Jun 27, 2014 - From staff reportsA spectacular thunderstorm sent rivers of rainwater rushing through Riverton roadways over the course of 15-20 minutes Thursday afternoon, bringing strong winds and hail to populated areas with a ferocity rarely witnessed in the vicinity.
National Weather Service meteorologist Paul Skrbac said the entire storm lasted almost an hour, but the bulk of the precipitation came down between 4:20 p.m. and 4:35 p.m.
"Those 15 core minutes were extremely intense," he said Friday. "For those who saw it, it was very tropical --just coming down in sheets with small hail and wind."
He was at home at the time and said he could "barely see" past his backyard in southwest Riverton.
"The driveway had maybe 3 or 4 inches of water on it for a while," Skrbac said.
Later, he drove through town and saw "curb high" pools along North Eight Street West.
A section of the Riverton Valley Irrigation District Canal near South Third Street West collapsed during the storm, sending mud sliding toward a small apartment building. The mud collected against the north wall of the building to a depth of more than a foot.
Residents of the building were advised to evacuate. The canal bank held sufficiently for water to keep flowing east, although it was over the embankment in a couple of areas. Workers shored up the area after the rain ended.
Rainwater inundated city streets with storm drains overflowing quickly in downtown Riverton and on North Federal Boulevard during the day's heaviest traffic period. Giant "fantails" of water were produced by vehicles whose drivers had no choice but to navigate the flooded streets.
No major road closures were reported, but motorists traveled in single lanes voluntarily along some portions of north federal.
Work was interrupted on both Riverton's summer road construction projects, South Federal Boulevard and Riverview Road. Motorists were advised to avoid both areas if possible as exposed dirt roadways turned to thick mud.
Flooding was reported in several neighborhoods and businesses in south Riverton as the water streamed to lower elevations. The Riverview Heights area south of Riverview Road was swamped, and business parking lots on South Federal Boulevard quickly turned to small lakes. Flooding was considerable at the Fremont County Fairgrounds as well.
Flooding conditions were made worse as the rushing water carried grass, leaves and other debris to the mouths of storm drains, clogging them and preventing better flow.
Championship games for Riverton Little League baseball were canceled Thursday as water collected on the baseball fields just east of Smith Road. The games will not be rescheduled.
Garden crops and structures sustained hail damage, with anecdotal reports delivered from many areas. The hail endured for longer than usual in a Fremont County storm. Most hail was estimated at about 1/4 inch, with some reports of 1/2-inch hail as well.
A similarly powerful storm system Monday brought hail measuring at a full inch to largely uninhabited areas of the county.
Skrbac said the system came from the Wind River Mountains and intensified when it hit some unstable air in the central part of the Wind River Basin.
"It peaked right on top of us," Skrbac said. "For once, Riverton got the brunt of the storm."
Usually, Lander sees more rain due to its proximity to the range, but on Thursday the Fremont County seat only recorded .18 inch of precipitation.
By comparison, Riverton recorded .39 inch of precipitation from the storm, with .64 inch at Riverton Regional Airport by Friday morning after rain returned during the night.
The normal value for the day is .03 in both towns. The record for Riverton was set at 1.04 inches in 1985.
Local reports from the Riverton Valley told of heavier rainfall. Several rain gauges in central Riverton recorded more than an inch of rain in about 30 minutes, according to witnesses.
Lightning also accompanied the system, Skrbac said, and a few areas in Fremont County experienced "severe" winds of 60 mph or stronger, according to the NWS.
No flooding predicted
As of 11:45 a.m. Friday, the Wind River below Diversion Dam was recorded at 4.84 feet --"near" the flood stage of 5.5 feet based on NWS data. Skrbac said the waterway may rise "quite a bit" over the coming days, possibly reaching 8 feet near Riverton.
The Wind River at Riverton was recorded at 7.61 feet at 11:15 a.m. Friday. Flood stage begins at 9 feet, with action stage set at 8 feet.
"By Monday morning it's forecast to get up to action stage," Skrbac said of the Wind River at Riverton. "It will continue its trend of the last couple days of getting a little higher each day. We're not forecasting it up to flood stage, but it's definitely on the rise one last time."
Dry weather soon
Scattered rain showers and isolated thunderstorms will continue to develop Friday, but Skrbac said those systems won't involve as much instability as Thursday's weather event.
The weekend will be drier, he continued, leading to cooler temperatures Monday. More isolated thunderstorms could hit central Wyoming on Wednesday and Thursday according to the NWS, followed by 90-degree weather leading up to the holiday weekend, Skrbac said.
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