Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

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Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Razor »

MS has issued a couple of warnings for a possible impact from a system that Tony has very astutely been talking about in recent days

Heavy rain for central New Zealand. Heavy snow for the upper South Island.

A low pressure system is expected to move across the upper South Island late Saturday and Sunday morning, bringing a period of heavy northerly rain followed by cold southerly rain. The heaviest falls are expected in Nelson, Marlborough and Wellington, where accumulations may approach warning criteria. A period of snow is likely about inland parts of Marlborough and North Canterbury, with a Watch in force for heavy snow above 400 metres.

Note, there is some uncertainty with the track of this low, and people are advised to keep up to date with the latest forecasts in case further areas are added, or upgraded to a Warning.

Heavy Rain Watch


Area: Nelson

Period: 11hrs from 1pm Sat, 30 Jul - midnight Sat, 30 Jul

Forecast: A period of heavy northerly rain, rainfall accumulations may approach warning criteria

Area: Marlborough

Period: 17hrs from 2pm Sat, 30 Jul - 7am Sun, 31 Jul

Forecast: A period of heavy northerly rain turning southerly overnight Saturday. Rainfall accumulations may approach warning criteria. Note, rain may fall as snow to about 200 metres south of Seddon from Saturday night as detailed in the Heavy Snow Watch.

Area: Wellington


Period: 18hrs from 2pm Sat, 30 Jul - 8am Sun, 31 Jul

Forecast: Periods of heavy rain. Rainfall accumulations may approach warning amounts.

Heavy Snow Watch

Area: Inland Marlborough south of Seddon, and Canterbury High Country north of Culverden


Period: 10hrs from 8pm Sat, 30 Jul - 6am Sun, 31 Jul

Forecast: A period of heavy snow above 400 metres, with lighter falls down to about 200 metres. Snow accumulation above 400 metres may approach warning criteria.

Issued: 9:37am Fri, 29 Jul
Next update: 9pm Fri, 29 Jul
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Razor »

MS also quite bullish in the modelled amounts for ChCH tomorrow evening, punting for 30mm+ in the 8 hours from 5pm. That would cause more issues after the month thats been for sure- but to me that seems on the high side?
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Bradley »

Both the GFS and EC deterministic and ensemble models are all predicting between 7-15mm on Saturday night so 30mm does seem on the high side that's for sure...
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Razor »

Bradley wrote: Fri 29/07/2022 11:05 Both the GFS and EC deterministic and ensemble models are all predicting between 7-15mm on Saturday night so 30mm does seem on the high side that's for sure...
Latest GFS run has dragged the system a bit further south which is of interest
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by spwill »

Some risk for thunderstorms/hail showing for the west of the North Island Sunday morning, the risk is mostly south of Auckland but see how the upper trough tracks in next model runs.
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by TonyT »

Bradley wrote: Fri 29/07/2022 11:05 Both the GFS and EC deterministic and ensemble models are all predicting between 7-15mm on Saturday night so 30mm does seem on the high side that's for sure...
Looking at the latest guidance, for Chch to midday Sunday GFS is at 30mm, EC, Access, icon 11-16mm, UKMO 9mm.
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Razor »

TonyT wrote: Fri 29/07/2022 12:43
Bradley wrote: Fri 29/07/2022 11:05 Both the GFS and EC deterministic and ensemble models are all predicting between 7-15mm on Saturday night so 30mm does seem on the high side that's for sure...
Looking at the latest guidance, for Chch to midday Sunday GFS is at 30mm, EC, Access, icon 11-16mm, UKMO 9mm.
Meanwhile MS has now dialled back to 10mm. Obviously a bit of uncertainly on the edges.
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by mikestormchaser »

The low pressure system is dragging it further south on latest guidance. Will be some heavy snow down to low levels tomorrow night/ Sunday.
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Awhituobs »

is this low #9 from the tasman for the month of July?
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by TonyT »

Predicted Christchurch precip totals to midday Sunday from the operationals - UKMO 27mm, GFS 21mm, EC 19mm, Access 16mm, Icon 15mm
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Razor »

Looks like those numbers have ramped up a bit too. Could be problematic in Chch tonight. Nasty little system for the farmers further afield too
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Cantab03 »

Razor wrote: Sat 30/07/2022 07:55 Looks like those numbers have ramped up a bit too. Could be problematic in Chch tonight. Nasty little system for the farmers further afield too
Yes metservice putting out 42mm for CHCH by midnight now on projection.
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Bradley »

Latest EC ensemble data has 0.1cm of snow in Christchurch for this event although only 5 out of 50 members is predicting snow for Christchurch so not great odds..
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Bradley »

Snow now forecast down to 100m for areas north of the Rangitata :eek:
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Orion »

The overcast has been descending all morning and the temperature quietly dropping over the past hour or so, currently 6.7°C and there may be a hint of precipitation in the air.
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Razor »

Very steady rain in Chch now. Was up port hills this arvo, there was rain to the north and rain to the south. Convergence zone material?
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Orion »

Rain has begun to patter on the roof here; wind SW; 5.6°C.
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Razor »

Heavy snow at friends place in Castle Hill
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Richard »

Sleet in the rain here now, 4deg.
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Bradley »

Cold air not penetrating at all here in West Melton, been 4.5C for nearly 2 hours…
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Razor »

Same in town, 6 degrees for past 4 hours
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Dean. »

3.7 degrees here at 109 metres in Ashburton...radar looking interesting offshore moving onshore
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Dean. »

2 degrees in Methven at 320 metres close to snow there
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Razor »

Bit of a bouquet to Tony on calling this one well in advance of anyone else really. Tricky wee beast to spot and I note that MS just put an Orange Warning up at 8pm tonight.

13mm here so far and very steady falls indeed. Will be a few issues in Chch with this rate. Just dropped a degree to 5
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Re: Developing Low Pressure - Upper South Island/ Lower North Island 30/31 July

Unread post by Richard »

Snowing here now and settling
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