Understanding BECMG, TEMPO, and PROB in IFR Alternate Planning

When filing an IFR flight plan in Canada, selecting a legal alternate isn't always as simple as checking the TAF for good weather. If a forecast includes BECMG, TEMPO, or PROB, those modifiers directly affect whether an airport qualifies as an alternate under the Canadian Aviation Regulations (CARs).

Photo by John McArthur on Unsplash

Here’s how each term works and what it means for your IFR alternate decision-making.

Why Do Alternates Matter?

Under CAR 602.122, unless specific conditions are met, IFR flights must file an alternate aerodrome that has forecast weather above alternate minima during the ETA.

However, forecasts using BECMG, TEMPO, or PROB must be interpreted according to specific rules outlined by Transport Canada, notably in the CAP GEN – IFR Alternate Aerodrome Weather Minima.

✈ BECMG (Becoming)

BECMG indicates a gradual and permanent change expected to occur over the specified time frame. The change is not instantaneous but occurs progressively during the window.

TAF Example 1 – Improving:

TAF CYVR 211738Z 2118/2218 2SM BR OVC006 BECMG 2120/2122 P6SM SKC

This means conditions are forecast to improve from 2SM and overcast at 600 feet to clear skies and +6SM visibility between 2000Z and 2200Z.

✅ If your ETA is after 2200Z, you can plan using the improved weather.

TAF Example 2 – Deteriorating:

TAF CYEG 101740Z 1018/1118 P6SM FEW020 BECMG 1100/1102 2SM BR OVC003

Conditions worsen to 2SM and overcast at 300 feet between 0000Z and 0200Z on the 11th.

❌ If your ETA is after 0000Z, you must assume the poor weather is already in effect.

🌩 TEMPO (Temporary)

TEMPO indicates temporary fluctuations in weather conditions, typically lasting less than 60 minutes. If the condition is expected to recur, the combined duration of all occurrences should not exceed half of the total forecast period defined by the TEMPO group.

TAF Example 3:

TEMPO 1309/1313 1SM BR OVC002

Between 0900Z and 1300Z, visibility may briefly drop to 1SM with an overcast ceiling of 200 feet.

❌ If your ETA falls within this window, you must plan for the worst forecasted conditions — even if they only occur briefly.

If a TEMPO indicates improving conditions, you have to assume those improving conditions won't happen. 

🎲 PROB (Probability)

PROB expresses the likelihood of certain temporary weather phenomena occurring.

TAF Example 4:

PROB30 0812/0815 1/2SM FG VV002

There is a 30% chance of fog and vertical visibility of 200 feet between 1200Z and 1500Z.

✅ When deteriming the alternate requirments for an airport with a PROB30 you can consider the landing minima on the approach plate. For most CAT 1 ILSs, this PROB30 would be acceptable.

✅ Summary: Alternate Suitability

BECMG (Improving)

Use the improved conditions at the end of the BECMG period.

BECMG (Worsening)

Assume the worse conditions begin at the start of the BECMG period.

TEMPO

Assume the worse condition during the TEMPO period. Improving conditions should not be considered.

PROB

PROB30 or PROB40 will only restrict an alternate based on the landing minma on the approach plate.

Source Documents

For more details, refer to:

Final Thoughts

BECMG, TEMPO, and PROB are more than just weather jargon — they’re legal indicators that affect IFR planning. Always:

✅ Plan using the most conservative interpretation.

✅ Cross-check your ETA with any forecast change windows.

✅ Use the CAP GEN to confirm what's legally allowed.

Being precise with your alternates isn’t just about compliance — it’s about safety and smart decision-making.

This article is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.



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