The METAR tells you what the weather was doing a few minutes ago. It can't tell you whether to set the alarm for tomorrow morning's flight or sleep in. For that you want the TAF (terminal aerodrome forecast), the forecaster's view of the weather at one airport for the next day or so.
It looks like a wall of code, but most of it is codes you already know from the METAR. Reading a TAF really comes down to the change groups: FM, TEMPO and PROB30 in the United States, plus BECMG and PROB40 in Canada.
How TAFs differ between the US and Canada#
United States. The National Weather Service (NWS) writes TAFs for the area within 5 statute miles of the centre of the airport's runway complex. A TAF ordinarily covers 24 hours (30 hours at some FAA-specified airports), and scheduled TAFs typically go out four times a day.
Canada. NAV CANADA's guide says Canadian TAFs have various validity periods of up to 30 hours. Two remarks are particular to Canada. RMK NXT FCST BY gives the time of the next forecast, and RMK FCST BASED ON AUTO OBS means the TAF was built partly on an automated (AWOS) observation.
A few basics hold in both countries, and they're the ones that trip people up:
- Every time is UTC. A validity period that ends at midnight UTC is written as 24, so 2912/2924 runs from 1200 UTC on the 29th to midnight.
- Visibility is in statute miles. P6SM means more than 6 statute miles.
- Cloud heights are above ground level, in hundreds of feet. SKC means sky clear.
Wind, visibility, weather and cloud use the same codes as a METAR, and our guide on how to read a METAR covers them. So what's genuinely new in a TAF is the header and the change groups.
A worked example from Pittsburgh#
This NWS example shows a front going through Pittsburgh. It's a good one to start with, because you can watch the weather change line by line:
TAF KPIT 231732Z 2318/2418 23010KT 4SM -SHRA BKN030
FM232200 28020G35KT P6SM OVC020
FM232330 30015KT P6SM SCT060
FM240500 30004KT P6SM SCT080=
| Group | Meaning |
|---|---|
| KPIT | Pittsburgh International |
| 231732Z | Issued at 1732 UTC on the 23rd |
| 2318/2418 | Valid from 1800 UTC on the 23rd to 1800 UTC on the 24th |
| 23010KT 4SM -SHRA BKN030 | To start: wind 230 degrees at 10 knots, 4 statute miles in light rain showers, ceiling broken at 3,000 ft |
| FM232200 28020G35KT P6SM OVC020 | From 2200 UTC on the 23rd: wind 280 degrees at 20 knots gusting 35, more than 6 statute miles, ceiling overcast at 2,000 ft |
| FM232330 30015KT P6SM SCT060 | From 2330 UTC: wind 300 degrees at 15 knots, more than 6 statute miles, scattered cloud at 6,000 ft |
| FM240500 30004KT P6SM SCT080 | From 0500 UTC on the 24th: wind 300 degrees at 4 knots, more than 6 statute miles, scattered cloud at 8,000 ft |
The first line after the validity period is the starting forecast. Every line below it is a change group, and each kind of group means something different.
FM: a new forecast from a set time#
FM followed by a day, hour and minute starts a fresh forecast. The NWS instruction says everything before an FM group is superseded by what follows it, and every element is restated even when it doesn't change. Transport Canada says the same of Canadian TAFs.
So each FM line in the Pittsburgh TAF stands on its own, and together they tell the story of the front. It brings the gusts and the lower ceiling at 2200. The gusts ease from 2330. And by 0500 the wind is down to 4 knots under scattered cloud at 8,000 ft. The front's been and gone.
TEMPO: spells that come and go#
TEMPO marks spells that come and go inside the window while the main forecast still applies. In both countries each spell lasts an hour at most (the NWS says one hour or less, Transport Canada less than one hour), and together the spells cover less than half the TEMPO period. The NWS adds that a TEMPO group has a greater than 50% probability of occurring and never runs longer than four hours.
That 50% is the bit we'd want every student to remember. The spells are forecast to happen, just not exactly when.
