end of the front. Precursors for development A preexisting frontal boundary, as defined in surface weather analysis, is required for the development of a mid-latitude cyclone. The cyclonic flow begins around a disturbed section of the stationary front due to an upper level disturbance, such as a short wave or an upper-level trough, near a favorable quadrant of the upper-level jet. However, enhanced along-frontal stretching rates in the lower troposphere can suppress the growth of extratropical cyclones. Vertical motion affecting development Cyclogenesis can only occur when temperature decreases polewards (to the north, in the northern hemisphere), and pressure perturbation lines tilt westward with height. Cyclogenesis is most likely to occur in regions of cyclonic vorticity advection, downstream of a strong westerly jet. The combination of vorticity advection and thermal advection created by the temperature gradient and a low pressure center cause upward motion around the low. If the temperature gradient is strong enough, temperature advection will increase, driving more vertical motion. This increases the overall strength of the system. Shearwise updrafts are the most important factor in determining cyclonic growth and strength. Modes of development A surface low can have a variety of causes for forming. Topography can force a surface low to form when an existing baroclinic wave moves over a mountain barrier; this is known as "lee cyclogenesis" since the low forms on the leeward side of the mountains. Mesoscale convective systems can spawn surface lows which are initially warm core. The disturbance can grow into a wave-like formation along the front and the low will be positioned at the crest. Around the low, flow will become cyclonic, by definition. This rotational flow will push polar air equator-ward west of the low via its trailing cold front, and warmer air will push poleward low via the warm front. Usually the cold front will move at a quicker pace than the warm front and "catch up" with it due to the slow erosion of higher density airmass located out ahead of the cyclone and the higher density airmass sweeping in behind the cyclone, usually resulting in a narrowing warm sector. At this point an occluded front forms where the warm air mass is pushed upwards into a trough of warm air aloft, which is also known as a trowal (a trough of warm air aloft). All developing low-pressure areas share one important aspect, that of upward vertical motion within