• Surf is up on the east coast with significant winds and seas this weekend.

  • Rain developing over WA, becoming more widespread during Friday.

  • Severe frosts for southern and eastern inland parts of Australia.

  • Nice drier surge for northern Australia.

Two large long wave troughs, one off the west coast of Australia and the other over the Tasman Sea are creating sizeable weather events over coming days.

Out east, a vigorous cold front will emerge out of the Southern Ocean tonight and rapidly move north through the Tasman Sea during Friday bringing a squally southerly to the NSW coast and significant seas. A major weather event with all modes of severe weather will occur in NZ on the western side of this wave.

Out west, a cold front building offshore with a trough, likely to tap into a reasonable moisture supply through the Indian Ocean bringing up the chances of rainfall later today but more likely Friday through the weekend. Severe weather also possible with that event during Sunday to Tuesday along the south coast with a slow moving low.

Over the south and eastern inland, with high pressure sandwiched in between the waves, stable sinking air will be in place with sunny skies away from the coastal areas, cold nights with lighter winds will lead to frost developing, especially Saturday, Sunday and possibly Monday. Severe frosts are also possible Sunday for inland NSW, VIC and eastern SA.

With all that dry colder air surging north through NSW, that will rotate around high pressure and bring a nice shot of drier air into the northern states and territory with significant reduction in the humidity values. Next week, it could actually swing to be quite cold under thick cloud cover however.

National Satellite - Valid Thursday May 27th 2021.

Cloud streaming into northern WA is near a trough which is activating the moisture into showers and thunderstorms. Some of this is shearing off into areas of anvil rain, forming what will be rain areas along this part of the coastline for the coming days (more on that in a separate post earlier today). A cold front is evident off the southwest of WA and has a band of thunderstorms along that. Otherwise cold air in the Tasmania region is producing showers, gusty winds and local hail. Light showers and drizzle over southern VIC in low cloud, but dry and sunny elsewhere with bone crushing subsidence under high pressure.

A closer look at WA and you can see the showers and thunderstorms flaring throughout the Indian Ocean which is likely to increase in coverage overnight with moderate to heavy falls possible across the state in coming day. But you can see that convergence of moisture streaming south into the front as it is moving eastwards.

Moisture pooling off WA this afternoon with widespread light to moderate rain shearing southeast from clusters of storms well northwest of Barrow Island. A few thunderstorms also west of Perth at the moment, may drift closer to the coast this evening in a weakening phase. Valid Thursday 27th May 2021 1pm WST.

Temperatures after that cold frosty start over southeastern inland areas this morning have moved towards the seasonal mark. It is quite cold and gloomy over parts of southeast coastal Australia with fresh westerly winds and passing showers and temperatures have not moved much overnight and throughout today. The mercury still pushing well above the average over northern Australia but conditions are set to cool down a tad with a dry surge this weekend into next week.

Temperatures as at 230pm EST - Near seasonal values for late May across most of the mainland.

Rainfall for the next 10 days - Valid Thursday May 27th 2021.

Signals are strengthening for rainfall to be heaviest about the south facing coastal areas of WA over the weekend, with a slow moving low pressure system within the region. Widespread rainfall developing along the front initially over the coming days, with the heaviest of the falls along the west coast from about Karratha down to the Capes. Some locations could see 50mm. Then the focus of the rainfall will follow the track of the low as it moves into SA and the eastern inland next week with moisture over northern Australia via the jet stream, being lifted by this unstable parcel of air, to see showers and thunderstorms hopefully break out over the wide areas of inland Australia. Showers also likely to increase about the QLD coast with a separate coastal trough through next week in advance of the low pressure trough/system.

Modelling for the next week.

Rainfall first - Models are largely split on where the rainfall descends next week so that is low confidence and will remain so for the next 4-5 days. High confidence over the WA rainfall event with some locations likely to see 100mm of rainfall or more over the next 3-4 days.

GFS 18z run for rainfall for the next 10 days -Valid Thursday 27th May 2021

High chance of heavy rainfall for the west coast, with low to very low confidence of rainfall for the remainder of central and eastern Australia with the placement of that rainfall likely to shift around violently from run to run.

Canadian Model also showing some falls but further south in QLD. Heavy falls for the west.

CMC 12z run - Rainfall next 10 days. Valid Thursday May 27th 2021.

Synoptic Scale and Upper Level Pattern.

Friday will see a major frontal passage to the east of the nation, rolling up through the Tasman Sea bringing with it powerful surf and gale force winds. The bulk of the wet weather will stay well east of Australia but the island groups and NZ could be hit pretty hard.

Hazardous surf warnings are likely to be in place for the weekend along the entire NSW coast with the areas around the Mid North Coast and Hunter likely to see the worst of the weather from this feature.

