expected height of the lowest level of icing in hundreds of feet above ground level (AGL). Finally, the "tL" is the thickness of the layer of icing in thousands of feet. An example of an icing forecast is as follows: 650104. Decoding the group, we see that a moderate icing potential is expected in clouds (code 5) from 1000 feet to 5000 feet AGL (4000 foot thickness as denoted by the ending "4").

Icing groups can be repeated as often as necessary to indicate more than one layer or type of icing. Coding policy states that if one layer of icing is expected to be greater than 9000 feet in thickness, two groups must be coded. Consider the following example: 630309 631203. Looking at the groups, we note that the forecaster is predicting that light icing is expected in precipitation (code 3 in both the first and second groups) and that the potential exists from 3000 feet to 15,000 feet. Notice that the ending "9" in the first group (indicating 9000 feet thickness) triggers the need for the second group.

Turbulence Group. The turbulence group follows the format 5BhBhBhBtL, where the "5" is the turbulence group indicator, the "B" indicates a severity and frequency potential as given in the appropriate international code table (see Attachment), the "hBhBhB" is the height of the lowest level of turbulence expressed in hundreds of feet above ground level, and, once again, the "tL" denotes thickness of the layer in which the turbulence is expected.

Let's examine how this group might be coded using a specific example. If a forecaster is predicting that frequent moderate turbulence will occur in clear air (code 3 from the table) and that the turbulence will extend from 8000 feet to 12000 feet AGL, he/she would code it as follows: 530804.

Like the icing group, if a layer of turbulence is forecast to exceed 9000 feet in thickness, a second group would be required.

Surface Temperature Group. The final optional group we will look at is the surface temperature group, coded as TTFTF/GFGFZ, where "T" is the indicator for the surface temperature group, the "TFTF" is the temperature in whole degrees Celsius (C), the "GFGF" is the valid time of the temperature forecast to the nearest whole hour UTC, and "Z" is the abbreviated symbol for UTC. Temperatures below zero are coded with a leading "M" (minus), and temperatures between +9 degrees C and 0 degrees C are preceded by a zero.

The following examples illustrate the coding of the surface temperature group:

T15/20Z - forecast temperature of 15 degrees C at 2000 UTC

T06/12Z - forecast temperature of 6 degrees at 1200 UTC

TM11/03Z - forecast temperature of -11 degrees C at 0300 UTC

That wraps up this edition of METAR/TAF Training Notes. In our next edition, we will focus our attentions on handling unscheduled TAF issuances (amended, corrected, and delayed).

Sources:

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

NWS, October 1995: Weather Service Operations Manual, Chapter D-31 (DRAFT).

Table 1733 - Type of Ice Accretion:

Code Type of Accretion

0 No icing

1 Light icing

2 Light icing in cloud

3 Light icing in precipitation

4 Moderate icing

5 Moderate icing in cloud

6 Moderate icing in precipitation

7 Severe icing

8 Severe icing in cloud

9 Severe icing in precipitation

Table 0300 - Type and Frequency of Turbulence

Code Type and Frequency

0 None

1 Light turbulence

2 Moderate turbulence in clear air, occasional

3 Moderate turbulence in clear air, frequent

4 Moderate turbulence in cloud, occasional

5 Moderate turbulence in cloud, frequent

6 Severe turbulence in clear air, occasional

7 Severe turbulence in clear air, frequent

8 Severe turbulence in cloud, occasional

9 Severe turbulence in cloud, frequent

X Extreme turbulence