Flood — Fremont, Wyoming

2016-05-07 to 2016-05-10 · near Sweetwater Station, Fremont, Wyoming

Event narrative

Widespread heavy rain of 2.50 to 4.50 fell in a broad swath across Fremont County during a 48-hour period from Friday evening, May 6, through Sunday evening, May 8. There was a peak rainfall amount of 6.88' at a rain gauge about 6 miles west of Fort Washakie on the Little Wind River. The heavy rain fell over the lower elevations of the Wind River Basin and in the lower foothills of the Wind River Range.

The heaviest rainfall occurred between 1900MST Friday, May 6, and 1700MST Saturday, May 7. The rain gauge at Lander Hunt Field reported 3.82' in this 22-hour period, with 1.12' falling between 0800MST and 1100MST Saturday. The heavy rainfall contributed to significant rises along the Little Wind River and its tributaries, particularly the Little Popo Agie River through Lyons Valley. The rapid river rises produced substantial flooding from Lyons Valley, to Hudson, to Arapahoe.

Overland flow was a significant contributor to the rapidly rising rivers. Recent rains had left soils relatively wet, so as rainfall piled up on Saturday morning water began to flow across fields, overwhelm ditches, and fill ravines. These waters made their way to the larger rivers and quickly filled them. By early Saturday afternoon, there were reports of water covering low-lying areas of roads north and northwest of Lander. The rain was so heavy that water seeped through the walls at the hospital in Lander prompting the evacuation of patients. Some basement flooding was reported in residential areas of Lander. By mid-afternoon Saturday, reports were received of flood waters on the Little Popo Agie River surrounding homes in Lyons Valley. Several rural roads were either covered with water or damaged by the flood waters in the valley. The flood waters on this tributary combined with the already high water on the Popo Agie River to cause water levels to surge in Hudson late Saturday evening and early Sunday morning. The subsequent flooding damaged or destroyed several mobile homes along the river on the west side of Hudson. Sunday afternoon saw the retreat of water levels on these rivers with a subsequent decrease in flood-related problems.

Multiple tributaries and overland flow to small creeks, streams, and ditches quickly filled the Little Wind River near Arapahoe downstream to around Riverton. The river topped flood stage of 8 feet at 0605MST Sunday, May 8, and climbed about 1 foot every three to four hours during the morning and early afternoon. The river crested at 10.19 feet between 1545MST and 1600MST Sunday as residents along the river scrambled to fill sandbags and move property to a safer location. Flood waters on the Little Wind River retreated slowly on Monday, May 9, before falling below flood stage at 0120MST on Tuesday, May 10. The river fell below action stage by the end of Tuesday.

Wider weather episode

An extremely slow-moving area of low pressure brought significant rainfall to the Wind River Basin during a 48-hour period from Friday evening, May 6, through Sunday evening, May 8. Showers and thunderstorms repeatedly tracked to the northwest along a track parallel to the Wind River Range. Rainfall totals of 2.50' to 4.50' were common within a 50-mile-long, 15-mile-wide swath along and northeast of the foothills of the Wind River Range. The heavy rainfall produced widespread flooding and caused the Little Wind River to reach its third highest all-time crest at 10.19 feet. It was also the first time the river crested over action stage of 7 feet in the month of May. Flooding along the river subsided on Tuesday, May 10.

View location on OpenStreetMap → (42.4975, -108.1528)

Source: NOAA Storm Events Database, event_id 640373. Narrative written by the NWS forecast office that issued the report.