1. Material Transmitted: Weather Service Operations Manual (WSOM) Chapter D-31, Aviation Terminal Forecasts.

2. Summary:

This chapter replaces, in its entirety, the June 20, 1996 draft of WSOM D-31, which became effective July 1, 1996. This new chapter, and the June 20, 1996 draft version of this chapter, are the result of the merging of codes and procedures used in the United States and those used internationally, into one new operations manual and incorporates revised requirements of the International Civil Aviation Organization (ICAO) and revised World Meteorological Organization (WMO) code formats.

National Weather Services (NWS) offices with terminal forecast responsibility are urged to study this chapter carefully. Several changes from the June 20, 1996 draft have been incorporated into this chapter, based on forecaster and customer suggestions. The major differences from the June 20, 1996 draft and this chapter are:

a. The chapter has been reorganized to clarify concepts and eliminate redundancy. As a result, most section numbers are different than those in the June 20, 1996 draft version of this chapter.

b. Sections 5.1 and 6.1 have been modified to emphasize and clarify the differences between them. Section 5.1 defines the minimum observational requirements to be considered by NWS regions as part of the process in evaluating requests to initiate new terminal forecast service. Section 6.1 describes the requirements for maintaining a specific forecast once service has been established. NWS forecasters should refer to Section 6.1 when they are debating whether or not there is sufficient data available to continue providing a particular terminal forecast.

c. In response to feedback from the Federal Aviation Administration and aviation customers, the use of the term BECMG has been restricted for use in NWS-prepared terminal forecasts as follows:

The duration of the change period covered by BECMG shall never exceed 2 hours.

BECMG should be used sparingly.

Forecasters should avoid using a BECMG group to forecast minimum conditions, especially visibility less than 1/2 statute mile.

Refer to Section 7.2.9 and 7.2.9.b.

d. The requirement for NWS offices using AFOS equipment to prepare and/or transmit terminal forecasts to manually type in the date-time group in the WMO abbreviated heading of amended, delayed, or corrected terminal forecasts has been eliminated. The NWS Telecommunication Gateway will assign the proper date/time to scheduled, amended, delayed and corrected forecasts issued by offices using AFOS equipment. Refer to Sections 8.1, 8.2, and 8.3.

e. The definition of very light winds, in the context of forecasting variable wind direction in terminal forecasts, has been changed from 6 knots to 3 knots in response to customer feedback. Three knots is also the threshold in the ICAO and WMO definitions of light winds. Also, expected variations in wind direction should be 180 degrees or more in order to be forecast as variable, again based on the ICAO and WMO definitions. Refer to Section 7.2.4.

f. The definition of gust has been restored to the traditional definition used in NWS products. The June 20, 1996 draft had inadvertently included the WMO definition of gusts. Refer to Section 7.2.4.

g. Significant weather in the vicinity (VC) has been restricted as follows: Refer to Section 7.2.6.a.

to forecast prevailing conditions only: the initial time period, FM and BECMG groups; and

for the following significant weather elements: fog, shower(s), thunderstorm

h. Section 8.3.a, including examples, was added to clarify procedures for correcting amended or delayed forecasts.

i. The remark "NIL AMD" has been restored. Refer to Sections 6.1, 6.1.1, and 6.1.3.

Numerous editorial changes were made and new examples were added in order to improve the readability of the chapter.

3. Effects On Other Instructions:

This Chapter supersedes the draft of WSOM Chapter D-31, dated June 20, 1996, in its entirety. The June 20, 1996 draft, in turn, superseded WSOM Chapters D-21, Aviation Terminal Forecasts, Issuance 94-10 dated July 11, 1994, and D-37, International Aviation Aerodrome Forecasts, Issuance 93-7 dated August 4, 1993, and Operations Manual Letter 2-96, dated April 25, 1996, in their entirety.

4. Effective date:

This Chapter becomes effective July 16, 1997 with the issuance of the terminal forecasts valid beginning at 0600 UTC.

/s/ Susan F. Zevin [for]

Elbert W. Friday, Jr.
Assistant Administrator
for Weather Services