Weather gurus differ on deluge intensity

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jeffsweather
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Weather gurus differ on deluge intensity

Unread post by jeffsweather »

Interesting story in the Southland Times today where Bob McDavitt and Augie Auer disagree on the intensity of the deluge in Belfour on Tuesday (4th).
http://www.stuff.co.nz/stuff/southlandt ... 11,00.html

"The short, sharp downpour cut off roads, buildings and communication systems across the province on Tuesday. Peter Davidson said yesterday he recorded 58mm of rain in 15 minutes at his Balfour property from about 6pm on Tuesday night. He had no doubt about the amount of rain, which added up to about 50mm in 15 minutes on another neighbour's gauge, Mr McKenzie said."
Mr McDavitt said he doubted the accuracy of the Balfour gauges. MetService equipment in nearby Lumsden recorded 24mm in the hour from 5pm to 6pm.


As we well know on this group, high intensity rainfall can vary widely over an area and as no officially calibrated rain gauge was in the deluge, it can't be counted as official.

Interesting the comments on summer...
The experts did not agree on the forecast either. Mr McDavitt thought summer was just around the corner: "just keep waiting".

But Mr Auer, who several months ago correctly predicted flooding in Southland around Christmas time, was pessimistic. "You might get the odd fine day or two but I've already written summer off."
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NZstorm
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Unread post by NZstorm »

I agree with Bob, 58mm in 15minutes at Balfour is not a believable rainfall reading. The 24mm in one hour is more in line with whats possible over Southland, especially given precipatable water rates and CAPE(convective energy) over the area on that day. :)
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Unread post by spwill »

Yes, 58mm in 15min is just too spectacular to be believable
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Unread post by 03Stormchaser »

it sounds like two people recorded similar amounts in the same area, so could be possible :roll:

yeah summer this year is a write off, might aswell cash it in and wait for next summer!
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Unread post by NZstorm »

The atmosphere was quite cold on that day too. The 500mb temperature was -21C and the freezing level looked to be around 2000m. PW was 21mm. Surface dewpoints were around 13C. The colder atmosphere makes it much less likely that such an intense shower (58mm in 15minutes) could have occured. If cb's got particularly active (higher CAPE due to strong surface heating) the resultant precipitation would have been hail stones, not water. The farmers would have been looking at white paddocks, not floods.

The ground down there is very sodden at the moment so any deluge would cause surface flooding. That explains all the surface water they reported.
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Unread post by 03Stormchaser »

NZstorm wrote:The atmosphere was quite cold on that day too. The 500mb temperature was -21C and the freezing level looked to be around 2000m. PW was 21mm. Surface dewpoints were around 13C. The colder atmosphere makes it much less likely that such an intense shower (58mm in 15minutes) could have occured. If cb's got particularly active (higher CAPE due to strong surface heating) the resultant precipitation would have been hail stones, not water. The farmers would have been looking at white paddocks, not floods.

The ground down there is very sodden at the moment so any deluge would cause surface flooding. That explains all the surface water they reported.
so how would you explain two people getting similar rainfalls??
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Unread post by NZstorm »

so how would you explain two people getting similar rainfalls??
I couldn't comment on the rain guages as I know nothing about them. But there is no doubt the area got some very heavy rain. I am just very sceptical on the quantities recorded. (50mm+ in 15mins).

:)
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Unread post by Gary Roberts »

Speaking from personal experience, I can say that the vast majority of people, if not actually everybody, always grossly overestimate the rate and totals when it comes to rain.

Our electronic gauges were often criticised for "under-reading", yet when we compared their data with that from manual gauges, they were actually over-reading slightly, due to splash (we had them sitting on wide, flat surfaces...they are now mounted on rigid poles).

I've yet to meet anybody who is accurate when it comes to rainfall estimation.

It looks heavy, it sounds heavy, it feels heavy...but is it really heavy?
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Unread post by Michael »

03 Stormchaser wrote:it sounds like two people recorded similar amounts in the same area, so could be possible :roll:

yeah summer this year is a write off, might aswell cash it in and wait for next summer!
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Unread post by Gary Roberts »

And the claim that Summer is just around the corner may well be accurate...it's been very down nice here the past few days, and still improving. The weather finally appears to be settling down.

I've never seen it so green around these parts.
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