Ten days of rain on the Front Range and Eastern Plains of Colorado reduced drought conditions in the state by more than 60%, continuing a statewide recovery from drought over the past year. The amount of the state experiencing drought conditions has dropped from 93% a year ago to just 11% today.
May is normally the wettest month in eastern Colorado. But a slow-moving storm, more typical of winter, combined with a summery pattern of strong daily heating and moisture streaming up from the Gulf of Mexico have produced an extraordinarily wet period in eastern Colorado.
Between May 10 and May 19, central Denver received almost 5 inches of rain, compared with a normal of 2 inches for the entire month. Southern metro areas saw around 7 inches of rain, and Castle Rock topped out at just over 8 inches, four times its monthly rain total of just under 2 inches. The northern Front Range, from Greeley and Fort Collins to Boulder, received about 4 to 5 inches of rain.
Farther east on the plains, rainfall has ranged from 2 to 3 inches, a mostly beneficial early-season boost to young wheat and dry rangelands for cattle grazing. Damaging hail did hit a few places across the plains, and scattered patches of standing water delayed planting and put young plants at risk of rot and fungal infections, according to Kat Caswell, agronomy specialist with the Colorado State University Extension in Weld and Washington counties.
“Phenomenal rainfall totals led to significant reductions in drought coverage, especially from eastern Colorado and northwestern Kansas into western North Dakota,” meteorologists Brad Rippey and Denise Gutzmer said in a May 18 post on the U.S. Drought Monitor website.
The percentage of Colorado experiencing drought dropped 63% in one week, to just 11% from 30%. This continues a yearlong recovery from drought. A year ago 93% of the state was in some state of drought. The extreme southeast corner of the state, which got only about an inch or less of rain recently, remains gripped by extreme and exceptional drought.
It is not unusual for Colorado to get a week of rainy weather in May, according to Chad Gimmestad, meteorologist for the National Weather Service in Boulder. While the May 10-19 window in Fort Collins, Denver and Colorado Springs was extraordinary, it did not break into the Top 20 rainiest 10 days on record, all of which occurred more than 20 years ago.
“It’s been glorious,” said Greg Heavener, warning coordination meteorologist with the National Weather Service in Boulder.
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The rainy period began with a storm system coming down from the Pacific Northwest, then tracking east along the southern border of Colorado. Winds in these systems rotate counterclockwise around the lowest pressure. So, as the system tracked along southern Colorado, the winds on the northern edge of the system circled around from the east, pushing moist air against the Front Range and causing it to rise, cool and drop rain.
In addition, the low pressure drew additional moist air out of the Gulf of Mexico, increasing available moisture and rainfall. With relatively slow winds high in the atmosphere, the storm hung around for a few days.
“We had a prolonged period of moderate to occasionally heavy rain due to just how slow the system was moving across the state,” Heavener said.
The storm dropped 4.4 inches on Denver, more than 30% of an entire year’s normal rainfall. The 2.92 inches that fell May 11 was the wettest day in Denver in the past 50 years. Colorado Springs got 3.18 inches May 11, the wettest day in May on record.
After all that rainfall, the soil was saturated and there was plenty of moisture available for daily storms caused by May sunshine heating and lifting the air.
“The sun comes out and you can start to bake and evaporate some of the ground moisture to develop showers and thunderstorms on a daily basis,” said Heavener.
In addition, regular seasonal winds from the south and southeast brought more Gulf of Mexico moisture, and a series of weak low-pressure disturbances during the period helped air rise and more rain to fall.
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Some drying has occurred recently, but off-and-on rain continues to figure in the forecast for the next week. Heavener said an El Niño system anticipated for later this summer could bring more rain to Colorado as it alters the path of the jet stream to pull in more rain from Mexico and the Gulf of Mexico.
“It’s actually likely to be more like a normal summer as far as thunderstorm activity goes,” said Heavener. “But it’s going to seem like a lot more because the past couple of years have been so dry.”