Author Topic: predicting imminent rainfall  (Read 9054 times)

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Offline m77

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Re: predicting imminent rainfall
« Reply #25 on: August 15, 2011, 02:25:19 AM »
or from dpd <= 2

   15m   30m   45m   1h   2h   6h   12h   24h   36h   48h
2   5.88%   10.09%   13.47%   16.26%   24.59%   43.30%   56.57%   68.65%   75.89%   80.66%
1.7   6.24%   10.76%   14.36%   17.28%   26.03%   45.15%   58.35%   69.54%   76.53%   81.20%
1.5   6.61%   11.28%   14.99%   18.02%   27.04%   47.04%   59.95%   70.49%   77.15%   81.85%
1.2   6.97%   11.91%   15.81%   18.97%   28.64%   49.76%   62.52%   72.15%   78.10%   82.89%
1   7.21%   12.20%   16.30%   19.56%   29.46%   51.09%   63.94%   73.38%   79.15%   84.10%
0.6   8.05%   13.84%   18.32%   22.05%   32.62%   55.34%   68.74%   76.64%   81.79%   86.49%
0.5   8.16%   14.07%   18.42%   22.04%   32.54%   55.10%   67.13%   75.67%   81.06%   86.07%
0.4   8.17%   14.19%   18.37%   21.88%   32.20%   54.98%   66.49%   75.49%   80.92%   85.99%
0.3   8.03%   13.26%   17.72%   21.30%   32.43%   53.24%   64.96%   73.77%   79.28%   85.19%
0.2   7.78%   13.17%   18.36%   22.36%   32.73%   52.89%   62.87%   72.85%   79.44%   82.83%
0.1   10.68%   16.50%   22.82%   28.16%   37.38%   58.25%   68.45%   78.16%   84.95%   86.41%
0   35.00%   50.00%   65.00%   80.00%   100.00%   100.00%   100.00%   100.00%   100.00%   100.00%
« Last Edit: August 15, 2011, 03:05:56 AM by m77 »
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Offline m77

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Re: predicting imminent rainfall
« Reply #26 on: August 15, 2011, 02:34:43 AM »
...leading to these rules of thumb when press <= 1010

dpd
2   50-50 within 12h   likely within 36h
1.7   50-50 within 12h   likely within 36h
1.5   50-50 within 6h   likely within 36h
1.2   50-50 within 6h   likely within 36h
1   50-50 within 6h   likely within 36h
0.6   50-50 within 6h   likely within 24h
0.5   50-50 within 6h   likely within 24h
0.4   50-50 within 6h   likely within 24h
0.3   50-50 within 6h   likely within 24h
0.2   50-50 within 6h   likely within 24h
0.1   50-50 within 6h   likely within 12h
0   50-50 within 30m   likely within 1h
« Last Edit: August 15, 2011, 03:06:10 AM by m77 »
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Offline m77

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Re: predicting imminent rainfall
« Reply #27 on: August 15, 2011, 03:05:30 AM »
i think the obvious step is to look at what happens to the %s as i vary pressure and DPD, to give a full look up chart.

i will probably lump pressures together eg 1005-1010, 1010-1015, 1015-1020
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Offline m77

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Re: predicting imminent rainfall
« Reply #28 on: August 15, 2011, 07:08:26 AM »
just realised those chances are for less than or equal to the dpd, as opposed to equal to it.

will correct and add in the pressure factor in the next hour or so.
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Offline m77

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Re: predicting imminent rainfall
« Reply #29 on: August 15, 2011, 11:14:51 AM »
yes our 2010 pressure average was 1014.5!  ?

what do you mean about 100% threshold. are you referring to the 0.6?
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Offline m77

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Re: predicting imminent rainfall
« Reply #30 on: August 15, 2011, 11:33:09 AM »
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Offline m77

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Re: predicting imminent rainfall
« Reply #31 on: August 16, 2011, 08:33:51 AM »
i am developing some expressions rather than using the % chances for a piece of php script.

any comments on these particular phrases and % thresholds?

or should i just quote the % chance category? eg "33-49%"

it is aimed at a local comunity and school rather than experts in meteorology.

40-59   even chance
60-79   likely
80+   extremely likely

are these too many?
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Offline m77

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Re: predicting imminent rainfall
« Reply #32 on: August 16, 2011, 08:44:55 AM »
using these cutoffs i get a nice summary table:

where E = even chance, L= likely and X = extemely likely

and 15m = next 15 minutes, 1h = next 1 hour etc etc

so E24h means evens chance of rain in the next 24 hours

« Last Edit: August 16, 2011, 03:15:22 PM by m77 »
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Offline m77

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Re: predicting imminent rainfall
« Reply #33 on: August 16, 2011, 08:55:57 AM »
so at the moment we have pressure = 1015.9 and DPD = 3.1 so that translates to

Even chance of rain in the next 48 hours


we will see!

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Offline m77

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Re: predicting imminent rainfall
« Reply #34 on: August 16, 2011, 03:15:47 PM »
second thoughts...

help - is this a simple way to notify locals of rain chances?

for example if a sentence read

"The chance of rain is 50:50 in the next 6 hours, LIKELY in the next 12 hours and EXTREMELY LIKELY in the next 24 hours"

does that mean anything or is it too much?

or should it be reduced to

"The chance of rain is 50:50 in the next 6 hours"

or even just

"The chance of rain is EXTREMELY LIKELY in the next 24 hours"
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Offline Cynjut

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Re: predicting imminent rainfall
« Reply #35 on: August 17, 2011, 09:18:21 AM »
The only problematic thing I can see here is the granularity of your report.

Your rain chances are 50% because the conditions at your station say that half the time these conditions exists it rains at your station.  From what I remember about the weather report (when I was a radio announcer), the likelihood at each locality also figures into that percentage.

