AIM 

4/20/23 

(a) 

As a supplement to an existing SIGMET, Convective SIGMET or AIRMET. 

(b) 

When an Inflight Advisory has not been issued but observed or expected weather conditions meet 

SIGMET/AIRMET criteria based on current pilot reports and reinforced by other sources of information about 

existing meteorological conditions. 

(c) 

When observed or developing weather conditions do not meet SIGMET, Convective SIGMET, or 

AIRMET criteria; e.g., in terms of intensity or area coverage, but current pilot reports or other weather 

information sources indicate that existing or anticipated meteorological phenomena will adversely affect the safe 

flow of air traffic within the ARTCC area of responsibility. 

2. 

The following example is a CWA issued from the Kansas City, Missouri, ARTCC. The “3” after ZKC 

in the first line denotes this CWA has been issued for the third weather phenomena to occur for the day. The “301” 

in the second line denotes the phenomena number again (3) and the issuance number (01) for this phenomena. 

The CWA was issued at 2140Z and is valid until 2340Z. 

EXAMPLE

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ZKC3 CWA 032140 

ZKC CWA 301 VALID UNTIL 032340 

ISOLD SVR TSTM over KCOU MOVG SWWD 10 KTS ETC. 

7

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1

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7.  Categorical Outlooks 

a. 

Categorical outlook terms, describing general ceiling and visibility conditions for advanced planning 

purposes are used only in area forecasts and are defined as follows: 

1.  LIFR (Low IFR). 

Ceiling less than 500 feet and/or visibility less than 1 mile. 

2.  IFR. 

Ceiling 500 to less than 1,000 feet and/or visibility 1 to less than 3 miles. 

3.  MVFR (Marginal VFR). 

Ceiling 1,000 to 3,000 feet and/or visibility 3 to 5 miles inclusive. 

4.  VFR. 

Ceiling greater than 3,000 feet and visibility greater than 5 miles; includes sky clear. 

b. 

The cause of LIFR, IFR, or MVFR is indicated by either ceiling or visibility restrictions or both. The 

contraction “CIG” and/or weather and obstruction to vision symbols are used. If winds or gusts of 25 knots or 

greater are forecast for the outlook period, the word “WIND” is also included for all categories including VFR. 

EXAMPLE

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1. 

LIFR CIG

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low IFR due to low ceiling. 

2. 

IFR FG

−

IFR due to visibility restricted by fog. 

3. 

MVFR CIG HZ FU

−

marginal VFR due to both ceiling and visibility restricted by haze and smoke. 

4. 

IFR CIG RA WIND

−

IFR due to both low ceiling and visibility restricted by rain; wind expected to be 25 knots or greater. 

7

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1

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8.  Inflight Weather Advisory Broadcasts 

a. 

ARTCCs broadcast a Convective SIGMET, SIGMET, AIRMET (except in the contiguous U.S.), Urgent 

Pilot Report, or CWA alert once on all frequencies, except emergency frequencies, when any part of the area 

described is within 150 miles of the airspace under their jurisdiction. These broadcasts advise pilots of the 

availability of hazardous weather advisories and to contact the nearest flight service facility for additional details. 

EXAMPLE

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1. 

Attention all aircraft, SIGMET Delta Three, from Myton to Tuba City to Milford, severe turbulence and severe clear icing 

below one zero thousand feet. Expected to continue beyond zero three zero zero zulu. 

2. 

Attention all aircraft, convective SIGMET Two Seven Eastern. From the vicinity of Elmira to Phillipsburg. Scattered 

embedded thunderstorms moving east at one zero knots. A few intense level five cells, maximum tops four five zero. 

3. 

Attention all aircraft, Kansas City Center weather advisory one zero three. Numerous reports of moderate to severe icing 

from eight to niner thousand feet in a three zero mile radius of St. Louis. Light or negative icing reported from four thousand 
to one two thousand feet remainder of Kansas City Center area. 

7

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1

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22 

Meteorology