Author Topic: predicting imminent rainfall  (Read 15180 times)

0 Members and 1 Guest are viewing this topic.

Offline m77

  • Forecaster
  • *****
  • Posts: 878
predicting imminent rainfall
« on: August 11, 2011, 05:13:48 PM »
I am interested in whether or not you can predict rainfall using data available from a pws, not hours ahead but maybe 5 mins in advance.

not sure what use it may be but its more of a challenge.

what sort of variables would be needed from a pws to give a better than 50.50 call of rain?
Weatherlink ip, Vantage Vue,

Offline SLOweather

  • Global Moderator
  • Forecaster
  • *****
  • Posts: 3456
    • Weatherelement Moline IL
Re: predicting imminent rainfall
« Reply #1 on: August 11, 2011, 06:12:04 PM »
Divide the current 10 minute average wind speed by 12. That gives the approximate wind run in miles for 5 minutes.

Estimate the current average wind direction.

Take the weather station that direction and that distance and set it down.

When the station gets wet, it'll rain at the original location in about 5 minutes.

 :lol:

Seriously, I don't know of anything a PWS can do with the granularity required for that kind of prediction. Around here (central California coast), I've noticed that in classic winter storms, (our rainy season), the barometer drops and drops, and it's windy. The rain generally doesn't start until the baro bottoms out and the wind dies down.

But trying to predict rain from those 2 observances is going to be pretty hard. I can get a better idea from looking at radar, or other close by stations if the rainfall is hard enough.

If it's really important, I'd consider a more elegant solution to what I described above. Figure out the prevailing directions that the storm comes from, and the average speed at which they travel.

Put one of those Hydreon RG-11 Optical Rain Gauges at the calculated location, set to the most sensitive "It's raining" setting, and use the relay contacts to signal you some how.

Offline SLOweather

  • Global Moderator
  • Forecaster
  • *****
  • Posts: 3456
    • Weatherelement Moline IL
Re: predicting imminent rainfall
« Reply #2 on: August 11, 2011, 06:36:34 PM »
I'm curious, what's your definition of high ground level dew point?

Two things are needed: (1) falling barometric pressure, and (2) high ground level dew point temperature.

I'm not sure exactly *how* but cloud level (meaning elevated atmospheric dew point temperature) also comes into play, because of the lapse rate of temperature with increasing altitude.

It's possible for "rain" to occur below a cloud, but no water to actually reach the ground, ie: virga. That occurs when the surface air is much drier (low dew point temp) than the upper air (high dew point temp).

Offline TNETWeather

  • Kevin Reed (KrelvinAZ)
  • Senior Contributor
  • ****
  • Posts: 193
  • Davis Vantage Pro2+ with full FARS
Re: predicting imminent rainfall
« Reply #3 on: August 11, 2011, 06:50:06 PM »
Around here normally you are only assured of getting rain when it hits your head.  Thunderstorm cells are very fickle and even watching radar can give you a false idea if it is actually going to rain in a location.

Can't tell how many times I've watched GRLevel3 radar and seen a cell approaching only to have it be done before it gets there, or skip over and regenerate past our location.

We have a location just north of us by about 3-4 miles that due to a little ridge in front of it, gets a lot of the moisture that passed over us and dumps on them on a very regular basis.  Great lightning shows from that area as well.

Its also no uncommon for a cell that ends up generating rain doesn't have any measurable rain from it leading up to when it actually starts raining.

I know a lot of places in the country when you see certain cloud types, its going to rain and rain over a wide area, but we get a lot smaller cells normally in the desert unless it is a fairly large storm.
All you need is Time, Aptitude and Desire ... and you can build just about anything...

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #4 on: August 12, 2011, 01:39:42 AM »
thanks, I think I may study

pressure
pressure tendency
air temp minus dew point
the above tendency
relative humidity
rh tendency

any others?
Weatherlink ip, Vantage Vue,

Offline d_l

  • Forecaster
  • *****
  • Posts: 1700
  • Slide Mtn - Mt Rose
    • Thomas Creek Estates neighborhood weather
Re: predicting imminent rainfall
« Reply #5 on: August 12, 2011, 11:26:21 AM »
Rapidly rising barometric pressures, rapidly dropping temperatures, and suddenly gusty winds are indicative of a microburst from a cell and may be accompanied or soon followed by rainfall.
--Dave--

Wireless VP2 w/ solar, 24hr FARS, Heater, (Envoy-WLIP)*3-Meteohub, plus custom VP2 @ 26', WL 6.0.4, WU & W4U=KNVRENO37 NetcamXL

People always talk about the weather, but they never do anything about it.  Not me.  I'm gonna measure it.  https://www.tceweather.com

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #6 on: August 12, 2011, 01:04:02 PM »
wow thanks guys.

I am still looking at a few things but at the moment I have a 10% ish strike rate of rain within 15 mins.

any advice is welcome.

I suppose one use could be to use your data to generate a "rain likely" warning on a website.

it interests me anyhow.

I will look at dl's advice on gusts, pressure rise and temp fall and see of the strike goes up.
« Last Edit: August 14, 2011, 05:23:40 PM by m77 »
Weatherlink ip, Vantage Vue,

Offline wuhu_software

  • Forecaster
  • *****
  • Posts: 689
    • WUHU Software Yahoo Group
Re: predicting imminent rainfall
« Reply #7 on: August 13, 2011, 09:56:10 PM »

I have always wondering if using fuzzy logic or a neural network system, developed using historical data, would produce good results. I looked around for weather prediction based on either of these concepts but did not find anything.

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #8 on: August 14, 2011, 03:41:46 AM »
thanks all.

I looked at lots of variables individually and in combination.