For a larger forecast reporting center (TV, NWS, Radio), the area is much larger.  For them, the percentage chance of rain isn't just the likelihood that the rain will fall at a specific location, it's that "within the sound of my voice", the rain will fail as a percentage.

So, if 20% of the listening area is "100%" going to get rain, the percentage was 20%.  If it's 50% in 50% of the listening area, it was "30%" (we always rounded up to 10s instead of using the things like "20-30%").  The trick, then, is for your prediction aggregater to take all of the "50%"s and bring them into a model that takes your data and the data from other stations like yours pull it into a forecast that works for them.  The larger the reporting area, the more stations would need to be contacted.

Other than that, this has been a fascinating process to watch.

Offline m77

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Re: predicting imminent rainfall
« Reply #36 on: August 17, 2011, 10:43:34 AM »
interesting idea, many thanks.

however i have no idea how big the actual audience is (and more importantly who cares!) and os i will assume calculations to be done on the school roof / rain gauge rather than any sort of catchment area as you suggest. rain fall distribution is patchy and hit and miss anyhow.

i have smoothened the data (as there was noise and inconsistencies - sometimes a decrease in DPD or pressure led to a slight decrease in the rain chance and not increase a sexpected)

by using Excel's data analysis tool i performed a bit of multiple regression to get best fit equations for the probabilities for each time period.

so there is a formula for the rain chance when given DPD and pressure for 1h, 2h, 6h, 12h etc etc

for example i found the 12h formula to be

12hchance = 11.129 + (DPD * -0.105 ) + ( pressure * -0.010 )

i used the 20% "description" cutoffs (Low, small, even, high, very high) and have now used php to lookup the pressure and DPD and put them through the formulae. This produces the chances shown in red on my own personal data page here:

http://www.maidenerleghweather.com/lastrecord.php

i do not broadcast this page as it's too much data but it has everything for me at a quick glance!!!

it's really just to get the php and formulae up and running!
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Offline m77

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Re: predicting imminent rainfall
« Reply #37 on: August 18, 2011, 01:22:56 AM »
I'm pretty sure that the 12h formula I posted won't work for you as its based on my data.

perhaps look at your own data and derive formulae and compare.
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Offline m77

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Re: predicting imminent rainfall
« Reply #38 on: August 18, 2011, 09:46:53 AM »
just struck me that the %s by hour don't mean much unless you relate them to the normal average chance of rain in that given period!

hence i have worked out the amount of times rain is more likely than normal

eg http://www.maidenerleghweather.com/lastrecord.php

1h%: 12

means there is a 12% chance of rain within the hour

but if you were picking blindly the rain would come 5.97% of the time within the hour.

so this is 2.0 x more likely than average

hence the addition of 1hx: 2

does this make more sense and is it more useful??
;-)


at the moment it is raining  :-P and the lines read:

1h%: 12 2h%: 18 6h%: 35 12h%: 48 24h%: 63 36h%: 74 48h%: 80
1hx: 2 2hx: 1.9 6hx: 1.8 12hx: 1.6 24hx: 1.4 36hx: 1.4 48hx: 1.3

so in retrospect, it is twice as likely - and it actually is!!!!

bit of backward thinking but we shall see from hereonin.
« Last Edit: August 18, 2011, 09:49:51 AM by m77 »
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Offline m77

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Re: predicting imminent rainfall
« Reply #39 on: August 22, 2011, 03:50:40 PM »
yes its a pretty good indicator I reckon.

the hourly predictor has a larger variation compared to the longer term 48h which fluctuates very little.

one addition that I will make is the historical change so that I can track how the percentage has changed periodically, perhaps every 15mins for the last 2h to are a tendency - a rise or fall in the percent.
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Offline m77

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Re: predicting imminent rainfall
« Reply #40 on: August 23, 2011, 03:29:21 AM »
ok my data page now looks like this:


chance1h%: 9 2h%: 14 6h%: 26 12h%: 38 24h%: 53 36h%: 63 48h%: 70
compare to average1hx: 1.5 2hx: 1.4 6hx: 1.3 12hx: 1.3 24hx: 1.2 36hx: 1.2 48hx: 1.1
historical 1h%9 9 8 8 8 8 8 8 9 9 8 7
historical 1h compare to av1.5 1.5 1.3 1.3 1.3 1.3 1.3 1.3 1.5 1.5 1.3 1.2

each value represents - now, 5m, 10m, 15, 20, 30m, 45m, 60m, 75m, 90m, 105m, 120m ago in time

from this data i can see that rain is becoming more and more likely - the chance has increased from 7/8% to a 9% and the av compare has risen from a 1.2x to 1.5x more likely than average

this way i can see what the trend / tendency is and can probably make more of a call on what's going to come.

check it out live at http://www.maidenerleghweather.com/lastrecord.php
« Last Edit: August 23, 2011, 03:32:40 AM by m77 »
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Offline m77

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Re: predicting imminent rainfall
« Reply #41 on: August 24, 2011, 02:00:27 PM »
humidty rise?
dpd fall?

why not? Im interested.
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Offline m77

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Re: predicting imminent rainfall
« Reply #42 on: August 24, 2011, 02:13:24 PM »
pressure drop?

or is it mostly convectional?

ours is frontal.
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Offline xykotik

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Re: predicting imminent rainfall
« Reply #43 on: August 25, 2011, 11:11:25 PM »
what sort of variables would be needed from a pws to give a better than 50.50 call of rain?

You don't need any fancy-schmancy variable whatchamacallits or historical data set thingamajiggers to know if it is going to rain.  The statistical probability for anything to happen is always 50/50.  It either will, or it won't.  Accuracy beyond that and you may be a labeled a witch, so be careful.


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