3.5 Significant Weather
The weather group consists of appropriate qualifier(s), weather phenomena and obstructions to vision that are included in TAFs when conditions are expected.
NWSI 10-813, Section 1.2.6 and Appendix E contain additional information on the significant weather group and allowable weather phenomena. Please reference the directive while completing this lesson.
Key points from NWSI 10-813:
- One or more significant weather group(s) is (are) required when the visibility is forecast to be 6 (six) statute miles (SM) or less.
- If no weather phenomenon is expected in the initial forecast period or subsequent FM groups, then the significant group will be omitted.
- When more than one type of significant weather is forecast in the same forecast time
period, the order will be:
- Thunderstorms with/without associated precipitation.
- Significant weather in order of decreasing dominance based on intensity.
- See Significant Weather table in NWSI 10-813 Appendix E.
- Precipitation weather events are encoded with:
- intensity or proximity.
- a descriptor ("Blowing", "Showers", "Thunderstorm").
- precipitation type all together with no spaces (e.g., -SHSN, TSRA, VCTS).
- Non-precipitation significant weather elements are encoded after any precipitation, in their own group, separated by a space (e.g., RA BR).
- Multiple precipitation elements are encoded in a single group (e.g., -TSRASN).
- up to three appropriate precipitation contractions can be combined in a single
group (with no spaces) with the predominant type of precipitation being first. - In this single group, the intensity will refer to the total precipitation and be used with either one or no intensity qualifier.
- Ensure TAF thunderstorm forecasts are consistent with other NWS Products. Review thunderstorm consistency page for additional information.
Examples of the Significant Weather group:
TSRA - (thunderstorm with moderate rain)
-SHSN - (light snow showers)
RA BR - [moderate rain and mist (visibility between 5/8 SM to less than 6 SM]
P6SM VA - [volcanic ash (VA) included when expected regardless of the visibility]
No Significant Weather (NSW)
The contraction "NSW" (No Significant Weather) will be used in a TEMPO group to indicate when significant weather included in the previous "FM" group is expected to end. "NSW" when used will appear in the same location where the significant weather and/or obstruction to vision would be found. "NSW" should not be used in a "FM" group.
Example of NSW:
Vicinity (VC)
Vicinity (VC) is defined as donut-shaped area between 5 and 10 SM from the center of the airport's runway complex. If fog, showers, or thunderstorms are expected in the "vicinity" of the airport and there is a greater than 50% chance the phenomena will occur for more than one-half of the forecast time period, then it should be included in the TAF.
Vicinity forecasts can be included in the initial forecast period and subsequent "FM" groups. It is not used in TEMPO or PROB30 groups.
Examples of Vicinity:
