Convective Initiation over the Serra do Mar and the 18:00 UTC TAF Window
It is 15:30 UTC on a summer afternoon at SBGR (Guarulhos, São Paulo). A surface cold front lies over northern Paraná, moving northeast at 15 kt. The 12:00 UTC SBGR sounding shows a surface temperature of 30 °C, a dewpoint of 21 °C, a lifted condensation level near 1,300 m, a level of free convection near 2,000 m, and a convective inhibition of 80 J kg⁻¹. The equilibrium level is near 12,500 m. Low-level flow is from the southeast at 8 kt, while the 700 hPa wind is from the northwest at 20 kt, giving strong directional shear in the lowest 3 km. The 10:00 UTC model run keeps the front 60 nm northwest of SBGR at 18:00 UTC. The TAF for SBGR valid 15:00–21:00 UTC currently carries only SHRA and no TS. The airport has one runway in use and no parallel taxiway, so any ground stop propagates quickly.
1. Using the thermodynamic and kinematic evidence, decide whether the TAF should be amended to include TSRA and, if so, in which time window. Justify the decision and state the residual uncertainty.
Read the worked answer
The sounding supports deep convection once the CIN is overcome. With a surface temperature of 30 °C and dewpoint of 21 °C, the LCL near 1,300 m and LFC near 2,000 m imply only about 700 m of dry ascent before free convection begins. The 80 J kg⁻¹ of CIN is modest and can be removed by roughly 1–2 °C of surface heating or by low-level convergence along the sea-breeze front from the southeast flow. The EL near 12,500 m indicates a deep, well-developed updraft, and the strong low-level directional shear (southeast surface versus northwest at 700 hPa) favours organised multicells or a short-lived supercell rather than isolated pulse storms. The approaching cold front provides the synoptic-scale forcing, but the model keeps it 60 nm northwest at 18:00 UTC, so the primary trigger at SBGR is likely the local convergence zone rather than the front itself. A reasonable amendment is to introduce a TEMPO or PROB30 TSRA from about 17:00 to 21:00 UTC, timed to peak surface heating and to the sea-breeze convergence maximum, with CB and reduced visibility in the remarks. The residual uncertainty is whether the model is displacing the front too slowly; if the front arrives earlier, the window shifts forward and coverage increases. The forecaster should also flag the possibility of hail and strong gusty surface winds given the shear and the deep mixed-phase layer.
2. The tower asks whether a ground stop is justified at 17:30 UTC if radar shows an echo with a 50 dBZ core at 8,000 m, 12 nm southwest of the airport, moving northeast at 20 kt. What is your operational assessment?
Read the worked answer
At 20 kt, a 12 nm echo takes roughly 36 minutes to reach the airport, so the earliest impact is around 18:06 UTC. A 50 dBZ core at 8,000 m is well above the freezing level in a deep convective column and indicates a mature cell with a strong updraft and likely hail aloft. The primary aviation hazards are microburst potential on descent, lightning within 5 nm of the movement area, and wind shear on final approach. Because the airport has a single runway and no parallel taxiway, a ground stop is operationally defensible now rather than after the cell is overhead, since the ramp cannot absorb a sudden closure without cascading delays. The recommended action is a precautionary ground stop of 30 minutes beginning at 17:45 UTC, with a review at 18:15 UTC, plus a low-level wind shear warning and a lightning advisory. The forecaster should confirm the cell's motion and whether it is part of a developing line; if a line is forming, the ground stop should be extended rather than lifted at the first gap.