Most people across NSW have restlessly slept through very warm temperatures over the last few nights, the prelude to extreme daytime heat as a trough crosses the state. Thankfully, a cool change today will see the end of the heatwave for most of the state. The heatwave is coming to an end in NSW after much of the state was classified as severe or extreme. As per the BoM's heatwave definition, minimum temperatures, as well as maximums at any given location, must be unusually high for three or more days. This has certainly been occurring over NSW, thanks to a very hot airmass being dragged across the state ahead of a slow moving trough. The warmest places on Monday night were in the northwest. Tibooburra had its warmest December night at least 100 years, only dropping to a balmy 33.5 degrees following a day which saw the mercury rise to 45.9. Cobar stayed above 30 on Monday night, the town's warmest December night in more than 50 years. Tibooburra reached the mid-forties again on Tuesday, along with many other parts of the state. Bourke, West Wyalong, Dubbo and Tocal all sweated through their hottest December day in 12 years. Closer to the capital city, Penrith and Richmond both had their hottest December day in at least 23 years, 44.1 and 42.9 respectively. The heatwave hadn't loosened its grip on Tuesday night, with many centres remaining uncomfortably warm. Trangie only dipped to just under 28 degrees, its warmest for December in 35 years. Cobar residents slept through their warmest December night in 23 years, whilst Bourke stayed above 30 degrees and like the previous day, was their warmest for December in 12 years. Sydney City had a midnight minimum of 26.4 but rose to almost 30 degrees at 3am thanks to a little increase in wind speeds. Thunderstorms are likely to develop over parts of greater Sydney and surrounds this afternoon, but not before a sultry day with temperatures in the high-thirties to low-forties. The trough will cross most of NSW today, leaving a cool change in its wake. As it moves over northeastern NSW, the trough will stall and increase instability, with severe thunderstorms a risk. Alongside the threat of damaging winds and large hail, bursts of heavy rain with the thunderstorms are a hazard. From now until the weekend, parts of the North Coast and Northern Tablelands will likely receive widespread falls of over 30mm, possibly over 60mm in some parts, likely causing flash flooding in areas without sufficient runoff capabilities.