Understand BECMG, TEMPO, and PROB with IFR Alternates

When calculating an IFR alternate for an IFR flight plan in Canada a pilot must ensure that the terminal area forecast (TAF) does not fall below the calculated alternate minima.

There is sometimes confusion with BECMG, TEMPO and PROB in regards to IFR alternates. The Transport Canada Aeronautical Information Manual defines them as follows:

BECMG-Permanent Change Group (Gradual): If a permanent change in a few weather elements is forecast to occur gradually, with conditions evolving over a period of time (normally one to two hours, but not more than four hours), the new conditions that differ from those immediately prior are indicated following “BECMG.” Normally only those elements for which a change is forecast to occur will follow “BECMG.” Any forecast weather element not indicated as part of the “BECMG” group remains the same as in the period prior to the onset of the change.

TEMPO-Transitory Change Group: If a temporary fluctuation in some or all of the weather elements is forecast to occur during a specified period, the new conditions that differ from those immediately prior are indicated following “TEMPO.” In other words, when an element is not indicated after “TEMPO,” it shall be considered to be the same as that for the prior period. The time period, as with”BECMG,” is indicated by two four-digits date/time groups following “TEMPO.” The first two digits of each group indicate the date, while the last two digits of each group indicate the time in whole UTC hours.

PROB-Probability Group: In order to indicate the probability of occurrence of alternative values of forecast groups, PROB30 (a 30% probability) or PROB40 (a 40% probability) is placed directly before the change group’s validity period and alternative value(s) to indicate that different conditions will occur within the specified time period. The time period is given in whole UTC hour values. For example, “PROB30 2817/2821” would indicate that between 1700Z and 2100Z on the 28th day of the month there is a 30% probability that the indicated weather will occur. The weather elements used in the PROB group are restricted to hazards to aviation, which include, but are not limited to, the following:

  • thunderstorms;
  • freezing precipitation;
  • low-level wind shear below 1 500 ft AGL; or
  • ceiling and visibility values important to aircraft operations (e.g. threshold such as alternate limits, lowest approach limits).


There are two ways to look at a BECMG period. The weather is either improving, or deteriorating. We never know exactly when the weather will improve or deteriorate during the BECMG period, so we must interpret the two differently. Remember also that “improving” or “deteriorating” weather is subjective and depends on the situation.

When the BECMG is forecasting improving weather we must presume that the weather will not improve until the end of the BECMG period. When the BECMG is forecasting deteriorating weather we presume that the weather will deteriorate at the beginning of the period.

Example 1:

TAF CYDP 171938Z 1720/1723 33018G28KT P6SM -SHRA FEW025 OVC080
BECMG 1720/1722 30012G22KT

In this example TAF we can see the BECMG period forecasted for the 17th of this month, between 20-22z. The winds will change from 330º at 18 gusting 28 kts. to 300º at 12 gusting 22 kts.

If the wind change in this BECMG period is an improvement, the wind is within the aircraft’s crosswind limit, the wind change is considered to take effect at 22z.

If the wind change in this BECMG period is a deterioration, wind is outside of the aircraft’s crosswind limit, the wind change if considered to take effect at 20z.

Example 2:

TAF CYLL 171738Z 1718/1806 14018G28KT P6SM SCT100 BKN220
FM172200 16015KT BKN080
BECMG 1801/1803 BKN007
FM180300 16010KT P6SM BKN007

In this example TAF we can see the BECMG period forecasted for the 18th of this month, between 01-03z. The ceiling will go from broken at 8000′ to broken at 700′.

As the ceiling in this BECMG period is a deterioration, the new ceiling is considered to take effect at 01z.


The weather during a TEMPO can not fall below your alternate minima for an aerodrome. Sometimes a TEMPO forecast will show an improvement. Credit for improving weather in a TEMPO can not be taken.

There is a level of uncertainty when it comes to legally defining what to do when TEMPOs  show improving weather conditions. Expand the information below to see how I derive this interpretation.

