The Unforecast Squall Line
You are planning a 1 h 40 min VFR cross-country in a light single. The TAF for departure is fine, but the area forecast mentions isolated thunderstorms developing in the afternoon. The current METAR at the destination shows 25 kt gusting 35 kt, visibility 8 km in light rain, FEW 2 500 ft, SCT 4 000 ft, and a recent thunderstorm. The radar picture you can access shows a line of cells 40 km west of the destination, moving east at about 25 kt. Your planned arrival time is 1 h 45 min from now. Your aircraft has no weather radar and no storm scope.
1. Estimate where the squall line will be at your planned arrival time and assess the risk to a VFR arrival.
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The line is 40 km west, moving east at 25 kt ≈ 46 km/h. In 1 h 45 min it will have moved about 80 km east, so it will be roughly 40 km east of the destination - i.e. it will have passed over or near the destination during your arrival window. That means the arrival is likely to coincide with the passage of the line: strong gusty winds, heavy rain, reduced visibility, possible hail and lightning, and low cloud. VFR arrival is not safe in that window. The risk is not just the storm itself but the outflow and gust front ahead of it, which can produce severe turbulence and wind shear well before the visible rain.
2. What planning options do you have before departure, and which would you choose?
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Options: (1) Delay departure so the line passes before you arrive - but the forecast is for afternoon development, so the window may be long. (2) Depart earlier to arrive before the line reaches the destination, accepting that you must be confident of the timing. (3) Divert to an alternate outside the line's path. (4) Cancel. The safest is to combine (2) and (3): depart early enough to arrive with at least 30 min before the line's estimated arrival, and nominate an alternate clear of the line. But the timing estimate has uncertainty of perhaps ±30 min, and the line may develop or move faster. Given no on-board weather capability, the conservative choice is to delay until the line has passed and the destination METAR shows improvement, or cancel. Do not launch on the assumption that you can outrun or weave around cells you cannot see.
3. If you were already airborne and approaching, what cues would tell you to divert rather than continue?
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Cues: a dark, sharply defined cloud base or a roll cloud/gust front ahead; sudden wind shift or increase in turbulence; lightning; a visible rain shaft or hail; a rapid drop in temperature; and any sign of a microburst (a sudden increase in headwind then a sharp decrease). Without radar or storm scope, you rely on eyes and the aircraft's behaviour. Any of these cues means divert to the alternate immediately, ideally to a field well clear of the line, and land before the cell arrives. Trying to scud-run under the base or thread between cells without on-board weather is a classic accident scenario; the correct decision is to give the storm a wide berth.