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3.4 Visibility

The initial time period and any subsequent FM groups will include a visibility forecast in statute miles (SM). The contraction "SM" is appended to the end of the visibility group (no space between the visibility and "SM").

Allowable forecast values for visibility are: P6, 6, 5, 4, 3, 2, 1 1/2, 1, 3/4, 1/2, 1/4 and 0

  Visibilities greater than 6 statute miles... Visibilities 6 miles or less...
Description ... are encoded as P6SM.
 ...must include at least one or more significant weather groups.
Example(s) ...P6SM BKB050

....3SM -SN BR...
...1/2SM FG...(FG is used for fog with visibilities 1/2 SM or less)
...5SM -TSRA...
...3SM -RA BR (light rain and fog, "BR" used with visibilities > 1/2 SM but < 6 SM)

The exceptions to this requirement are drifting dust (DRDU), drifting sand (DRSA), drifting snow (DRSN), shallow fog (MIFG), partial fog (PRFG), and patchy fog (BCFG). These may be forecast with prevailing visibility of 7 statute miles or greater.

Note on Fractions: If whole and fractions of statute miles are forecast, the whole mile and fraction must be separated by a space (e.g., 1 1/2SM...as opposed to 11/2SM)

Significant weather phenomena can be found in NWSI 10-813, Section 1.2.6 and Appendix E.