The precipitation downdraught associated with an individual cell tends to be concentrated towards the leading edge of the storm where the cold heavy outflow spreads out at sea level forming a small high pressure cell, a meso-high, 10 – 15 nm across. The dense air lifts the warmer, moist air in its path and may initiate a self amplifying convective complex, in which neighbouring storm cells consolidate into a towering squall line of large thunderstorm cells ranged across the prevailing wind direction. At locations in the path of the squall line the resultant line squall occurs as a sharp backing in wind direction, severe gusts, temperature drop, hail or heavy rain and possibly tornadoes. If the squall line is formed in an environment of strong mid-level winds the surface gusts may exceed 50 knots.
Squall lines vary in length, some of the longest being those which develop in a pre-frontal trough 50 -100 nm ahead of a cold front. These squall lines may be several hundred nautical miles in length and 10 – 25 nm wide moving at typically 25 knots. The pre-frontal lines form ahead of the front as upper air flow develops waves ahead of the front; downward wave flow inhibiting and upward wave flow favouring, uplift. Squall lines are a common northern Australia and north into Malaysia and Sumatra feature developing along active areas of the Inter Tropical Convergence Zone, within the feeder bands of tropical storms, along sea breeze fronts or other convergence zones and in the summer heat trough. In south-east Australia they may also be associated with fast moving winter cold fronts, producing severe winds and heavy rainfall.
During daylight hours the squall line may appear as a wall of advancing cloud with spreading cirrus plume but the most severe effects will be close to each of the numerous cumulonimbus cells. The convective complex releases a tremendous amount of latent heat and moisture which may be sufficient to generate a warm core mesoscale cyclone lasting several days.
Sea breeze fronts
In coastal areas differential diurnal heating promotes development of on-shore breezes which, during the day, grow in strength to ‘moderate breeze’ and, due to coriolis effect, begin to back. The surface wind is a resultant of the sea breeze vector and the gradient wind vector. In hot land conditions the sea breeze front, a density current, can travel 100 – 200 nm inland by midnight if not blocked or diverted by terrain.
The cool air lifts the warmer inland air and, if conditions are suitable for deep convection, a squall line may develop and propagate along the convergence line of the surface flow. Opposing sea breeze fronts, such as occur in Cape York, may cause strong convergence disturbances when they meet. Along the eastern Queensland coast, typically between September and March, storm lines of cumulonimbus up to 100 nm in length form inland in mid to late afternoon then move towards the coast, being out to sea by mid-evening. Such storm lines may be difficult to avoid if encountered unexpectedly.