Monday will be an action-packed day for much of NSW as surface and upper level troughs generate widespread thunderstorms, and Tropical Cyclone Gita sends big waves to the coast. A surface trough will linger over much of NSW on Monday, which alone can produce showers and thunderstorms. In addition to this trough, a pool of cold air in the upper atmosphere, otherwise known as an upper trough, will exacerbate the instability, and combined will likely trigger thunderstorms over a large area. Thunderstorms look to spark up during the morning over southern parts of the state, possibly stretching up to Sydney, as the upper trough migrates north from over the Southern Ocean. A high pressure ridge over the southeast is helping to provide moisture for storm development, with onshore winds a result of the anticlockwise airflow around the high. As the upper trough continues to move north, thunderstorms become a risk for areas in the north of the state. The combination of a surface and upper trough are conducive to severe thunderstorm development, and throughout the day the most likely places to see severe thunderstorms are over the ranges and the western slopes and plains. Damaging winds, heavy rain and large hail are possible in this synoptic pattern. Far western parts of NSW will miss out on most of the action, however isolated storms are possible. Moving offshore, Tropical Cyclone Gita continues to generate very large waves which are currently pounding the northeast coast of NSW. Gita will continue to track south towards New Zealand over the coming days, and as it does the wave heights will increase significantly. Waves of 2-3 metres are likely on Monday, and as a result a Hazardous Surf Warning is current for the whole NSW coast. Unfortunately for surfers, especially those south of the Mid North Coast, strong onshore winds followed by a gusty southerly change will make conditions messy. Even bigger wave heights are expected on Tuesday, between 3-4 metres, so experienced surfers may be able to find some joy in protected southern corners of beaches. As always, when severe weather and surf happens, keep up to date with warnings at http://www.weatherzone.com.au/warnings.jsp