Today I revisited the code on the Seasonality forecast server that decides on precipitation intensity. I.E., how much rain does it take to have a drizzle, light rain, moderate rain, or heavy rain? This is forecasted in 12 hour blocks, and I take the sum of precipitation over that time frame to get a total precipitation for that block of time.

Previously the intensity chart looked something like this:

• 0 – 3mm: Drizzle
• 4 – 7mm: Light Rain
• 8 – 25mm: Moderate Rain
• > 25mm: Heavy Rain

What I found was the GFS model output would often place a millimeter or two of precipitation at random locations and this would trigger a “drizzle” forecast far too often. I decided that I needed to revisit the precipitation intensity chart.

I ended up finding this page at the MetOffice (United Kingdom). They have about twice as many intensity categories, from Very slight to Downpour, so I tried to adapt them to my intensity names (which match those of the NDFD forecasts put out by the U.S. National Weather Service). I ended up using these values:

• 3 – 6mm: Drizzle
• 7 – 12mm: Light Rain
• 13 – 25mm: Moderate Rain
• > 25mm: Heavy Rain

We’ll try these out for awhile to see how accurate they are. If you’re a Seasonality user, these changes were made all on the forecast server, so no software update is necessary.