I only looked at scenarios where it wasn't already raining, to make it fair.

the strongest variables were pressure and (air temp much minus dew point).

wind speed, direction, humidity and temp didn't really have as much of an effect.

when pressure was equal to or below 1010mb and t-dp was equal to or below 0.6 I got the following

rain within 15 mins 1 in 12
within 30 ...1 in 7
within 45...1 in 5
within 60...1 in 5
within 120...1 in 3

each is four times more likely than choosing blindly.

when temp = d point and pressure <= 1010 the odds were
1 in 3, 1 in 2, 2 in 3, 8 in 10 and 100%.

pretty pleased with that study.
« Last Edit: August 14, 2011, 04:33:08 AM by m77 »
Weatherlink ip, Vantage Vue,

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #9 on: August 14, 2011, 07:40:04 AM »
ok, extended the study a little to look further ahead.

when pressure <= 1010 and (T-DP) <= 0.6

the chance of rain (>=0.2mm) within...

15m   8%
30m   14%
45m   18%
1h   22%
2h   33%
6h   55%
12h   69%
24h   77%
36h   82%
48h   86%


obviously i'd like to expand my analysis onto more than 3 months worth of data to see how it stacks up all year.

Weatherlink ip, Vantage Vue,

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #10 on: August 14, 2011, 07:44:10 AM »
... and when pressure <= 1010 and T-DP =0

15m   35% - which is 16 x more accurate than picking at random!
30m   50%
45m   65%
1h   80%
2h   100%
6h   100%
12h   100%
24h   100%
36h   100%
48h   100%


(although this scenario occurred only 20x, whereas the previous one happened 2600x)
« Last Edit: August 14, 2011, 07:45:58 AM by m77 »
Weatherlink ip, Vantage Vue,

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #11 on: August 14, 2011, 02:15:54 PM »
my mistake - that data is based on 466 days worth of 15 min logging, so a year and 3 months or so.
Weatherlink ip, Vantage Vue,

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #12 on: August 14, 2011, 04:15:39 PM »
thanks Tele man.

you are correct, this rule of thumb that I have suggested will not work at another location easily. there will be many factors influencing local conditions and thus rainfall.

our altitude is about 90m and average pressure is 1014.

when the pressure <=1010 and dpd is <=0.6 I think there is a good chance (70%) of rain within 12 hours. when dpd is zero rain is likely within an hour (80%).

I will follow this rule of thumb with interest.

what does tdp stand for exactly?



Weatherlink ip, Vantage Vue,

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #13 on: August 14, 2011, 04:19:27 PM »
i'd also be interested to hear if anyone else can test this using their own data and seeing how results differ, using pressure and dpd.
« Last Edit: August 14, 2011, 05:11:58 PM by m77 »
Weatherlink ip, Vantage Vue,

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #14 on: August 14, 2011, 04:21:43 PM »
I think it is also important to say HOW MUCH MORE the conditions improve your prediction by.

my first was four times more likely than average and the stricter condition was 16x

Weatherlink ip, Vantage Vue,

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #15 on: August 14, 2011, 05:03:19 PM »
dry bulb!

got it.
Weatherlink ip, Vantage Vue,

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #16 on: August 14, 2011, 05:37:42 PM »
yes thanks I use the cloud base calculation.

0.6 tdp is equivalent to a cloud base of around 94m agl for me with anything lower increasing the chances of rain
 when pressure is under 1010.

so extending the rule of thumb... cloudbase under 100m (clouds this low probably indicate pressure <1010) brings rain within 12h. sounds quite like a no brainer when you put it like this!
« Last Edit: August 14, 2011, 05:48:47 PM by m77 »
Weatherlink ip, Vantage Vue,

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #17 on: August 14, 2011, 05:54:28 PM »
do you mean the curves for forecasted times ahead or as pressure and tdp alter?

hours ahead chances are posted above?
Weatherlink ip, Vantage Vue,

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #18 on: August 14, 2011, 06:04:28 PM »
ie posts 13, 14.
Weatherlink ip, Vantage Vue,

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #19 on: August 14, 2011, 06:09:00 PM »
good question if understand it correctly.

so you are asking what variables would be present for the probability to be x in y hours?

what do you think?

I have an excel sheet with altering probabilities for time as you tweak the factors.

?
Weatherlink ip, Vantage Vue,

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #20 on: August 14, 2011, 06:10:02 PM »
...OK, it looks like only a "two-line" curve, correct?

AZ CWOP - DW6988 (inactive)


?
Weatherlink ip, Vantage Vue,

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #21 on: August 14, 2011, 06:20:54 PM »
ah ok.

I would have to alter it 0.1 at a time and record probability.

not on pc so could do it tomorrow.

Weatherlink ip, Vantage Vue,

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #22 on: August 14, 2011, 06:27:42 PM »
what do you predict or no idea?

will post in about 10 hours when on pc.

i will even go from higher than 0.6 and will draw graphs for 15mins, 30 mins etc
Weatherlink ip, Vantage Vue,

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #23 on: August 15, 2011, 01:34:35 AM »
this site uses 4F. my calculation was 0.6C

0.6C is about 1F
4F is about 2.2C


and that looks like they can detect mist whereas I am going for rain. interesting setup though.
« Last Edit: August 15, 2011, 02:28:05 AM by m77 »
Weatherlink ip, Vantage Vue,

Offline m77

  • Forecaster
  • *****
  • Posts: 878
Re: predicting imminent rainfall
« Reply #24 on: August 15, 2011, 02:19:43 AM »
   15m   30m   45m   1h   2h   6h   12h   24h   36h   48h
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%
Weatherlink ip, Vantage Vue,