A SIGMET is the most consequential weather product a pilot is handed. It is also written like a telegram somebody was paying for by the character.

Most explanations tell you what a SIGMET is and stop there. A SIGMET decoder has to go further and take a real bulletin apart line by line. That is what this page does, so the next time you meet AREA EMBD TS MOV FROM 28020KT. TOPS ABV FL450. you know exactly what it is telling you to do.

A SIGMET warns all aircraft about severe hazards. An AIRMET covers lower intensities that matter mainly to lighter aircraft. A Convective SIGMET covers thunderstorms and automatically implies severe turbulence, severe icing and low level wind shear without ever naming them. A CWA is the two hour tactical version issued by one air traffic control centre.

What's New in SIGMET and AIRMET Products for 2026

The SIGMET has not changed. The AIRMET has.

US AIRMETs are now issued graphically, as G-AIRMETs, in three named products: SIERRA, TANGO and ZULU. A text bulletin used to cover a large region for six hours. A G-AIRMET is a set of snapshots at three hour steps instead, each with its own polygon.

That is a real improvement in precision and an awkward change for anybody used to reading text. A G-AIRMET has no telegraphic body to decode. The hazard, the severity, the base and the top all arrive as structured fields. The skill it asks for is reading a map in time steps, not a paragraph.

The text SIGMET is untouched, which means the shorthand below is still exactly what you will be handed for the hazards that matter most.

What a SIGMET Decoder Has to Take Apart

Here is a complete Convective SIGMET, as transmitted:

WSUS32 KKCI 290355 SIGC CONVECTIVE SIGMET 22C VALID UNTIL 0555Z MN IA SD FROM 40NW RWF-20SSW RWF-40ESE FSD-30N FSD-40NW RWF AREA EMBD TS MOV FROM 28020KT. TOPS ABV FL450.

Six pieces, and each one answers a different question:

• The communications header, which says who issued it and when
• The product and its series number, which says what kind of advisory this is
• The expiry, which says how long it is good for
• The affected states, which says roughly where
• The boundary, which says exactly where
• The body, which says what is happening and what it is doing next

A SIGMET decoder that only expands the abbreviations in the last line has done the easy part. The header and the expiry carry as much decision value.

The Header: WSUS32, KKCI and the Series Letter

WSUS32 KKCI 290355 is the WMO communications header. KKCI is the Aviation Weather Center in Kansas City, which issues all US convective advisories. 290355 is the day and time of issue in UTC, so the 29th at 0355Z.

CONVECTIVE SIGMET 22C is the product and the series. The number counts up through the day and resets. The letter is the region:

• C central
• E eastern
• W western

So 22C is the twenty second convective advisory for the central region that day. A high number early in the day tells you something on its own.

VALID UNTIL 0555Z is the expiry, and this is the one people skim. A Convective SIGMET is valid for two hours at most, and a new set is issued hourly. That short life is not red tape. It reflects how fast the weather it describes can change. An advisory you pulled during planning may already be replaced by the time you taxi.

The Area Line and Why It Belongs on a Map

MN IA SD is the affected states: Minnesota, Iowa, South Dakota. A coastal advisory may say something like FL AND FL AL CSTL WTRS instead, and note that FL there is Florida, not a flight level. Context tells you which, and nothing else does.

FROM 40NW RWF-20SSW RWF-40ESE FSD-30N FSD-40NW RWF is the boundary, as a chain of fixes. Read one segment at a time: 40 nautical miles north-west of RWF, then 20 south-south-west of RWF, then 40 east-south-east of FSD, and so on back to the start.

Be honest about this one. The boundary is better as a shape than as words. A decoder should show you the fix list and not pretend to summarise it. The useful version is the polygon, drawn on the chart you already have open.

The Body: Contractions That Change the Decision

This is where the shorthand lives, and where a decoder earns its keep.

The words that describe coverage:

• ISOL isolated
• WDLY SCT widely scattered
• AREA an area of
• LINE a line of
• EMBD embedded

The words that describe what it is doing:

• MOV LTL moving little
• MOV FROM 28020KT moving from 280 degrees at 20 knots
• DMSHG diminishing
• WKN weakening
• INTSFYG intensifying
• CONTG continuing
• EST estimated

And the vertical extent:

• TOPS ABV FL450 tops above flight level 450
• TOPS TO FL420 tops to flight level 420
• SFC-060 from the surface to 6 thousand feet

So AREA EMBD TS MOV FROM 28020KT. TOPS ABV FL450. reads as an area of embedded thunderstorms, moving from 280 degrees at 20 knots, with tops above flight level 450.

