Rainfall accuracy

In my previous analysis of forecast accuracy, I didn't look at the prediction for rain. Part of the reason for this is the fact that some of the services don't provide their rainfall reports and/or predictions in an easy to analyse format e.g. it might say 'light drizzle', 'cloudy' or heavy rain.

The one exception to this rule is the yr.no forecast service which provides rainfall predictions and reports in mm/hour. I thought it would be too strict to expect the forecasted mm/hour to match so I instead translated into rain or dry values.

This means that we can have 4 situations:

1. pred_rain => we predicted rain and we got it
2. unpred_rain => we didn't predict rain but we got it anyway
3. pred_dry => we predicted dry weather and we got it
4. unpred_dry => we didn't predict dry conditions but we got it anyway

I analysed 1 day and 7 day predictions for a selection of cities and I noticed a few key facts:

• Rain forecasts in Luxor are 100% accurate, but this is because it never rained at all in Luxor during our time to track. This is in stark contrast to the results from other cities I looked at.
• In Dublin, forecasts are about 82% accurate for the next day and 66% accurate for one week in advance.
• In Galway, forecasts are about 75% accurate for the next day and 60% accurate for one week in advance. This is only slightly better than random coin tossing which we would expect to  provide 50% accuracy

Here is the raw output if you want to look closer:

1 Days in advance for Dublin
Total 2316
pred_rain 240 10.36%
pred_dry 1662 71.76%
unpred_dry 216 9.33%
unpred_rain 198 8.55%

Correct 1902 82.12%
Incorrect 456 19.69%
 
7 Days in advance for Dublin
Total 2232
pred_rain 84 3.76%
pred_dry 1398 62.63%
unpred_dry 408 18.28%
unpred_rain 342 15.32%

Correct 1482 66.40%
Incorrect 492 22.04%
 
1 Days in advance for Galway
Total 2346
pred_rain 330 14.07%
pred_dry 1422 60.61%
unpred_dry 276 11.76%
unpred_rain 318 13.55%

Correct 1752 74.68%
Incorrect 606 25.83%
 
7 Days in advance for Galway
Total 2256
pred_rain 180 7.98%
pred_dry 1164 51.60%
unpred_dry 474 21.01%
unpred_rain 438 19.41%

Correct 1344 59.57%
Incorrect 654 28.99%
 
1 Days in advance for Luxor
Total 2346
pred_rain 0 0.00%
pred_dry 2346 100.00%
unpred_dry 0 0.00%
unpred_rain 0 0.00%

Correct 2346 100.00%
Incorrect 0 0.00%
 
7 Days in advance for Luxor
Total 2238
pred_rain 0 0.00%
pred_dry 2238 100.00%
unpred_dry 0 0.00%
unpred_rain 0 0.00%

Correct 2238 100.00%
Incorrect 0 0.00%
 
1 Days in advance for Coral Springs
Total 2364
pred_rain 708 29.95%
pred_dry 996 42.13%
unpred_dry 402 17.01%
unpred_rain 258 10.91%

Correct 1704 72.08%
Incorrect 1110 46.95%
 
7 Days in advance for Coral Springs
Total 2268
pred_rain 594 26.19%
pred_dry 846 37.30%
unpred_dry 498 21.96%
unpred_rain 330 14.55%

Correct 1440 63.49%
Incorrect 1092 48.15%
 
1 Days in advance for Perth
Total 2382
pred_rain 168 7.05%
pred_dry 1974 82.87%
unpred_dry 108 4.53%
unpred_rain 132 5.54%

Correct 2142 89.92%
Incorrect 276 11.59%
 
7 Days in advance for Perth
Total 2262
pred_rain 132 5.84%
pred_dry 1740 76.92%
unpred_dry 228 10.08%
unpred_rain 162 7.16%

Correct 1872 82.76%
Incorrect 360 15.92%