Center Weather Advisory: The CWA Decoded
A Center Weather Advisory is the most local and the most urgent of the area hazard products, and it is the one general aviation pilots read least often.
It is written by a meteorologist sitting inside an air traffic control centre, about that centre's airspace, for the next two hours. That origin explains almost everything about how it reads.
What a CWA is for
Each air route traffic control centre has a Center Weather Service Unit, and a meteorologist works in it alongside the controllers.
Their job is to tell the centre what the weather will do to the traffic. When something is building that will hit operations, they put out a Center Weather Advisory.
It does three jobs. It warns of weather that meets SIGMET criteria and is about to start. It covers weather already there that no SIGMET has been issued for. And it flags weather below SIGMET criteria that will still snarl the airspace.
That third purpose is the one with no equivalent elsewhere.
The two hour window
A CWA is valid for two hours, and no more.
That is short even by the standards of the convective products, and it follows from the job. The meteorologist is describing what is happening now and what happens next, not building a forecast for the afternoon.
It also means a CWA is always current when you read it or close to expired. Any CWA you find is either live or nearly finished, so the issue time matters as much as the text.
They are unscheduled. There is no clock they go out on. One appears because somebody decided the centre needed to know.
How it differs from a SIGMET
The two products overlap and the differences are worth holding clearly.
Area is the first. A CWA covers one centre's airspace. A SIGMET covers a broader region under a national system, and an international SIGMET covers a whole Flight Information Region.
Threshold is the second. A SIGMET fires only at the severe end. A CWA can be issued for conditions that would never meet SIGMET criteria, if those conditions will still cause a problem for the traffic.
Timing is the third. A CWA is often the early warning. The centre meteorologist sees the line building and issues an advisory, and the SIGMET follows once the criteria are properly met.
So a CWA with no matching SIGMET is not a contradiction. It is usually either an early call or a sub-SIGMET problem.
Reading one
The layout is compact and the parts are consistent.
A header names the centre and gives a sequence number and the valid period. Then the body describes the condition, the area, and how it is moving.
Movement uses the toward form. MOVG NE 25KT means the condition is heading
northeast at 25 knots. That is worth stating plainly, because international
SIGMETs often write MOV FROM and mean the opposite.
Our decoder tracks which of the two forms a product used, rather than guessing, so a CWA and a SIGMET do not end up described the same way when they mean different things.
The text is written in plain words rather than heavy code, because a controller has to read it at speed.
Why it is worth reading
The audience shapes the content, and that makes a CWA useful in a way the other products are not.
It is written for the people who run the traffic flow. They care about what moves and what stops.
So it names the things that close routes and stack arrivals. A line of storms across an arrival path. Ice at the levels where aircraft hold. Shear on the approach in use.
Those are the items a controller acts on first.
That is not a weather report so much as a traffic report. It tells you what the sky is doing. It also tells you what that will do to the flow you are joining.
If you are flying into busy airspace, a CWA is often the earliest sign that the next two hours will be difficult.
Verdict on center weather advisory
Read a Center Weather Advisory as the local, short-range, early-warning product. Two hours, one centre, written by somebody watching the traffic.
Check the issue time first, because the window is short. Read the movement as toward, not from. And treat a CWA with no matching SIGMET as a live signal rather than an error, because it may be the first warning of something a SIGMET will confirm within the hour.
Frequently asked questions
What is a Center Weather Advisory?
An unscheduled warning issued by a meteorologist working inside an air route traffic control centre. It covers conditions affecting that centre's airspace within the next two hours, and it often precedes a SIGMET.
How long is a CWA valid?
Two hours. It is a short-term product by design, written for conditions developing now rather than forecast for later in the day.
How is a CWA different from a SIGMET?
A CWA covers one centre's airspace and lasts two hours. A SIGMET covers a wider region under a national system. A CWA can be issued for conditions that do not meet SIGMET criteria, or as an early warning that one is coming.
Who writes a CWA?
A meteorologist in the Center Weather Service Unit, sitting inside the air route traffic control centre. They work alongside the controllers, which is why the product is aimed at traffic flow as much as at individual aircraft.
Does MOVG NE mean moving toward the northeast?
Yes. A CWA uses the toward form, so MOVG NE means the condition is heading northeast. That is the opposite of a SIGMET written as MOV FROM, which gives the direction it comes from.