Author Topic: DarkSky current conditions accuracy  (Read 312 times)

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Offline daneast

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DarkSky current conditions accuracy
« on: February 02, 2019, 08:09:58 AM »
Hey wx folks. I've switched over to DarkSky in place of WU for a lot of my data (but I'm hoping to eventually transition to NWS once they finish their APIs that have been in beta for years).  Today I noticed quite a large deviance in current conditions for a given location.  I checked DarkSky against as many other sources as I could find for the current conditions for my town, and DarkSky indicated 5 degrees warmer than most (DarkSky reports 32F, where other sources are reporting 27F).  In another location in the region DarkSky is reporting 5 degrees colder (30F vs 35F).

I assume this might be due to the way they interpolate conditions between stations and what stations they are using.

I'm curious if those of you using DarkSky have made any observations about its accuracy.  Thanks!

Offline saratogaWX

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Re: DarkSky current conditions accuracy
« Reply #1 on: February 02, 2019, 12:09:03 PM »
They don't describe exactly how they pick a station close to the lat/long of the forecast to display .. it may be as simple as the nearest one which could be a METAR or MADIS ingress station.  They do show the distance from the forecast point to the conditions point.

For my particular forecast point in Saratoga, the conditions appear to be from my station likely via CWOP->MADIS as the point is 0km away.
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Offline ConligWX

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Re: DarkSky current conditions accuracy
« Reply #2 on: February 02, 2019, 12:33:14 PM »
DS has been pretty good so far with reports and forecasts here. The have not noticed a big difference in unit values at all.

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Offline daneast

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Re: DarkSky current conditions accuracy
« Reply #3 on: February 19, 2019, 09:50:16 PM »
So in the last week, after I released an update to my iPhone app that integrated Darksky weather, I'm seeing multiple cases in which Darksky forecast / current conditions are totally blowing it.  The issue is precipitation type.  My coverage area is the western part of Virginia. The forecast difficulty here in winter is almost always the type of precipitation we will get.

Last week Darksky predicted 2-4" of heavy snow, where no other weather forecasts said snow (and the NWS watch was for mixed winter weather - sleet, freezing rain type stuff).  For the entire day Darksky said it was snowing.  Minutely said things like "Heavy snow ending in 30 minutes", etc.  We did not get one flake of snow from that system.  The ground temperature was a little below freezing, and we got lots of rain, with some freezing rain with ice accumulation, which is very typical here.

Now today we are getting snow.  2-4" of it.  It is snowing right now.  DarkSky's current condition and minutely forecast: "Heavy rain for the hour".  The temperature is 36 F but we are getting snow.  As forecast by the NWS and others.

So I can only conclude that Darksky is extrapolating minutely type forecasts from radar, and naively determining the precipitation type by the ground temperature.

After some searching I came across this research document from 2016.  Darksky consistently ranked second to last (7th out of 8 ) in weather forecast accuracy.  https://www.forecastwatch.com/wp-content/uploads/Three_Region_Overview_2010_201606.pdf

I am in the process of reimplementing my backend to work with the new NWS raw forecast data.  I won't have "minutely" data, but the hourly has looked very good in my tests.  It is not great when a forecast is wrong, but it's downright embarrassing when the current conditions are totally wrong as to the precipitation type - all day long.

I just wanted to point this out for those using Darksky.  Perhaps in areas where the precipitation type is more simplistic (all rain or all snow) the accuracy is better, but for this part of the country, where cold often meets warmth, they have been exactly backwards in the precip type.
« Last Edit: February 19, 2019, 09:51:58 PM by daneast »