NSW - SEVERE WEATHER RISK - ADDITIONAL FLASH AND RIVERINE FLOODING.

The wet season keeps on keeping on for the east coast with yet another wave of heavy rainfall and flooding potential this week leading to some areas once again recording rainfall totals in excess of 200mm for the period. This could lead to additional flooding and flash flooding.


The risk for the system is broad and associated with a deep upper trough slowly moving through the eastern inland. This trough will tap into the moisture left over from the weekend feature and that should be enough to produce widespread falls for areas hit hard by March flooding.


Severe weather is a moderate chance at the moment but this could still change as we move through the days ahead, either reducing or increasing. The period of April in these shoulder seasons more generally, can be volatile, so careful consideration of the data is required before pulling the trigger so to speak, and issuing warnings too far out for too many regions at once.


I can be more specific probably from tomorrow once I can see the system in real time and the data from the upper air network is fed back into the model data. That sharpens the forecast up and will answer the question, is what we are seeing a copy cat system that happens in the wake of a prolonged period of severe weather or is it a real event that is going to bring further above average rainfall?


Lets take a look at the data


Severe Weather Watch - Wednesday through Saturday

Areas of heavy rainfall likely to develop in association with a deepening upper trough and low through inland NSW. This will likely link into a moist onshore easterly flow and increase rainfall over saturated catchments leading to additional flood risks. Thunderstorms will also form over inland areas, leading to scattered thunderstorms, some of these could be severe and the colour of green over inland areas could be expanded and increased in chances in the next day or so.

Heavy Rainfall Leading to Additional Flooding

Heavy showers developing on Tuesday night will likely turn to areas of rain with thunderstorms with moderate to heavy falls accumulating from Wednesday through Saturday. Daily rainfall totals of 50-100mm for some parts of the southern and central coast could see weekly totals clear 300mm in areas that are saturated. A major flood event is possible over these areas, but will come down to placement of the upper level system. So if you are living in the zones identified here, be aware of the risk developing and stay close to forecasts, official warnings and advice from authorities. I am giving you a heads up well ahead of the authorities, not to hype the event but to risk assess and mitigate danger.

Heavy Rainfall Leading to Flash Flooding

With the saturated soil and catchments, the rainfall forecast will result in the rivers and streams/creeks responding rapidly to moderate to heavy rainfall. Some areas could be upgraded to very high to dangerous flash flooding based on some of the data coming in so these forecasts will be updated and more localised forecast charts will be drawn if required.

Rainfall Next 7 Days

Rainfall remaining very much above average for the eastern third of NSW and into the ACT if the data sets do verify. Some modelling has rainfall further west, however that idea is not as supported like the rainfall being heavy along the coast. The zone of the heaviest rainfall is forecast to be between Newcastle and Narooma at this time. The northeast should also remain on alert, but current trends are pushing the heaviest of the rainfall further south at this time. Inland areas, you rainfall numbers will vary with thunderstorms, with the uneven distribution of rainfall likely under this upper low. Conditions should ease by Sunday but not totally clear with another system likely to emerge early next week.

DATA

12Z Euro Surface Pressure Pattern and Rainfall Distribution - Next 10 Days

We can clearly see the low is moving away today and weakening as it is absorbed into the westerly wind belt. Drier weather developing through the region. From Tuesday you can see the upper level trough south of SA coming into the eastern inland of the nation with the system becoming cut off and slow moving through inland NSW for the remainder of this week. Some heavy rainfall is possible about the coast and thunderstorms over the inland may turn severe at times from Thursday into the weekend. But the overall pattern carries a low confidence which hopefully will shift towards a moderate confidence as better data analysis is drawn into the models this week.

12z Euro - Upper Air Pattern - Next 10 Days

Noting that all of the instability is found in the upper levels and will be the driving force for this unsettled spell of weather. The slow migration of the system through the southeast and eastern inland and position will dictate the spread of rainfall. Right now using the higher resolution Euro, it does appear that this event could be drawn out through to the weekend before the system is swept out to the east during the weekend and into next week ahead of another system.

12z Euro Rainfall Accumulation - Next 10 days

I think playing the rainfall progression through the week shows you the coverage of rainfall and how it evolves. Frequent showers or areas of rainfall along the east coast will bring moderate to heavy rainfall, some very heavy falls possible about the Central NSW Coast at this time. But that will shift around. Over the inland, uneven distribution of rainfall is expected with the scattered showers and thunderstorm activity likely to get underway from Wednesday but maybe increasing Thursday through the weekend. Some of the storms could drop a month's worth of rainfall. But at this time, far western areas are dry. But as per the other models that may not be the case so pay attention to the forecasts as the data starts to improve as forecasts will change!

The static image for the next 10 days

18z GFS Rainfall Accumulation - Next 10 days

The latest GFS showing that enhanced rainfall risk along the east coast from Sydney and western Sydney where there could be some major flood potential once again with the risk extending down to the South Coast. On this basis, the northeast of NSW would be spared. Also noting a follow up system is due during the early part of next week through SA which could bring more inland rainfall.

12z CMC Rainfall Accumulation - Next 10 days

Showing the upper low further north and tapping into widespread moisture left over by this current event, leading to rain further inland with some areas getting 1-2 months of rainfall. Noting the heavy rainfall bias near Ulladulla to Wollongong. That area is likely to see heavy shower activity this week. Northeast areas on this current guide would be spared. Also has another system next week approaching the west.

18z ICON - Rainfall Accumulation - Next 5 days

Latest German showing fairly heavy rainfall along the coast from Sydney through to Ulladulla which seems to be a theme on the modelling. Thunderstorms over the northern and central inland could deliver some a months worth of rainfall and reasonable falls for the ACT too and surrounds.

I will have more on this in the state based forecast this evening when the next data set is in.

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