A student pilot has some mid-afternoon cross-country flying planned on a day expected to produce fair weather after the morning fog burns off. One more flight to an airport 50 nautical miles north should satisfy the cross-country requirement for a private pilot certificate. Once this trip is safely in the logbook, it will be time to go ahead and schedule a checkride, the instructor has said.
The latest terminal aerodrome forecast is generally encouraging. But unlike the serene drift of the previous day’s outlook briefing, the TAF portion that runs through midnight local time contains several elements that might affect the planning, or even shape the decision whether to go: "261727Z 2618/2718 29010KT P6SM SCT025 BKN050 FM262000 33005KT P6SM VCTS SCT025 BKN050CB FM270000 33004KT P6SM BKN250…"
Perhaps that scattered cloud layer at 2,500 feet is the remnant of the morning fog. But clearly, the weather window begins to close at 2000Z when there will be thunderstorms in the vicinity, and when that broken layer at 5,000 feet could include cumulonimbus clouds.
Well, that forecast is more than an hour old. Let’s check the latest surface observation: 261853Z 27010G16KT 10SM FEW038 SCT050 SCT110 BKN200 24/14 A2969 RMK AO2 SLP051 MOD CU N T02390139.
It seems to tell an encouraging story about winds and clouds—but there, buried in remarks, is an important item: Already there are moderate cumulus clouds to the north (the student pilot’s direction of flight.)
That means the clouds’ vertical development has already gone beyond what pilots and meteorologists refer to as “fair-weather cumulus” or “summer puffies.” That common type, you have learned from experience, signals a lumpy ride if you must fly beneath them in the updrafts, although they pose no major hazard.
“But, if the clouds are growing like castles in the air, the thermals are soaring high and could become thunderstorms. They are to be avoided,” wrote Jack Williams in "The Weather Never Sleeps."
Is this a go or a no-go scenario? It would help to update your briefing, taking a look at the current radar, more surface observations, and any current pilot reports.
But now that it is clear that the development of today’s clouds is going beyond a benign degree of vertical development, earlier than expected, be warned that rough weather could entrap the unwary.