TAF KDDC 221130Z 2212/2312 29010G25KT P6SM SCT025
TEMPO 2215/2217 30025G35KT 1 1/2SM SHRA BKN010 ...
This Dodge City example (shortened in the NWS instruction) forecasts temporary spells between 1500 and 1700 UTC of gusts to 35 knots, 1 1/2 statute miles in rain showers and a ceiling at 1,000 ft. Anything a TEMPO group doesn't restate stays as forecast before it.
Cloud is the exception. When the cloud changes, both the NWS instruction and Transport Canada have every layer repeated, including any that isn't changing. So read the cloud in a TEMPO group as the whole sky during those spells.
PROB30 and PROB40: a stated chance#
PROB gives the probability that the conditions after it happen in the window. NAV CANADA puts it well. PROB30 means a 30% chance of the condition occurring in that window, not that it occurs 30% of the time.
United States. PROB30 is the only probability group in NWS TAFs. It's used for a 30% chance of a thunderstorm or precipitation event and the wind, visibility and cloud that go with it, and it covers six hours or less. The NWS doesn't combine PROB30 with TEMPO.
TAF KROW 021726Z 0218/0318 30008KT 5SM HZ BKN030
PROB30 0304/0306 27020G45KT 1SM TSRA OVC012CB ...
That's a 30% chance at Roswell, between 0400 and 0600 UTC on the 3rd, of a thunderstorm with gusts to 45 knots, 1 statute mile in rain and an overcast ceiling of cumulonimbus at 1,200 ft. We wouldn't shrug off a 30% chance of that.
Canada. Canadian TAFs use PROB30 and PROB40. Transport Canada says a probability under 30% doesn't justify a PROB group, and at 50% or more the forecaster uses FM, BECMG or TEMPO instead. A PROB can follow a TEMPO, but it must fall entirely inside or entirely outside the TEMPO hours.
BECMG in Canadian TAFs#
You won't see BECMG from the NWS, but Canadian TAFs use it. Transport Canada describes BECMG as a lasting change that happens gradually, normally over one to two hours and never more than four, and normally only the elements that change are listed after it. The change can happen at any point in the window.
Transport Canada also explains how to read a BECMG when you're choosing an IFR alternate. If conditions are improving, the new conditions apply when the change period is complete. If they're getting worse, they apply from the beginning of the period. That guidance is written for IFR alternates. We'd read every BECMG that way anyway, VFR included.
A full Canadian example#
NAV CANADA's guide decodes this TAF for Moncton (set out here one group per line, with the remarks line omitted). It's winter, and it shows:
TAF CYQM 291145Z 2912/3012 24010G25KT WS011/27050KT 3SM -SN BKN010 OVC040
TEMPO 2918/3001 1 1/2SM -SN BLSN BKN008
PROB30 2920/2922 1/2SM SN VV005
FM300130 28010KT 5SM -SN BKN020
BECMG 3006/3008 00000KT P6SM SKC
| Group | Meaning |
|---|---|
| 291145Z | Issued at 1145 UTC on the 29th |
| 2912/3012 | Valid from 1200 UTC on the 29th to 1200 UTC on the 30th |
| 24010G25KT | Wind 240 degrees true at 10 knots gusting 25 |
| WS011/27050KT | Low-level wind shear from the surface to 1,100 ft AGL, with the wind at 1,100 ft from 270 degrees true at 50 knots |
| 3SM -SN BKN010 OVC040 | 3 statute miles in light snow, ceiling broken at 1,000 ft, overcast at 4,000 ft |
| TEMPO 2918/3001 | Between 1800 UTC on the 29th and 0100 UTC on the 30th, temporarily: 1 1/2 statute miles in light snow and blowing snow, ceiling broken at 800 ft |
| PROB30 2920/2922 | A 30% chance, between 2000 and 2200 UTC, of 1/2 statute mile in snow with a vertical visibility of 500 ft |
| FM300130 | From 0130 UTC on the 30th: wind 280 degrees at 10 knots, 5 statute miles in light snow, ceiling broken at 2,000 ft |
| BECMG 3006/3008 | Between 0600 and 0800 UTC on the 30th: calm, more than 6 statute miles, sky clear |
Using the TAF to decide whether to fly#
Say you've got two flights in mind and the Moncton TAF in front of you. Here's how we'd read each one.