GFS 00Z model run - 500mb flow pattern at 18000ft -Valid Thursday 27th May 2021 - Friday

A strong cold front pushing through the Tasman Sea surging southerly winds with high seas developing over the entire basin. Meanwhile a strong high pressure ridge will move into the southern Bight and keep skies clear. Out west significant cold front on the approach with a wave of low pressure likely to form on the front this weekend, with rainfall increasing with thunderstorms. The main feature for the nation is that significant wave heights over the Tasman Sea and a damaging and hazardous wave event.

Significant Wave Height forecast for Friday through Sunday across Australia. Note the swells in the Tasman reaching extreme levels over the coming days. Valid Thursday May 27th 2021.

Out west we will have another strong cold front on the approach with strong gusty northerly winds turning westerly. Along with that cooler change, widespread rain and thunderstorms expected through Friday afternoon and into the weekend, with locally heavy falls developing along the south and west coasts. Early next week, near the track and western side of the low pressure, heavy rainfall, damaging winds and hazardous surf can be expected.

GFS 00z Model run 500mb flow pattern at 18000ft -Valid Thursday 27th May 2021 - Saturday

A low will continue to deepen over the Tasman Sea with significant waves peaking over the entire basin. High pressure continuing to control the entire nation away from the southwest, with cold nights and sunny days with light winds. A low pressure system looks to develop on a frontal wave off the coast on Saturday with widespread falls over the west and south of the state.

GFS 500mb flow pattern at 18000ft -Valid Thursday 27th May 2021 - Sunday

A low pressure system will move through the southwest of the nation during Sunday, with widespread rain and thunderstorms and heavy falls continuing, but some clearance likely for the northern and central parts of the coast. A ridge of high pressure will maintain sinking air over the south and east meaning cold nights and clear skies to continue. Some frosts could be severe. (more information on the frosts in another post). Low pressure in the Tasman Sea will lift out but move a surge of drier cooler air through the northern parts of the nation.

GFS 500mb flow pattern at 18000ft -Valid Thursday 27th May 2021 - Monday

The upper low over southern WA with a surface low stacked underneath will bring severe weather risk to southern coastal areas. Flash flooding and damaging winds are possible. High pressure over the southeast will control the weather with a cold start and sunny day for Monday. Seas over the Tasman Sea will begin to ease, with the low moving off east of NZ. Heights over the Top End have come down with temperatures likely to feel better for the first part of next week.

GFS 500mb flow pattern at 18000ft -Valid Thursday 27th May 2021 - Tuesday

Upper low moves along the south coast and becomes cut off from the westerly flow moving more north of east on Tuesday. That will take the rain focus from the west coast out to southern and eastern WA and moving into western SA. Severe weather is a risk along the south coast of WA early on Tuesday. A cloud band may start to develop over the NT and northern SA as the low moves into the interior. High pressure in the east remains in place with calm conditions developing for the east coast. Seasonal conditions for most locations.

GFS 500mb flow pattern at 18000ft -Valid Thursday 27th May 2021 - Wednesday

The upper low begins to weaken over western SA with widespread cloud developing over northern and central Australia in response to moisture pooling over the northern regions thanks to jetstream dragging in the moisture from the Indian Ocean. At this stage more cloud than rainfall is likely to form over SA and the NT, but this could change. The forecast from this point becomes very low.

Moisture will be key to the development of the rainfall band through inland parts of the nation, and there is going to be a bit to work with. At this stage with the trigger still a fair way west of the moisture, the rainfall may remain patchy over inland parts of SA and the NT as mentioned above but could become more widespread as the upper low gets a kick from a front moving over southeast Australia during later next week. But the confidence on all the systems, their positions and phases will not become higher until next week. Something to keep watch on.

Precipitable Water Values - GFS 00z Run Valid Thursday May 27th 2021.

Showing significant moisture being drawn into WA and fed into a deep low off the south coast. More moisture being streamed in on the upper northwest winds over northern Australia, then being drawn south into northern SA and western QLD and NSW as the upper low lifts out into the central parts of the nation. From here, it is a very low confidence forecast in terms of rainfall. Green shading good moisture supply, brown shading not so great.

GFS 500mb flow pattern at 18000ft -Valid Thursday 27th May 2021 - A week from today.

Deep low south of Tasmania becomes the driving force for the weather over the southeast, knocking the high into the Tasman, sending a weak cold front through the region with windy showery weather. The remnant upper low over SA moves northeast through interior, running into moisture that is streaming over northern Australia with rainfall likely to break out over parts of northern SA, eastern NT, QLD and northern NSW in response. Timing of the front will determine how far south this rainfall will descend through NSW and SA, but for now it is likely to stay north. But once again a low confidence forecast is being applied to this.

GFS 500mb flow pattern at 18000ft -Valid Thursday 27th May 2021 - Taking it out 10 days.

Upper low over QLD opens into an upper trough still driving clouds and showers through the inland and along the coast, some could be heavy. A zonal flow to the south of the nation with a few embedded fronts likely to cause showers and windy weather over southeast Australia. A wave tries to form along a front over SA which may produce more widespread rainfall. Dry for the remainder with a ridge building in over WA. Cool in the east, warm in the west.

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