Active Weather
This is the most active region in the country this morning, and the activity is concentrated in Montana.
The Storm Prediction Center's Day 1 outlook, issued at 7:40 PM Central Sunday and valid through 7 AM Central today, places a slight risk of severe thunderstorms over parts of western and central Montana. The stated hazard is widely scattered large hail with hailstones up to two inches, developing this evening and continuing overnight. Slight is the second of five levels.
A marginal risk, the lowest level, extends across the northern and central High Plains. The population centers named inside it are Denver, Aurora, Fort Collins, and Lakewood in Colorado, and Salt Lake City in Utah. The hazards there are described as more sporadic large hail and severe wind gusts.
The forecaster discussion adds detail on the eastern half. Isolated to widely scattered storms developed along a lee trough from western North Dakota into northeast Colorado, in an environment supporting supercells with hail potentially over two inches and locally severe wind gusts. A tornado was reported with a slow-moving supercell over McKenzie County, North Dakota. The discussion expects the tornado threat to fade overnight as the inflow air dries and the atmosphere caps, while the hail and wind threat lingers from eastern Montana into the western Dakotas.
The National Weather Service public alert feed, read at about 1 AM Eastern, counts twelve active alerts in Montana, four in Wyoming, two in Colorado, and one in South Dakota. Utah and North Dakota show none at that hour.
On the rainfall side, the Weather Prediction Center's discussion issued at 12:58 AM Eastern today holds a marginal risk of excessive rainfall over Montana and Utah through this morning, and raises northern Montana to a slight risk for Tuesday. Wednesday holds a marginal risk over northern Montana.
As with all overnight outlooks, the daytime update for Monday issues later this morning and may shift these areas.
Infrastructure
Three named large fires are burning in this region, according to the National Interagency Fire Center situation report dated Sunday September 6 at 7:30 AM Mountain time.
The Aspen Acres fire in Colorado stands at 102,004 acres and 82 percent containment, with 915 structures threatened. That structure count is the highest on the national list, and it is the reason this fire stays in the brief even at eighty-two percent contained. Containment describes the perimeter, not the risk to what sits inside and beside it.
The Sand Creek fire in Montana is at 35,490 acres and 36 percent containment, with four structures threatened. Thirty-six percent means most of the edge is still open, which is why a hail-and-wind night matters here beyond the hail itself. Outflow winds from thunderstorms are the mechanism that pushes fire lines in unexpected directions.
The Needles fire in South Dakota is at 4,000 acres and 30 percent containment with structures threatened.
The national wildfire preparedness level is 4 on a five-step scale, according to the National Interagency Fire Center situation report dated Sunday September 6 at 7:30 AM Mountain time. That report counts 75 uncontained large fires nationally, with 53,165 fires and 8,302,657 acres burned so far this year, running at 125 percent and 158 percent of the ten-year average.
No grid emergencies or water system disruptions are reported. The Federal Emergency Management Agency's declarations page could not be read this run, so this brief reports no change either way on open declarations or fire management assistance grants.
Public Health
The national AirNow site carries a standing notice this morning that wildfire smoke is affecting air quality in multiple states. Specific readings for Montana, Colorado, and South Dakota were not obtained this run. With three named fires burning in the region, the Fire and Smoke Map at fire.airnow.gov is the practical place to get a current local number.
No boil-water advisories were identified across the six states.
The Centers for Disease Control and Prevention's Health Alert Network archive could not be read this run, so this brief makes no claim about national health advisories in either direction.
Watch Item
The overlap worth naming today is fire and thunderstorm in the same airspace. Montana has a slight risk of severe storms and an active fire at thirty-six percent containment. Thunderstorm outflow is wind that arrives ahead of the rain, from a direction the fire has not been running, at speeds that move a line faster than crews can reposition. That is the mechanism behind most of the surprise growth days in a fire season, and it is why a storm forecast in a fire region reads differently than the same forecast anywhere else.
For households near the Sand Creek fire, that means the relevant preparation today is the evacuation-ready one rather than the hail one. Vehicle fueled and parked facing out. Documents, prescriptions, and pet supplies in one place. Phone charged before the evening rather than during it.
For households near Aspen Acres in Colorado, eighty-two percent containment is genuinely good news and the number to watch is whether it holds. Containment percentages that fall are the signal worth acting on.
For everyone else in the region, the hail preparation is short. Vehicles under cover, patio furniture in, and stay inside and away from windows once it starts. Two-inch hail is large enough to injure someone caught outside, and the window for moving a car closes before you hear it on the roof.
Later in the week the pattern turns wetter. The Weather Prediction Center raises northern Montana to a slight risk of excessive rainfall for Tuesday and holds a marginal risk there Wednesday. Rain on a burn scar behaves differently than rain on soil: the ground repels water rather than absorbing it, and debris flows can start with rainfall amounts that would be unremarkable anywhere else. If you live below a scar from this season or the last, that Tuesday risk is the line in this brief that applies most directly to you.
Explore Today
Defensible space, evacuation timing, and how to read containment numbers as they change.
What hail and straight-line wind actually do, and the short list of things worth doing before a storm arrives.
Why rain on a burn scar is a different hazard than rain on soil, and what a downhill household does about it.