PROB30 vs PROB40: What the Two Probability Groups Mean

PROB30 vs PROB40 is the narrowest comparison in a TAF. The two groups do the same job, in the same format, ten percentage points apart.

What makes them worth a page is not the gap between them. It is what their existence tells you about how much the forecaster actually knows.

What PROB30 vs PROB40 each say

PROB30 means a thirty percent chance that the conditions which follow will occur during the period given. PROB40 means forty percent.

The layout is the same for both. The word, then a four-digit start and end, then the weather being forecast. PROB30 1418/1422 2SM TSRA is a three in ten chance of two miles visibility in thunderstorms and rain, between 1800 and 2200 on the fourteenth.

Neither group replaces the surrounding forecast. They sit on top of it. The main body says what is expected, and the probability group says what might happen instead during that slice of time.

Why the scale stops at forty

There is no PROB50, PROB60 or PROB20, and the reason says a lot about how a TAF is built.

Above forty percent a forecaster is expected to commit. If a condition is more likely to happen than not, it belongs in the body of the forecast as a TEMPO period or a FM group. Hedging at that point would be avoiding the job.

Below thirty percent, the chance is too small to be worth the ink. A one in five possibility of rain would clutter every forecast in the world without changing anybody's plan.

So the probability groups cover a narrow band of genuine uncertainty. Likely enough to matter, not likely enough to promise.

What the group is really telling you

Read a PROB group as a confidence statement about the forecaster, not only as a statement about the weather.

A PROB30 for thunderstorms means the ingredients are there and the timing or the trigger is not settled. Somebody looked at the model and could not say whether the cap would break. That is different from a quiet forecast, and it is different from a confident one.

It also means the situation is worth re-checking. A forecast with probability groups in it is a forecast likely to be amended. The uncertainty that produced the group tends to resolve one way or the other within a few hours.

If you are planning a flight for later in the day, a PROB group is a signal to pull a fresh TAF before you go rather than trusting this one.

How to plan against a probability group

The safe reading is simple. Plan as if the condition happens, then be pleased when it does not.

Two in five is not a remote chance. It is close to a coin toss with a slight lean. If the conditions in a PROB40 would put the destination below your personal minimums, the flight needs the fuel and the alternate to cover it.

PROB30 deserves the same treatment when the outcome is severe. Thirty percent of a thunderstorm over the field at your arrival time is not a number to plan against lightly. The cost of being wrong is not symmetric.

Where it describes something mild, the sums change. A temporary drop to four miles in mist is not the same bet. Match the caution to the consequence.

Combining PROB with TEMPO

You will often see PROB30 TEMPO together, and the pairing changes the meaning slightly.

TEMPO on its own means fluctuations expected to last under an hour at a time, and to cover less than half the period. PROB30 TEMPO means a thirty percent chance that those brief fluctuations occur at all.

So the probability attaches to whether it happens, and the TEMPO describes the form it takes if it does. Nested that way, the group is saying: probably not, but if so, in bursts.

PROB never combines with FM, because FM is a firm change to a new steady state and a firm change cannot be thirty percent likely.

Where you will not see them

Some services limit the groups further, and it helps to know when one is absent.

In the first nine hours of a US TAF, PROB groups are not used at all. The forecaster is expected to commit that close in. So a probability group in a US forecast always sits well out in time.

Military forecasts often avoid them too. Many use a firm TEMPO instead, on the view that a crew needs a decision rather than odds.

That makes the absence of a PROB group weak evidence. It may mean the weather is settled. It may just mean this service does not write them at this range.

Verdict on PROB30 vs PROB40

The ten point gap between PROB30 vs PROB40 should not drive different decisions. Both mean the forecaster saw a real possibility and could not commit to it.

Plan for the described conditions in either case. Weight your caution by how bad the outcome would be, not by the number. And treat any TAF carrying a probability group as one worth refreshing before you fly.

Last updated August 29, 2026