Weather Nerd Corner 1 - Forecasting, Weather, Climate, and Models

Originally sent via email - October 2, 2019, with some edits for grammar and context.

Today I want to talk about forecasting and models.

Forecasting is a complex process that I'm only beginning to learn. The main thing to understand is that our forecasts are only good out to about 3-5 days. It's not uncommon for confidence to be low in a forecast 1 day out. Sometimes we can have relatively high confidence in a forecast beyond 5 days if there's really good model agreement run after run and if climatology favors the forecast, but in general don't put too much stock in forecasts beyond 5 days.

Climate is what you expect the weather to be year after year for the same period of time for a given location. For instance in the middle of July we really don't have to look at the temperature or rain forecast, we all know it's going to be hot with a chance of thunderstorms. Weather is what you actually get. For example we are running well above average temperature-wise and below average for precipitation this time of year. Climate says it shouldn't be this hot and dry, but that is the weather we're getting. With this in mind this is why we can sometimes have confidence in a forecast out beyond 5 days. If our past experience (climate) for a given time of year agrees with the long term forecast then that increases our confidence. When models start hinting at us getting snow in the deep south beyond 5 days we have good reason to doubt that, even though it may be correct.

I'll end with a brief discussion about models. Our forecasting is heavily reliant on computer forecast models. I think we all get the idea, you give a super computer some data and a program spits out a forecast. I'll go in to some more detail on this in the future. In the outlook I emailed today I mentioned the GFS and Euro. These two models are for the most part the best operational global models. GFS is the Global Forecast System. This model is produced and run by the National Weather Service. It produces forecast products for the entire globe and it runs 4 times a day. Its data is completely free to obtain, not counting the taxes you pay to keep it going. Euro is the nickname for ECMWF which is the European Center for Medium-Range Weather Forecasts. It is also a global model with the main model running 2 times a day. ECMWF is government supported but they are basically an independent organization. Some of their products are free but most have to be paid for and they are expensive. I subscribe to a service that buys ECMWF products and then sells subscriptions to users like me so that we share the costs.