TEMPO Interpretation[expand] The explanation of a TEMPO in the AIM under MET 3.9.3 (page 145) explains that a TEMPO “is only used when the modified forecast condition is expected to last less than one hour in each instance, and if expected to recur, the total period of the modified condition will not cover more than half of the total forecast period.” When the TEMPO conditions are not happening, the forecasted conditions prior to the TEMPO are what is expected. Since we don’t know when these TEMPO conditions will occur we have to use worst case scenario. That being said, in the opposite situations when an element of a TEMPO improves we also have to use worst case scenario since we don’t know when in the TEMPO period this will happen.[/expand]


TAF CYQM 171738Z 1718/1818 34006KT P6SM SCT006 OVC012
FM172300 01008KT 6SM BR OVC006 TEMPO 1723/1806 3SM -DZ BR BKN003 OVC006
FM180600 04005KT 3SM BR OVC003 TEMPO 1806/1813 OVC006
FM181300 07008KT 5SM BR OVC006
FM181600 08006KT P6SM OVC010

In the first TEMPO of this example TAF temporarily between 23z and 06z the visibility will drop from 6 miles to 3 miles, light drizzle and the ceiling will drop from 600′ to 300′. Since the ceiling is below required alternate minima, this aerodrome could not be used as an alternate during this period.

In the second TEMPO of this example TAF temporarily between 06z and 13z the ceiling will climb to 600′. Normally, an aerodrome with a forecasted ceiling of 600′ can be used as an alternate, if the aerodrome has a usable ILS approach, but since the 600′ is a TEMPO during a forecast ceiling of 300′, credit can not be taken.


PROB is like a TEMPO but with one distinct difference. When a PROB is found in a TAF it can not fall lower then the landing minima not alternate minima.

Like a TEMPO, credit for improving weather in a PROB can not be taken.


TAF CYDQ 171839Z 1719/1805 03008KT P6SM SCT040 BKN080 TEMPO 1719/1803
5SM -SHRA BR SCT020 BKN040 PROB30 1721/1803 2SM RA BR OVC007
FM180300 VRB03KT P6SM SCT040 BKN080

In this TAF the PROB30 between 21z and 03z is 2 miles visibility, rain, mist and a ceiling of 700′. This aerodrome, Dawson Creek, has only two non-precision approaches. Without the PROB30 this aerodrome would be a legal alternate. Normally, 2 miles visibility and a ceiling of 700′ would disqualify Dawson Creek from being a legal alternate. Since these values fall in a PROB and they don’t fall lower then the landing minima, Dawson Creek is still a legal alternate.

Below is an exert from an NDB approach in Dawson Creek. The landing minima are highlighted in the red boxes. Remember that a value of 552 would be rounded to 600.


12 thoughts on “Understand BECMG, TEMPO, and PROB with IFR Alternates”

  1. Nice website, along with my study guides the examples on your website are quite helpful. Thank you.

    HOWEVER. I do believe you have made a minor mistake on this posting. You mention “Since these values fall in a PROB and they don’t fall lower then the landing minima, Dawson Creek is still a legal alternate.” Having just read your “Calculating Alternate Weather Requirements for Aerodromes With TAFs” posting, it mentions if the Aerodrome has NON-PRECISION ONLY AVAILABLE the Weather Requirements to land as an Alternate are 800-2. Except we also would add 300-1 to the 600(552 Rounded up) and 1 3/4. Giving, 900 and 2 3/4. Thus for a “PROB30 1721/1803 2SM RA BR OVC007” we’ve now gone below minimums, and this airport is no longer a viable legal alternate.

    Otherwise, do let me know if I’m making a complete ass of myself.. Thanks & Cheers!

    1. Hi Nick,

      Thanks for your post. I believe, and I may also be wrong, that you are getting mixed up with PROB. You are absolutely right about your calculation of rounding 552 feet to 600 feet and adding 300 feet when choosing an alternate based on a TAF. Any forecasted weather in that TAF for Dawson Creek must be at or above the published alternate minima requirements. However, a PROB must be at or above the landing minima.

      Published Alternate Minima: 900 & 2 3/4
      Published Landing Minima: 600 & 1 3/4

      You are not alone in being confused about this. After learning all the rules about calculating alternate weather requirements for TAF, a PROB can be hard to get your mind around. The rational is that a PROB has a very low chance of happening, but just in case it does happen, at least you will be able to land with 600 & 1 3/4.

      My last note is not to be confused with PROB of improving weather. If you have a TAF with terrible weather, but a PROB in the middle with improving weather, you can not take credit for that improvement.

      Feel free to post or email me if you have any more questions.

