Heat is building alongside the threat of severe thunderstorms over much of NSW and the ACT today and over the coming days. Many parts of northern NSW remains in the grip of an extreme heatwave, with severe or low-intensity heatwave conditions blanketing most of the remainder of NSW and ACT. Today, most centres in Upper and Lower Western NSW will reach the mid-forties, whilst the mercury will rise to about 40 for areas adjacent to either side of the ranges, including western Sydney. The forecast of 35 degrees for Canberra is nine above the December average. Similar to Sunday evening, thunderstorms will ignite over the ranges, slopes and parts of the coasts. The risk of some of these storms being severe is high. Severe storms may generate over a very large area, triggered by a broad low pressure trough covering most of NSW and the ACT. Damaging winds, heavy rain and large hail are all possible during severe thunderstorms. Heat will linger in many places on Tuesday and Wednesday as northwest winds ahead of an approaching trough and cold front keeps the mercury well above normal. There is a good chance both Canberra (high thirties) and Cobar (mid forties) could break their December heat records on Tuesday. Penrith may have its hottest December day in 23 years. Thunderstorms are also likely to spark up over the ranges and adjacent areas. A cool change will sweep across NSW and the ACT during Wednesday, although most areas will still be warm-to-hot before relief comes in the form of a southerly wind change.