Rain coming to the Rocky Flats National Wildlife Refuge Golden, Colorado on June 3, 2019.
From a bomb cyclone (or two?) of spring snowfall to a soggy May, it’s been a wet and cold spring. Will that, however, translate into a soggy summer?
There are a lot of meteorological clues one can look through to try and gather what our cold, soggy spring might mean for the Front Range’s summer.
The first way to start off could be by taking a look at previous wet springs and to see if they translated into a wetter summer as well.
From 1981 through 2010, 15 Denver springs finished with above and 15 finished with below average precipitation (a combination of rainfall and snow equivalent rainfall) over the combined months of March, April and May. In those 15 months with above average spring moisture, nine also had above average summer precipitation. So that’s a nudge in the direction of a wetter spring, but it’s obviously far from a sure deal.
The clearer trend, however, appears to lie with below average springs. Of those 15, 11 went on to have below average moisture during the summer months as well. So again, a nudge in favor of a wetter summer, based on those two data points alone.
Another clue to dive into is the El Nino/La Nina pattern in the central Pacific Ocean. According to the National Oceanic and Atmospheric Administration (NOAA), the central Pacific Ocean is officially experiencing a weak El Nino, and the majority of computer models expect that to continue through the summer and into the autumn and winter.
So what kind of a predictor is El Nino?
The answer: murky, but again leaning in favor of a wetter summer. From 1981-2010, 18 El Ninos of varying strengths took place. In those 18 years, 12 resulted in above average precipitation for Denver. In years with strong or very strong El Ninos, however, six of eight summers finished with above normal precipitation. Only six of 10 finished that way in weak to moderate El Nino events.
A stronger El Nino would appear to trend towards a wetter summer – perhaps due to an enhancement of tropical moisture. There’s a strong correlation between El Ninos and an active Eastern Pacific Ocean hurricane season, and the Front Range can sometimes receive glancing bits of tropical moisture from the remnants of those storms.
But, Denver is only one point on a map, and the time frame used to try and figure this out only stretches 30 years. In other words, don’t exactly take these trends to the bank.
One issue with spring-to-summer seasonal forecasting is that a lot of the weather patterns in spring aren’t the same ones for summer. The annual monsoon is the seasonal reversal of wind patterns that draws up moisture from the Gulf of Mexico and the Pacific Ocean, and it’s a primary driver for mid-to-late summer moisture.
The summer monsoon is also notoriously fickle to forecast, as we outlined earlier this spring. Summer tends to be filled with highly localized, hit-or-miss thunderstorm events. That can lead to a wide and localized variation in rainfall amounts, especially for July and August, during the monsoon’s peak.
That said, there are studies that show that increased soil moisture in mountainous terrain could enhance thunderstorm-type rainfall. Added soil moisture increases the amount of moisture available to evaporate, boosting the amount of water content in the air. With off-the-charts snowpack levels delaying the big spring melt, soil moisture will likely remain high for most of the state deeper into summer than usual.
From a broad perspective, history, snowpack and the current El Nino trend might lend a nudge in favor of a wetter summer. But it’s far from a sure deal. In the near term, at least, June looks wet and cool.
One quick logistical note: meteorologists count spring by full months, rather than astronomically. In other words, it’s not the March 20-to-June 21 spring that’s counted, rather, it’s by month. So, March, April and May constitute meteorological spring, and June, July and August count as meteorological summer.