Each August, as we start hearing outlooks for the upcoming winter season, we find it amusing to go back and look at the forecasts from this time last year.
I’m sure some of you find it incredible that forecasters even attempt a forecast 4-6 months out when they struggle to get the forecast right 4-6 days out.
A recent YouTube video forecast for the upcoming season already has over half a million views. Maybe we are hoping to see our area shown with a forecast for above-average snowfall, and we hope there is some validity to the science behind that forecast. Or nervously laugh it off if the forecast is for below-average snowfall.
One thing is for sure, skiers wish they could know the forecast for next season 6 months in advance, especially as they are buying their seasons pass and new gear in the off-season.
“What’s going to happen this winter?” is the #1 question we as forecasters get this time of year. More specifically here in California, it has been, “what does the cool summer so far mean for snow this winter?”.
Can Forecasters Actually Make an Accurate Forecast 3-6 Months before Winter?
The track record for U.S. forecasters making accurate winter predictions has been so bad in the past, that here at OpenSnow we don’t even attempt to publish an official forecast for the upcoming season. Our forecasters will analyze other forecasts and make their own guesses for their region as we get closer to winter, but we focus on shorter-range snow forecasts aimed at accuracy.
When I first started forecasting I remember being told, “the forecast models lose 10% of their accuracy for each day they forecast out into the future”.
That would mean 0% accuracy beyond 10 days!
The actual stats are a bit different than this general rule of thumb, but 10 days is still about the limit of a skillful forecast for individual storm systems.
While we cannot predict specific storms more than 10 days out, forecasters do try to predict general trends months in advance, such as the chance that an area will be colder or warmer than average, or wetter or drier than average.
What Did the Forecast Models Show Us Last Year?
The long-range forecast models are heavily used by forecasters when they are putting together their winter forecasts. One of the first things they look at well in advance of the upcoming season is the forecast for the sea surface temperatures, especially in the ENSO regions along the equator which will signal a possible El Niño or La Niña.
This time last year (August 2018), the forecast models were pointing towards a weak to moderate El Niño for the 2018-19 winter season.
This is shown by the red color west of South America, denoting warmer-than-average water temperatures.
Most of the winter forecasters and forecast models will look at the forecast for El Niño and compare it to historical weather patterns during similar El Niño events. Here is a typical weak El Niño weather pattern for the U.S. based on historical records.
We saw the forecast models like the CFSv2 predicting a similar wet pattern across the southern U.S. for last winter.
Typically we would see a warmer western U.S. and a colder eastern U.S. The CFSv2 was predicting warm temperatures for the entire U.S.
Meanwhile, WeatherBell’s Pioneer model was predicting a more typical pattern with colder air in the eastern U.S.