A cold front is forecast to move across the coastal area of China two to three days later.
The leading edge of the cold front will produce some thunderstorms and showers. But why? It is often mentioned in television broadcast that cold air lifts warm air upwards as a cold front moves closer, thus generating unstable weather. But this is not the whole picture.
The picture below is the forecast sounding generated by the GFS model. It shows the temperature and wind profile (with height) at a location.
How to interpret the sounding:
1. Saturation: First the green line represents the dew point temperature, whereas the red one is the forecast temperature. Low pressure values (in hPa) denotes higher altitudes. You should notice that the two lines are very close to each other, indicating high values of relative humidity. This is favorable for rainstorm activities.
2: Instability: As a general rule of thumb, if the air column is close to saturation, the air becomes unstable if the slope of the temperature plot is not as vertical as the purple line (especially when the red line lies to the left of the purple one)
The purple line is called the moist adiabatic lapse rate (MALR). A hypothetical saturated air parcel will evolve according to MALR. Thus, if MALR is steeper than temperature plot, the hypothetical air parcel will be warmer than surrounding air, and it will continue to rise upwards as just a hot air balloon does. This upward motion releases instability and may lead to thunderstorms.
3. Wind profile: Generally, warmer air is found on the equatorward side of a cold front. This gives an average westerly vertical shear across the front. Southerly winds near the surface transport warm air northward towards the front. This movement of warm air will force air to rise upward too (due to the Omega Equation). In addition, wind direction turns clockwise with height, which will also enhance upward motion by generating vorticity to support convection.
Other factors that reinforces storm activities (not shown) includes: presence of an upper level trough is present, presence of a strong convergence zone at lower levels, and whether the area concerned is near the entrance of a jet streak, etc.