The National Weather Service in Riverton, Wyoming, has updated the Flash Flood Warning for southern Fremont County. The warning, reissued at 7:29 PM MDT on Saturday, September 19, keeps the warned area — rural southern Fremont County near Lyons Valley — under alert until 9:30 PM MDT, with 2 to 3 inches of rain already down and another 1 to 2 inches possible.
Six named drainages are in the warned area, led by the Little Popo Agie and Popo Agie Rivers — in this country, that means creek-overflow jobs on rural properties and washed-out access roads long after the rain stops. This is a live snapshot as of 6:31 PM PDT (7:31 PM MDT) — conditions change fast, so always verify with the National Weather Service for your specific county or ZIP code.
Southern Fremont County, WY — Flash Flood Warning
Issuing office: NWS Riverton, WY
Issued: 7:29 PM MDT Saturday, September 19, 2026 (update/continuation of the 6:25 PM MDT warning — VTEC /O.CON.KRIW.FF.W.0003)
Expires: 9:30 PM MDT Saturday, September 19, 2026
Severity / certainty / urgency: Severe / Likely / Immediate
The hazard, in the alert’s own words: “Flash flooding caused by thunderstorms.” At 7:29 PM MDT, Doppler radar indicated thunderstorms producing heavy rain across the warned area. Between 2 and 3 inches of rain had fallen. Additional rainfall amounts of 1 to 2 inches are possible in the warned area. Flash flooding is ongoing or expected to begin shortly.
Area covered: mainly rural areas of Southern Fremont County near Lyons Valley.
Named streams and drainages:
- Little Popo Agie River
- Popo Agie River
- Grant Young Ditch
- Monument Draw
- North Fork Ballenger Draw
- Government Draw
Expected impacts: flash flooding of small creeks and streams, highways, streets and underpasses as well as other poor drainage and low-lying areas.
NWS instruction: “Turn around, don’t drown when encountering flooded roads. Most flood deaths occur in vehicles. Be aware of your surroundings and do not drive on flooded roads. Please report observed flooding to local emergency services or law enforcement and request they pass this information to the National Weather Service when you can do so safely.”
Stay safe tonight
Rural county roads are the trap in a warning like this. Turn around, don’t drown — most flood deaths occur in vehicles, and a rural road that dips through a draw or crosses an irrigation ditch can go from passable to lethal in minutes. Never drive through flooded roadways, and don’t assume a road you drove yesterday looks the same tonight.
If water gets into your home or outbuildings:
- Stay out of standing water where electricity may be present. Do not enter a flooded basement or shop until the water has receded and the power is confirmed off.
- Photograph everything before you touch anything — walls, floors, belongings, the high-water line. Documentation matters for insurance, and you only get one first look.
- Then call a local restoration contractor. Water left sitting starts a second wave of damage — swelling, wicking, and mold growth — within a day.
What this means for restoration teams
A rural creek-and-drainage flood warning in Fremont County produces a specific job mix: stream-overflow water in basements and crawlspaces of properties near the Popo Agie system, outbuilding and shop damage along the draws, and washed-out rural driveways and access roads that delay everyone — including the restoration crews. With 2 to 3 inches fallen and up to 2 more possible, expect saturated ground that keeps seeping into foundations overnight even where structures didn’t take direct water.
Damage categories to expect:
- Interior water intrusion in basements and crawlspaces of properties near the named drainages.
- Saturated drywall, insulation, and flooring in low-lying rooms — and outbuildings, shops, and ag structures that rarely have the same moisture protection as homes.
- Mold risk within 24–48 hours wherever water sits; rural properties with limited ventilation need fast drying.
- Sewage backup in low-lying areas if septic systems or storm drains surcharge.
- Access-road washouts — crews may need to reach properties by alternate routes, and job-site access should be confirmed before dispatch, not after.
The play: stage extraction and drying equipment for the Riverton/Lander corridor now, keep an on-call rotation through the morning, and capture the address, water source, and affected areas on the first call. Creeks crest after the rain stops — the demand window here runs through the night and into tomorrow as people wake up to what the water did. Field discipline: photo-document before mitigation, moisture-map every affected room, and keep the homeowner’s payment responsibility clear in writing before work starts.
FEMA & federal assistance
If the flooding damages your home and you need help beyond insurance, the FEMA Helpline is 1-800-621-3362 (7 a.m. to 10 p.m. local time, 7 days a week). You can also apply at DisasterAssistance.gov, through the FEMA mobile app, or in person at a Disaster Recovery Center — find one at fema.gov/drc.
Basic eligibility: U.S. citizenship or qualified status, identity verification, proof of occupancy or ownership, and unmet needs not covered by insurance. Apply as soon as it’s safe to do so — don’t wait for a disaster declaration to be finalized for your county.
Looking ahead
The warning expires at 9:30 PM MDT, but the six named drainages — especially the Little Popo Agie and Popo Agie Rivers — will keep running high after the rain stops. Rural roads through draws and low-water crossings can stay dangerous for hours after expiry. Watch for additional rounds of storms tonight; with saturated ground, any new cell re-floods fast.
For restoration operators, the demand window on a rural creek-flood event lags the warning: tonight’s emergency calls, tomorrow’s discovery calls as water recedes and damage surfaces, and a long tail of crawlspace and outbuilding jobs as people check properties they don’t live in. Keep crews available through the weekend.
Sources & how to verify
- NWS alert: Flash Flood Warning issued September 19 at 7:29PM MDT until September 19 at 9:30PM MDT by NWS Riverton WY
- Issuing office: NWS Riverton
- Check your county or ZIP: weather.gov
To verify for your own location, go to weather.gov, enter your county or ZIP code, and look for the current warnings box — or check the NWS Riverton office page linked above.
This post is for informational purposes and is not an official weather warning. Always follow instructions from local authorities and the National Weather Service.
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