MetService has issued a RED Weather Warning for Canterbury!

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Batman

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#2716142 31-May-2021 13:01

larknz:

They actually say somewhere on their web site that the predicted rainfall is a computer prediction. I'm not sure whether or not the computer looks out the window.

This is from the metservice website

you see, if the computer says no rain and the text says rain, you've got everything covered.

if the computer says rain and the text says no rain, you've got everything covered.

it's like economists saying - the house price will go down, but if everybody buys then it will go up. then at other times it says house prices will go up, but if nobody buys it will go down.

before the days of the internet it used to be called astrology.

Fred99

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#2716149 31-May-2021 13:32

Weather forecasters usually call what they produce "forecasts" - not "predictions".

Over the past 30 years, they've become much more accurate.

It's fairly obvious from that image from earlier this morning that a small difference in the path of the low pressure system has a major impact on wind direction (thus precipitation) over small distances, so if you expect pinpoint accuracy in forecasts, you're not going to get it.

tdgeek

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#2716153 31-May-2021 13:48

Fred99:

Weather forecasters usually call what they produce "forecasts" - not "predictions".

Over the past 30 years, they've become much more accurate.

It's fairly obvious from that image from earlier this morning that a small difference in the path of the low pressure system has a major impact on wind direction (thus precipitation) over small distances, so if you expect pinpoint accuracy in forecasts, you're not going to get it.

Not expecting pinpoint. If they say rain today possibly heavy, I don't expect to see 0.0 all day. If today, 31 May, they say rain all day at times heavy, I don't expect to see 0.4 all day. Invariably each 0.4 or thereabouts turned into an actual of 10 to 15 times higher

Get it more representative, or forget the numbers just say light. medium and heavy rain

msukiwi

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#2716236 31-May-2021 15:52

Almost 50mm here since 0700hrs so far.

Water is still across the Street. No Hi-Vis etc seen since 1000hrs!

(I think they have given up - the last ones from the CCC said they know what the problems is but there is no money to fix it!)

neb

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#2716281 31-May-2021 18:19

tdgeek:
Whats with Metservice rain data? OFTEN they say its rain maybe heavy yet the hourly forecast rainfall is almost always 0.0. Today its rain and at times heavy yet every hour is less then one mm. When the hour has passed we see a true figure of 3.0 to 5.0 mm.

May I offer in response Neb's Universal Rain Prediction Service (NURPS), guaranteed to be no less accurate than the Metservice and frequently more accurate:

Chance of rain, possibly heavy in places

This works for any location and any date or time.

If there's one thing that meteorologists have become quite good at it's predicting probabilities of weather events. If a meteorologist says there's a 40% chance of rain tomorrow, the chance of it raining is indeed about 40%. This is so remarkable they've been extensively studied to see how they manage this.

However, the next step is how it's reported. In the US it was presented as absolutes or 'chance of X' before 1965, when they switched to percentages. In a lot of Europe percentages started to appear around 1990 (France notably is still percentage-free, because it's France). In NZ, the Metservice still uses the dumbed-down version that most of the rest of the world (or at least the documented world, places like Uganda do it slightly differently) has abandoned because it doesn't work. Thus NURPS, using the Metservice way of presenting weather info and guaranteed never wrong while at the same time totally useless.

tdgeek

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#2716360 31-May-2021 19:23

NURPS! Love it! However, I do feel that 0.4mm per hour isnt quite heavy rain as forecast. Nor does 0.0 all day when they have said rain today, heavy at times, but it's 0.0 all day??? Im sure its not that hard to be close than 5% to 10% accurate. In any case, rain today at times heavy is enough, don't bother with the silly predictions

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tdgeek

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#2716438 1-Jun-2021 07:16

Canterbury and Christchurch rainfall breakdown over the last 7 days. Where I am about 5.5 inches. I recall ChCh has 26 inches per annum so about 1/5 of the annual rainfall in 3 days

https://www.ecan.govt.nz/data/rainfall-data/

Fred99

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#2716489 1-Jun-2021 08:04

tdgeek:

Canterbury and Christchurch rainfall breakdown over the last 7 days. Where I am about 5.5 inches. I recall ChCh has 26 inches per annum so about 1/5 of the annual rainfall in 3 days

https://www.ecan.govt.nz/data/rainfall-data/

So pretty much as forecast for Chch city, as well as being pretty much as forecast for the areas which prompted the red alert.