Afternoon convection over a valley route
At 0600 local a plains aerodrome reports surface temperature 24°C, dewpoint 20°C, and a light southerly wind. The 0600 sounding is: 1000 hPa (150 m) 23/19; 850 hPa (1,500 m) 15/12; 700 hPa (3,000 m) 5/−1; 500 hPa (5,600 m) −11/−22. A shallow inversion is present near 1,700 m. The forecast maximum surface temperature is 33°C by 1500, and a weak shortwave trough will cross the area between 1400 and 1600. A VFR flight is planned from 1400 to 1700 along a valley route with a minimum terrain clearance of 1,000 ft; the aircraft has no weather radar and no oxygen. The 0600 observation shows scattered cumulus at 4,000 ft and no CB. The surface wind is expected to remain light and variable until the trough arrives.
1. Using the sounding, assess whether deep moist convection is likely by 1500. Show the lapse-rate and parcel reasoning.
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Layer lapse rates are about 5.9°C/km (1000–850 hPa), 6.7°C/km (850–700 hPa), and 6.2°C/km (700–500 hPa). These are near or above the saturated adiabatic rate, so the air is conditionally unstable, and surface heating will steepen the low layers. For a surface parcel at the forecast 33°C/20°C, the LCL is about 125 × (33 − 20) ≈ 1,625 m, just below the 1,700 m inversion. Lifting dry-adiabatically to 1,500 m gives 33 − 9.8 × 1.35 ≈ 19.8°C against an environment of 15°C, so the parcel is about 4.8°C warmer. The inversion is a cap, but it is shallow and the parcel is already buoyant at its base; surface heating plus the shortwave trigger should break it. Above the LCL the parcel remains warmer than the environment at 700 and 500 hPa, so deep convection is likely, with CB tops above 500 hPa.
2. What hazards would the 1400–1700 VFR flight face, and how would you plan around them?
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Once CB develops, the hazards are not limited to cloud: severe turbulence, hail, lightning, strong up- and downdrafts, microburst wind shear near the surface, icing above the freezing level, and reduced visibility in rain. The valley route with 1,000 ft terrain clearance leaves little room for a downdraft, and the aircraft has no weather radar, so avoidance must be visual and early. I would plan to fly before 1300, or delay until evening, and identify a diversion aerodrome outside the storm area. If cells are already growing, I would not enter the valley; I would route around them by at least 20 nm, avoid flying under anvils, and keep an escape route. The lack of oxygen also rules out climbing over a mature CB.
3. At 1200 the surface observation is 29°C/14°C. How does that change your assessment?
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The dewpoint has fallen from 20°C to 14°C, so the surface spread is now 15°C. The LCL rises to about 125 × (33 − 14) ≈ 2,375 m if the surface still reaches 33°C, well above the 1,700 m inversion. A higher LCL means a deeper dry layer, more entrainment, and less CAPE; the cap is also stronger because the parcel must be lifted farther before it becomes saturated. Deep convection is no longer the most likely outcome; scattered towering cumulus or isolated weak cells are more probable, and storm coverage should be reduced. I would still avoid any developing cell, but the VFR flight is less exposed than in the 0600 scenario. I would update the decision using the 1200 observation and the latest radar/lightning data, not the 0600 forecast alone.