Two of those words deserve a second look.

Embedded means the cells sit inside a larger cloud mass. You cannot see them and pick your way between them visually. The see and avoid option is off the table, which is a different flight from one with scattered cells in clear air.

Moving from is not moving toward. A SIGMET gives the direction the weather is coming from, the same convention as a wind. A CWA often writes movement the other way, as MOVG NE AT 15KT, meaning heading north-east. Reading one as the other puts the weather 180 degrees from where it will be.

What a Convective SIGMET Already Implies

This is the part that catches experienced pilots, not just new ones.

A Convective SIGMET does not list turbulence. It does not list icing. It does not list wind shear. Read it quickly and you may conclude the advisory is only about thunderstorms.

It is not. Any Convective SIGMET implies severe or greater turbulence, severe icing and low level wind shear. They are never spelled out because they are assumed to be present. The silence is not reassurance, it is convention.

So a decoder that lists only what the text names can still leave you with the wrong picture. Good output says what the product implies, not just what it prints.

SIGMET Versus AIRMET, and the Three G-AIRMET Colours

The split is by severity and by who is affected.

A SIGMET concerns all aircraft. It is issued for:

• Severe or extreme turbulence not associated with thunderstorms
• Severe icing
• Dust storms or sandstorms lowering visibility below 3 miles
• Volcanic ash

A Convective SIGMET is a separate product for thunderstorms, issued hourly with that two hour life.

A G-AIRMET covers conditions hazardous mainly to lighter aircraft and to pilots operating without an instrument rating. Three products:

• SIERRA IFR conditions and mountain obscuration
• TANGO moderate turbulence and sustained surface winds of 30 knots or more
• ZULU moderate icing and freezing levels

The shortcut worth keeping: the intensity words move with the product. Moderate icing is a ZULU G-AIRMET. Severe icing is a SIGMET. Know which product you are holding and you already know roughly how bad it is.

What a SIGMET Decoder Should Do With a CWA

A Center Weather Advisory is issued by the Center Weather Service Unit at an ARTCC, for that centre's own airspace, and it is valid for up to two hours.

ZAN CWA 107 VALID UNTIL 290600 FROM 25SW TKA-10NE PAQ-25S ANC-25NNW ENA-25SW TKA AREA LGT TO OCNL MOD TURB SFC-060. OCNL LLWS +/- 20 KT RPRT BY ACFT. WKN.

Decoded: light to occasional moderate turbulence from the surface to 6 thousand feet, with occasional low level wind shear of plus or minus 20 knots reported by aircraft, weakening.

RPRT BY ACFT is the phrase to notice. That wind shear figure is not modelled and it is not forecast. An aeroplane measured it. A CWA that cites aircraft reports describes something that already happened to somebody. That puts it closer to a PIREP than to a forecast.

A CWA is tactical. It exists because something is happening now that the six hourly products did not see coming. That is why it is worth a second look shortly before you go.

The One Line Everybody Skips

SIGC on its own line is the bulletin identifier for the central region collective. It looks like noise, and for a single advisory it is. It matters when you are handed a whole file of them, because it tells you at a glance which regional set you are reading before you get to the series letter.

The same is true of the numbers in the communications header. WSUS32 follows the WMO bulletin naming scheme: WS for SIGMET, US for the United States, and the digits for the region and the sequence. You will never need to decode it in flight. You will want a decoder to keep it visible rather than strip it, because it is how you tell two similar looking advisories apart when they are stacked in a briefing pack.

Reading a Second Bulletin, Start to Finish

The shorthand only sticks once you have done it twice. Here is a different one:

CONVECTIVE SIGMET 21C VALID UNTIL 0555Z TX FROM 30SE JCT-30ENE SAT-50SSE SAT-50NNE LRD-30SE DLF-30SE JCT DMSHG AREA TS MOV LTL. TOPS TO FL420.

Line by line:

• Convective advisory 21 for the central region
• Good until 0555Z, so about two hours from issue
• Over Texas
• Bounded by a chain of six fixes, starting 30 nautical miles south-east of JCT
• A diminishing area of thunderstorms, moving little, with tops to flight level 420

Compare that with the first example. Same product, same structure, but three words change the picture entirely. DMSHG instead of nothing. MOV LTL instead of a direction and a speed. TOPS TO instead of TOPS ABV.

Diminishing, stationary, and with a stated top rather than an open ended one. It is still a Convective SIGMET, and it still implies severe turbulence and severe icing inside the area. But it is a very different two hours from a growing area tracking at 20 knots with tops above the level you can reach.

