Summer rainfall outlook still uncertain
By Bob Berwyn
SUMMIT COUNTY — The biggest uncertainty for the course of this summer’s fire season is whether the monsoon will arrive on schedule — or perhaps even a bit early — to soak Colorado with beneficial rains, and for now, the answer is still uncertain.
The larger Pacific weather patterns are in a transitional phase. With winter’s La Niña officially over, it’s unclear if and how quickly an El Niño might form, or whether neutral conditions will persist over the Pacific for the next few months.
The El Niño-La Niña cycle drives not only the winter storm track, but also has an effect on summer weather patterns. El Niño is marked by warmer-than-average sea surface temperatures in the eastern equatorial Pacific, while La Niña is signalled by cooler waters.
Some forecasters have suggested that a quick shift to a strong El Niño could bring better chances for a solid monsoon season to Colorado, but forecasting skills for the summer rains are not completely reliable.
Monsoon moisture generally arrives in Colorado in mid-July, rotating clockwise around an area of high pressure to the south and east of the state, but the exact trajectory of that flow is hard to pinpoint in advance.
National Weather Service forecasters in Boulder say that post-La Niña monsoons often shift farther west, delivering the best moisture over Arizona and Nevada. And an early forecast by AccuWeather.com suggests that, unlike the past two years, heavy monsoon downpours will target portions of the Southwest deserts.
With La Niña fading, Accuweather forecasters said the eastern Pacific could spawn more tropical activity, playing a role in the Southwest monsoon.
“The number of tropical storms and hurricanes will be higher in the eastern Pacific,” said Dan Kottlowski a senior meteorologist with the private forecasting company. “This increases the probability of recurving tropical systems into Mexico, which could allow more moisture to reach into the Southwest.”
The monsoon is expected to kick in around the typical time in the middle of July, mainly focused over Arizona.
“The monsoon moisture will gradually spread from Arizona to Utah and Nevada,” said long-range forecaster Paul Pastelok . “It will take some time for this to occur.”
While the mountains of the Southwest will receive the most rainfall, there will be chances at locally heavy showers and thunderstorms for cities such as Phoenix and Tucson.