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Chockstone Forum - General Discussion

General Climbing Discussion

Weather forecasts

1:48:19 PM
Ok forecasts are forecasts, indications, can't be trusted, blah blah, I get that. But can anyone explain why the following weather forecast (from Elders, I beleive it is a BOM source? most are the same) describes Tue as "Rain" when it has a 90% chance of 5-10mm, but describes Wednesday as "Possible shower" when it has same 90% chance and a greater 10-20mm. WTF?

Possible explanations:

1. Description is daylight hours (e.g. mostly sunny) but chance/amount is 24hr?
2. One describes spread out precipitation the other concentrated.
3. I'm a retard
4. They're all retards

2:01:29 PM
Rain generally equates to widespread, continuous precipitation. Showers have gaps between them. The summaries are usually hopeless by themselves though, the words give a much better idea of what's going on.
2:18:23 PM
There is a great comment on reddit that explain those percentage :

I haven't had time yet to check the links posted in the main post : , but I imagine that would be interesting to watch.

2:34:35 PM
Sometimes it's so bad that it rains between showers.

2:35:16 PM
Just use a website that breaks it down throughout the day

Examples for Katoomba this wekend...
2:35:33 PM
+1 That explains it really well.

On 26/09/2013 gaetanr wrote:
>There is a great comment on reddit that explain those percentage :
3:09:38 PM
"Probability of Precipitation Forecast
As part of the NexGenFWS, rainfall forecasts for major centres are presented as chance of any rain and rainfall amount for day one of the forecast."

"Chance of rain describes the likelihood of receiving a measurable amount of rain (>0.2mm) during the day at that location. For example, if the chance of rain for Mildura is 30%, it means that on 3 out of 10 days with similar weather conditions rainfall will be measured in the Mildura rain gauge. Where there may be a 30% chance of any rainfall, there is also a 70% chance of not receiving any rainfall at all."

Precipitation, often short-lived (but may last half an hour) and heavy, falling from convective clouds. Usually begin and end suddenly."

Precipitation of liquid water drops greater than 0.5 mm in diameter. In contrast to showers, it is steadier and normally falls from stratiform (layer) cloud."

3:12:59 PM
Fair enough. I am the retard for not just googling the answer.

3:22:55 PM
This was quite interesting too from BOM glossary, different to how I initially interpreted it.

Rainfall amount

Rainfall amount indicates the likely amount of rain in millimetres for the forecast period. Sometimes rain falls in a patchy pattern across an area with some locations receiving a heavy shower while an area nearby might miss out completely. On these days the rainfall range may be quite large, e.g. 5 to 30 mm. When steady rainfall is expected over a wide area, the range may be smaller, e.g. 10 to 15mm.
10:44:21 AM
I believe that these figures are generated from their computer modelling and do not have input from from an actual forecaster . the figures come from the agriculture part of the web site . elders use this data
2:07:58 PM
On 27/09/2013 lacto wrote:
>I believe that these figures are generated from their computer modelling
>and do not have input from from an actual forecaster .

You can clearly see the Manatee taking the weather data from the pile and placing it into the "for upload to bom website" pigeonhole:

Online Now
2:33:07 PM
Bahaha gold!

There are 12 messages in this topic.


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