A VFR arrival at 2030 UTC on the 29th. This one's easy, if disappointing. The base forecast is already a 1,000 ft ceiling in snow with wind shear below 1,100 ft. The TEMPO brings an 800 ft ceiling and blowing snow, and the PROB30 gives a 30% chance of half a mile in snow, with a vertical visibility of 500 ft, right at your arrival time. The base forecast alone makes this a no-go for VFR, before you even weigh the PROB30.
A VFR departure early on the 30th, UTC. This one's more hopeful. After the FM at 0130 the ceiling is broken at 2,000 ft in light snow, and the BECMG between 0600 and 0800 clears it. Conditions are improving, so plan on the clear sky only from 0800 UTC, the end of the window. In winter that's 0400 Atlantic Standard Time in Moncton, so the BECMG is done well before a normal morning departure. For a very early start, though, a plan built on 0600 UTC is assuming the best case.
Pittsburgh makes a different point. Say you plan to land back there at 2130 UTC. You'd be planning to land 30 minutes before a forecast frontal passage with gusts to 35 knots. FM times are a forecaster's best estimate, and fronts can arrive early. Leave early enough to be on the ground well before 2200, or don't go. Put 28020G35KT through the crosswind calculator for the runway you expect.
The same approach works for any TAF:
| Group | How to plan on it |
|---|---|
| FM | Treat the time as firm but not exact, and leave a margin either side. |
| BECMG, getting worse (Canada) | Assume the worse conditions from the start of the window. |
| BECMG, getting better (Canada) | Assume the better conditions only from the end of the window. |
| TEMPO | Expect the spells. If they are below your minima at your departure or arrival, you need a plan that works if one turns up: wait it out, hold, or divert. |
| PROB40 (Canada) | Plan as though it will happen. |
| PROB30 | Read what it is a chance of. In a US TAF it is always a thunderstorm or precipitation event, so read the wind, visibility and ceiling that come with it. |
The NWS tells its own forecasters that the lowest conditions anywhere in a TAF, including those in PROB and TEMPO groups, drive users' operational decisions. We think pilots should read it the same way.
Brief the whole time you might be flying, from departure to well after your planned arrival, because flights have a habit of running late. Check the issue time too. TAF AMD means the forecast has been amended, so always brief from the latest one. On a cross-country, the flight planner puts the departure and arrival TAFs side by side, so you're reading both for the same hours.
When your airport has no TAF#
Plenty of smaller airports don't issue a TAF, and they're often the ones we'd most like a forecast for. Read the TAFs for the nearest airports that do, ideally on both sides of you and upwind. For the route, use the GFA (Graphical Forecasts for Aviation) on aviationweather.gov in the US, and NAV CANADA's graphic area forecast (GFA) in Canada. Bear in mind a TAF only covers the airport it's written for. The weather between two good TAFs can still close a route.
See the forecast on a timeline#
Type an ICAO code into the METAR and TAF decoder and it shows the raw TAF, each period in plain English with visibility in statute miles, and a timeline coloured by flight category (VFR, MVFR, IFR and LIFR). Prevailing conditions are shown above and the TEMPO and PROB periods below, so the hours that drop to IFR stand out.
For a cross-country, print the flight planner as your briefing. It puts the latest METAR and TAF for your departure and arrival and the wind on every runway on one page.
Holding aircraft for weather in Volanti#
A METAR can only tell a club about weather that's already arrived. A TAF gives you notice, and that's worth using. In Volanti, an administrator can hold an aircraft for a set period with weather recorded as the reason, so nobody books into the snow. When a booking is cancelled for weather, the aircraft or instructor can be held for everyone rather than only for the member who cancelled. And when the hold is released, the waiting list runs again on its own. See how in booking and scheduling.