Over the last 2 days, Sydney's suburbs have been getting drenched by heavy showers but if you've been looking at the synoptic chart to find answers on where it's coming from, you're probably still scratching your head. This is because the synoptic chart only shows the conditions at the surface. Synoptic charts from the last 2 days have shown onshore winds on the NSW coast but this isn't enough to cause the intense downpours that we've seen.
One of the main triggers of these heavy showers is actually located about 4-5km above the surface in the form of a "cold pool" aloft. As this "cold pool" moved over the very warm sea surface temperatures (24 degrees at the moment!), this large temperature difference caused the instability to trigger thunderstorms. The warm sea surface temperatures have also lead to higher evaporation, providing more available moisture for these showers.
As a result, buckets of showers have soaked Sydneysiders with coastal suburbs copping the heaviest falls. Cronulla received its highest 2 day rain in 7 years with 160mm, while Sydney city received 62mm over 48 hours. Falls have been significantly lighter away from the coast with Penrith only getting 1mm. Yesterday was also the coldest day of the year in Sydney with the city shivering in a maximum of only 15 degrees .
The slow-moving cold pool has now moved offshore so showers are easing, but a repeat of this current pattern is expected on Friday and Saturday. Like the last 2 days, it will be cold and unstable so a few thunderstorms and heavy showers are likely.
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