It’s been a while since the last blog – I’ve been making the most of the exceptionally good weather. Forecasts for the last few days have included the possibility of thunderstorms and showers, so perhaps a few words on weather decision making are in order.
Some weather decisions are straightforward. If the observed cloud base or visibility are below VFR minima or your personal minima, you don’t go. If the cross wind is greater than the demonstrated aircraft limit, or your personal limit, you don’t go. You may think twice if the 2000 ft winds are greater than 35 knots: you could anticipate turbulence, ground speed into wind will be painfully slow, and the offset between track and heading may be confusing for cross-country navigation.
It gets a bit trickier if the forecast is poor but set to improve or vice versa. A BECMG trend in a TAF will 
always give a time range – so how to tell when the weather will improve or worsen? Here are a few tactics:
- Look at the actuals (METAR’s) upwind, and see what the trend is there. Similarly, you can call your destination airfield and ask what they observe.
- Look at the visible satellite images on the Met Office website to see what the cloud trend is.
- Examine the rainfall radar and observe how quickly the rain is coming in, and whether if is worsening or dissipating.
- If similar trends have been forecast several days running, it may be reasonable to assume that the timing will mimic previous days. For example, in winter low cloud or fog early in the morning may be forecast to lift to reasonable VFR conditions later. Successive days have a habit of repeating themselves.
- Use a more sophisticated tool like Autorouter’s GRAMET tool (details on our Planning >> Weather page) for another interpretation of the forecast data.
- Keep the back door open – it is better to fly towards forecast poorer weather, so you have the option of turning back. If you fly away from worsening weather, there is a chance that it will have arrived at your home airfield before you get back!
Another source of uncertainty are PROB30, PROB40 and TEMPO. With student pilots, we will often err on
the cautious side i.e. assume these things will happen and not send them on a solo cross-country. But for personal flying, what shall we do? Here are a few tips:
- ISOL and OCNL in the F215 mean exactly that – the phenomenon (typically showers) will be well-spaced, and it should be possible to fly around them.
- TEMPO in a TAF means not more than 1 hour at a time, and not more than half the forecast period. So TEMPO +SHRA means we could avoid them by delaying our departure, or avoid a heavy shower at our destination by orbiting clear, if fuel state allows.
- Weather radar is often helpful, as it can give an impression of how heavy and extensive the showers are, and also if they are intensifying or dying out.
- And lastly, the trend observed on previous days may – if other conditions are the same (think temperature, pressure, air-mass type) – may also inform your decision.
Happy flying!