  2. Phew, I’m glad we got that squared away before the INRAT. That could be a sneaky question.
    I see now, thanks for the clarification. I hope others see my error as its quite easy to look past that minor detail, since in reality we are looking at these airports as a viable alternate, and you have to consider the Published Landing Minima when your ETA falls within a PROB.

    One other thing. I was looking at the AIM – RAC 3.14, regarding TEMPOs, it only mentions: “(c) the forecast TEMPO condition shall not be below the published alternate minima requirements for that aerodrome” I could not find the part you mention above saying “Sometimes a TEMPO forecast will show an improvement. Credit for improving weather in a TEMPO can not be taken” or anything along those lines. Are you able to reference that rule, I’d just like to have to have rules down pat, you understand. Don’t get me wrong, it makes perfect sense why that would be the case, however, you know how TC is, “its in the book for a reason!” lol Thanks.

    1. Glad I could help.

      Both the AIM and the CARs reference the CAP, which is unusual, since CARs is normally the source of guidance.

      602.123 No pilot-in-command of an aircraft shall include an alternate aerodrome in an IFR flight plan or IFR flight itinerary unless available weather information indicates that the ceiling and visibility at the alternate aerodrome will, at the expected time of arrival, be at or above the alternate aerodrome weather minima specified in the Canada Air Pilot.

      If you look in the CAP GEN at point 6 under “Operating Minima – Alternate”, just before Noise Abatement Procedures it explains.

      Hope this helps.

  3. I just check my CAP (Effective SEP 2012) and then the most recent CAP on the TC website, exactly where you referenced (Point 6) and it doesn’t mention anything about “Credit for improving weather in a TEMPO can not be taken.”
    I too am sure I’ve seen this somewhere but having checked all my references: AIM – MET/RAC and the CAP GEN, it does not mention this. Perhaps its in the CARS ? Is there anywhere else you think this might be mentioned? I’m thinking now that I might have read it here! lol Would be nice to get this squared away.

    1. You are right, I can’t find anywhere where it spastically explains this. My reference would be to the RAC MET section where it defines a TEMPO. Because a TEMPO is not expected to last any more than a hour there would be no way to say at what time the weather would improve, if an improvement is noted in the TEMPO.

  4. “Credit for improving weather in a TEMPO can not be taken….Like a TEMPO, credit for improving weather in a PROB can not be taken.”

    Would you please refer me to the CARs pertaining to improving weather in a TEMPO or PROB. I can’t find it.


  5. I should have read the posts above. Seems like everyone else was asking the same question about improving weather. Your interpretation makes perfect sense but I have never done this in the industry and was wondering if TC had changed the rules.


  6. Well done for posting some important clarifications and the subsequent debate.

    This is the sort of TAF we have to put up with in the UK:

    LONDON/GATWICK EGKK 130504Z 1306/1412 05010KT 9000 FEW010 BKN040 TEMPO 1306/1311 05015G25KT 3000 +SHRA TSRA BKN010 BKN030CB PROB40 TEMPO 1306/1313 BKN006 BECMG 1312/1314 9999 BECMG 1314/1317 VRB03KT TEMPO 1314/1319 2000 +SHRA TSRA BKN006 BKN040CB BECMG 1319/1322 3000 BR BKN004 PROB40 TEMPO 1400/1407 0300 FG VV/// BECMG 1404/1406 19010KT BECMG 1406/1409 9999 NSW BKN010

    LONDON/HEATHROW EGLL 130458Z 1306/1412 06010KT 8000 -RA BKN009 BECMG 1306/1309 9999 NSW FEW012 SCT016 PROB30 TEMPO 1306/1309 SHRA TSRA BKN011 BKN040CB TEMPO 1309/1311 05015G25KT 4000 +SHRA TSRA BKN012 BKN040CB PROB40 TEMPO 1317/1320 4000 +SHRA TSRA BKN012 BKN040CB BECMG 1320/1323 VRB04KT 5000 HZ BKN010 PROB30 TEMPO 1323/1407 4000 BR BKN007 BECMG 1407/1410 22010KT 9999 NSW SCT010

    – and that is in August!

    Being pedantic, when you next amend your blog could you amend “exert” to “excerpt” in: “Below is an exert from an NDB approach in Dawson Creek”? !!!

    Best Wishes,


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