Convective SIGMET: Thunderstorm Warnings Decoded
A convective SIGMET is the thunderstorm warning, and it works differently from every other weather product you read.
It is short, it expires in two hours, and it leaves out most of the hazards it is warning you about. All three of those are deliberate.
What triggers a convective SIGMET
There are four triggers, and any one is enough.
A line of thunderstorms at least 60 miles long, with storms affecting at least 40 percent of its length. An area of thunderstorms covering at least 40 percent of a region. Embedded or severe thunderstorms anywhere. Or hail three quarters of an inch or larger, or tornadoes.
Notice how low that last bar is. A single severe storm, or one tornado, produces a convective SIGMET on its own. The other triggers are about scale, and that one is about severity.
Why the text leaves out the hazards
Read a convective SIGMET and you will notice it rarely mentions turbulence, ice, shear or hail. That looks like an omission. It is a convention.
A thunderstorm is assumed to carry all of them. Severe or greater turbulence, severe icing, low-level wind shear and hail come with the storm by definition, so listing them every time would add words and no information.
So the presence of the storm is the warning. If a convective SIGMET covers your route, you should assume the full set of hazards is in it, whether or not any of them appear in the text.
That is the opposite of how you read an AIRMET, where the named hazard is the hazard.
The two hour window
Every convective SIGMET expires two hours after it is issued, and that is much shorter than any other product.
Storms build and die on that timescale. A cell that did not exist an hour ago can be at its peak now and gone in another hour. A six hour forecast of a thunderstorm would be wrong for most of its span.
They are reissued hourly, at 55 minutes past, for each of the three regions the country is split into. Between those times, special bulletins go out when something new develops or an existing warning needs changing.
The practical rule follows from that. A convective SIGMET more than an hour old is stale. Pull a fresh one before you commit to a route.
Reading one
The format is compact and always in the same order.
A header gives the region and a sequence number. Then the valid time. Then the area, either as a line with a width or as a boundary of navigation fixes.
Then a description of the storms: whether they are a line, an area or isolated cells, how they are moving, and what the tops are.
Tops matter more than pilots expect. A storm topping at 45,000 feet is not one you climb over in anything. It also tells you how strong the updraught is, which tells you how bad the air around it will be.
Movement matters just as much. A line moving at 40 knots will be somewhere quite different by the time you arrive.
What to do with one
The standing advice is to avoid thunderstorms by 20 miles, and a convective SIGMET is the product that tells you where to start measuring from.
That 20 miles is not conservative. Hail is thrown out of the top of a storm and can fall in clear air well outside the cloud. Severe turbulence extends beyond the visible edge for the same reason.
Do not try to fly between cells in a line unless the gap is wide and you can see all the way through. Gaps close, and a gap that closes with you inside it is the classic way into the worst air.
If the storms are embedded, visual avoidance is not available at all. That is the case where the SIGMET and the radar picture are the only information you have.
Where the radar picture fits
A convective SIGMET tells you a storm area exists. Radar tells you where the cells are inside it right now.
Use both. The warning is written by a forecaster and covers a broad box. The radar is current and sharp, but it only shows what is falling, not what the air is doing around it.
Onboard radar has a further limit worth knowing. It sees the nearest heavy rain and can be blocked by it, so a second cell hiding behind the first may not paint at all. That is called attenuation, and it is why a gap on the screen is not proof of a gap in the sky.
Treat the SIGMET as the boundary and the radar as the detail inside it.
Verdict on convective SIGMET
Treat a convective SIGMET as a complete hazard package rather than as the short text it appears to be. The storm implies severe turbulence, severe icing, shear and hail, whether or not the words are on the page.
Check the issue time first, because two hours is the whole span of the product. Read the tops and the movement, then plan to be 20 miles clear of anything it describes.
Frequently asked questions
What triggers a convective SIGMET?
A line of thunderstorms at least 60 miles long, an area of storms covering at least 40 percent of a region, embedded or severe thunderstorms, or hail three quarters of an inch or larger, or tornadoes.
How long is a convective SIGMET valid?
Two hours. Storms move and grow far too fast for a longer window, so the product is reissued hourly with a fresh look, and special bulletins go out between those times when needed.
Does a convective SIGMET mention hail and turbulence?
Usually not, and it does not need to. Severe turbulence, severe icing, low-level wind shear and hail are all implied by the presence of the thunderstorm itself, so the text does not list them.
What does embedded mean?
The thunderstorm is buried inside other cloud, so you cannot see it and avoid it visually. Embedded storms are especially hazardous because a pilot in cloud may fly into one without any warning.
Is a convective SIGMET the same as a regular SIGMET?
No. Convective SIGMETs cover thunderstorms and are issued hourly on their own schedule. Regular SIGMETs cover severe turbulence, severe icing, dust, ash and other non-convective hazards, and are issued as needed.