What to Do With One

Decoding is the easy half. The harder half is knowing what changes.

• Check the expiry before anything else. If it has under half an hour left, look for the replacement rather than planning against it
• Read the vertical extent against your planned altitude. SFC-060 matters enormously to a light aircraft and very little to traffic in the flight levels
• Treat EMBD as a hard stop for visual avoidance. Scattered cells you can see are a routing problem; embedded cells are not
• Take the implied hazards seriously. A Convective SIGMET means severe turbulence and severe icing whether or not the words appear
• Look for a CWA covering the same area. It is more recent, more local, and often cites what aircraft have actually reported

None of that requires memorising the contraction list. It requires knowing which four or five words change the decision, and letting a decoder handle the rest.

Where SIGMETs Come From and How Often They Land

Knowing the issuing pattern tells you when to look. That matters more than it sounds.

Convective SIGMETs come from the Aviation Weather Center at Kansas City. They go out hourly at 55 minutes past, with an unscheduled special whenever the weather demands one. That hourly rhythm is why an advisory can be stale within minutes of you reading it.

Non-convective SIGMETs are issued as needed rather than on a clock. They run for four hours at a time, and six for hurricanes and volcanic ash. They do not arrive on a schedule you can plan around. They arrive when somebody decides the hazard has crossed a line.

G-AIRMETs go out every six hours, with amendments as needed. Each one carries snapshots out to twelve hours in three hour steps.

CWAs have no schedule at all. A centre issues one when something is developing in its airspace that the wider products missed. That is exactly why a CWA is often the freshest thing on the page.

Why an Unknown Contraction Should Stay Unknown

There is a design decision buried in every decoder. It is worth knowing which side yours took.

The contraction list is long and it is not closed. Forecasters write regional phrasing, terrain references and shorthand that no table covers. A decoder meets a word it does not have, and it can go one of two ways.

It can guess, match on something close, and produce fluent prose that reads convincingly. Or it can leave the word exactly as written, and let you see there is a gap.

In a hazard advisory the second is the only defensible choice. Prose that quietly renames a hazard is worse than a line with an unexpanded abbreviation in it. The first hides its own failure. The second does not.

The same logic covers severity. A decoder should never report severe unless the advisory used the word, because severe is the word that decides whether a flight happens.

FAQ

What is the difference between a SIGMET and an AIRMET?

A SIGMET concerns all aircraft and covers severe hazards: severe icing, severe or extreme turbulence, dust and sandstorms, and volcanic ash. An AIRMET covers lower intensities that matter mainly to lighter aircraft and to pilots without an instrument rating.

Does a Convective SIGMET mention turbulence and icing?

No, and it does not need to. Any Convective SIGMET implies severe or greater turbulence, severe icing and low level wind shear. Those hazards are never listed separately in one, so the absence of the words is convention rather than reassurance.

What does TOPS ABV FL450 mean?

Cloud tops above flight level 450. A flight level is hundreds of feet, so FL450 is forty five thousand feet. Tops above that means climbing over the cells is not realistically available to you, so the plan has to be around them.

How long is a Convective SIGMET valid?

Two hours at most, and a new set is issued hourly. The short life reflects how quickly convective weather changes. An advisory pulled during planning can be superseded before you taxi, so it is worth re-checking close to departure.

What is a CWA and who issues it?

A Center Weather Advisory, issued by the Center Weather Service Unit at an air route traffic control centre for that centre's airspace. It is tactical, valid for up to two hours, and often cites conditions that aircraft have already reported rather than forecast conditions.

What does EMBD mean in a SIGMET?

Embedded. The thunderstorms sit inside a larger mass of cloud, so you cannot see them and navigate visually between them. It removes the see and avoid option and is one of the most decision-relevant words in the whole bulletin.

Verdict on Reading Hazard Advisories

A SIGMET decoder is worth having, but the reason to learn the shorthand yourself is that these products are short and time critical. You will often read one on a phone, on a ramp, deciding whether to go.

Four things carry most of the value. The expiry, because two hours is not long. The word EMBD, because it removes visual avoidance. The direction convention, because from and toward are 180 degrees apart. And the fact that a Convective SIGMET implies severe turbulence and severe icing without ever printing the words.

Paste one into the WxDecoder decoder. It expands the contractions, pulls out the hazard, the altitude band and the expiry, and leaves the boundary as the fix list it is rather than turning it into prose that would read well and help nobody. Where a contraction is not in the table, it stays as written, because a confident wrong expansion in a hazard advisory is worse than an honest gap.

Last updated August 29, 2026