Storms today, showers to follow…

A discussion on altocumulus castellanus, storms, and the Flood Watch from the BoM...

Arriving in town today I could see some interesting clouds in the sky – altocumulus castellanus. I love those clouds for a couple of reasons…their beautiful name (which roughly translates from latin as high accumulating (growing) clouds that looks like the top of a castle) but also because they signify instability higher up in the sky…and with the right amount of morning sunshine that can indicate a good chance of storms in the afternoon. I took a photo but it looks fairly average, so sharing this image instead which is a much better example of them:

I also spotted this image from the UK Met Office, which I felt little choice but to share – it’s the same clouds but from above:

So the local environment (Bellingen Shire on a sunny January day with heaps of sunshine, moisture and obvious instability up high) combine well with the forecast models, which all show storms developing today. Here’s the forecast lightning animation from the European model – nothing right now but building quickly through this afternoon before clearing north:

….and that clearance north is a sign that the trough bringing this unsettledweather will also be moving north. Once it clears through we’ll lose most of the storms (and worth noting again that whilst some of us will likely see good falls, others could miss out) but will then be into a showery and cooler regime for a few days. With a very warm ocean, onshore winds and some remnant cool air up high, we could well see some of the higher totals come from this post-trough showery regime. The BoM has a Minor Flood Watch out for our local rivers:

Flood Watch for the Orara and Bellinger Rivers
Issued at 9:57 am EDT on Tuesday 5 January 2021

FLOODING POSSIBLE FOR THE ORARA AND BELLINGER RIVERS FROM LATE TUESDAY

Rainfall associated with a low pressure trough has the potential to cause minor riverine flooding across the Orara and Bellinger River catchments from late Tuesday.

A broad low pressure trough extends across eastern New South Wales through to the northern inland. This trough will move northeast on Tuesday and stall, bringing unsettled weather which will persist on Tuesday and Wednesday. Heavy rainfall associated with this trough is expected to impact the Mid North Coast resulting in river level rises from late Tuesday.

Catchments likely to be affected by flooding are currently saturated following rainfall over the last few weeks

Catchments likely to be affected include:

Orara River: minor flooding
Bellinger and Kalang Rivers: minor flooding

As the BoM notes, the catchments are saturated and so respond very quickly to rainfall. Yesterday in fact we had both a final flood warning for the Bellinger as well as a Flood Watch for upcoming flooding. A La Nina is still in play and models are suggesting that we’ll continue to see wetter than average conditions over the coming few months – which means the ongoing risk of flooding with rain events. In the short term however it looks like once we get through this wet week we could see less in the way of showers (though still some around) and more sunshine through the latter part of the weekend.

Images: UK Met Office / Tropical Tidbits

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