Wildfire Black Box · TB-WBB-USA-2021-MARSHALL
Marshall Fire 2021
Two investigated ignition sites, a mountain-wave windstorm, rapid grass-to-neighborhood transition and an evacuation clock reconstructed from official records.
Official perimeter and loss context. The incident team reported 6,026 acres and 100% perimeter containment on 3 January 2022; heat remained inside that perimeter. Boulder County’s operational review records two fatalities. The National Weather Service’s retrospective counted 1,084 homes and seven commercial structures destroyed, plus 149 homes and 30 commercial structures damaged. Boulder County’s later recovery dataset lists 1,109 homes destroyed. These are source-specific counts with different dates and categories; they are not silently combined. Incident team · County AAR · NWS · County recovery.
What the investigations establish
Origin and cause
In June 2023 the Boulder County Sheriff’s Office concluded that two fires eventually merged. One began at a residential property where a legal burn from the previous week reignited in high wind; a second began at arcing power lines south of Marshall Mesa Trailhead. Investigators examined coal-seam activity but found no evidence of surface heat capable of ignition, while acknowledging it could not be completely ruled out. Prosecutors filed no criminal charges under the applicable evidentiary standard. Sheriff’s release.
Weather and fuel
NWS describes a mountain-wave event with gusts of 70–100 mph at the foothill base, and a reported peak gust of 115 mph near Highway 93 and 72 around noon. A wet first half of 2021 grew grass; the unusually dry second half left it receptive to fire. A point gust is not the wind speed at every burning parcel. NWS event analysis.
Known At — the operational clock
The following times come from the county’s operational after-action chronology (local MST). They describe reports and decisions; they do not prove exact flame arrival at each property. The sheriff’s two-origin conclusion was reached in 2023 and was unavailable to responders on 30 December 2021.
- 11:05 a.m. Marshall Fire call dispatched, while another fire, Middle Fork, also drew resources.
- 11:23 a.m. Fire command requested additional units; the fire was moving east toward homes.
- 11:38–11:50 a.m. Evacuations began in Old Marshall, Cherryvale and nearby areas according to the county chronology.
- 12:03–12:19 p.m. Reports placed rapid movement toward Superior, evacuations near Sagamore and Costco, and flames at Sagamore.
- 12:15 p.m. Evacuation orders went through Everbridge for unincorporated Boulder County and Superior; door-to-door actions continued. Issuance is not proof of delivery or receipt by every person.
- 12:35 p.m. A call for Spanish Hills evacuation came when the fire was already there, according to the after-action table.
- 2:30 p.m. Evacuation of Avista Hospital began.
- Evening to 31 December. Winds and forward spread subsided; snow later helped stop the wildland fire’s forward spread. This did not remove all heat within the perimeter.
- 3 January 2022. The incident team listed 6,026 acres and 100% perimeter containment while crews continued to address heat around structures.
The NWS retrospective estimates fire start at approximately 11:41 a.m.; that estimate and the after-action dispatch time refer to different observations and should not be made identical.
Forensic Reconstruction
- Two initial pathways. A covered legal burn reignited and power-line arcing ignited a separate fire, as determined in the later sheriff investigation. The precise moment and place where the two fire fronts merged is not assigned here.
- Wind and receptive grass. Extreme downslope winds and dry fine fuels supported rapid eastward spread. The NWS weather mechanism explains the broad forcing, not a house-by-house ignition time.
- Transition into built areas. Fire crossed from grassland exposure to dense neighborhoods in Superior and Louisville. FEMA examined building performance and the unusual urban conflagration; structure, fence, vegetation and ember pathways require parcel-level evidence before attribution to any individual loss.
- Evacuation under compression. County records show orders, door-to-door action and fire arrival unfolding within minutes in some areas. The review does not compute an individual clearance time or a counterfactual fatality number.
- Containment and residual heat. The 3 January perimeter status did not mean every structure site was cold or safe for return.
Evolution of Knowledge
Crews and law enforcement faced an evolving fire, extreme wind, impaired visibility and simultaneous demands. Origin was not yet established.
The perimeter was reported at 6,026 acres and 100% contained; the public cause field still said “Under investigation.” Residual heat and access controls remained.
The county after-action report assembled dispatch, evacuation and response events, documenting the sequence rather than a complete reconstruction of every person’s received warning.
The sheriff identified two ignition sites that merged and the district attorney’s no-charge decision. The result updates causal knowledge; it does not rewrite what operators could know on day one.
Fire DNA — comparative signature
A qualitative public signature, not a numerical severity score or an unreviewed 24-axis vector. Dimensions without checked evidence remain Unknown.
Two investigated origins, later merging; exact merge time: Unknown in this public review.
Extreme mountain-wave wind and unusually dry grass after a wet growing season, per NWS.
Rapid grass-to-urban exposure documented by FEMA; parcel-level ignition pathways: Unknown here.
Compressed warning and evacuation chronology; individual alert receipt and universal route clearance: Unknown.
Two fatalities in the county AAR, major housing loss; count definitions change with assessment date and unit.
Long-running rebuilding tracked by Boulder County; this record does not convert permits into completed recovery.
Material claim register
| ID | Claim and boundary | State | Primary source |
|---|---|---|---|
| M001–M002 | 6,026 acres; 100% perimeter contained on 3 January, with residual heat. | Reported | Incident team |
| M003 | Two fatalities recorded in operational review. | Reported | County AAR |
| M004–M005 | Two fires merged: covered legal burn reignition and arcing power lines; no criminal charges filed. | Reported | Sheriff |
| M006–M007 | Mountain-wave gusts of 70–100 mph in foothill zone; 115 mph peak near Highway 93/72; seasonal grass fuel context. | Reported | NWS |
| M008–M010 | 11:05 dispatch; 11:38 initial evacuations; 12:15 Everbridge orders and subsequent fire reports. | Reported | County AAR |
| M011 | NWS retrospective: 1,084 homes and seven commercial structures destroyed. | Reported, source-specific | NWS |
| M012 | Later county recovery listing: 1,109 homes destroyed. | Reported, later dataset | County recovery |
| M013 | Rapid grass-fire to urban-conflagration transition studied for building performance. | Reported analytical scope | FEMA/USFA |
Contradictions, unknowns and limits
- Damage counts: NWS’s retrospective home tally and Boulder County’s later recovery property listing have different dates and possible unit definitions; this record shows each with attribution and does not calculate a blended total.
- Clock: dispatch, origin estimate, first visible smoke, neighborhood exposure and individual evacuation are distinct timestamps.
- Ignition: the sheriff’s evidence supports two starts and a later merger, not a universal single-source attribution.
- Warning: issuing Everbridge orders or going door to door does not prove what each resident saw, heard or could do.
- Weather: regional peak gusts cannot be assigned to each home or treated as a modeled spread speed.
- Accountability: a decision not to file criminal charges is not a determination about every possible civil claim.
Primary sources and record governance
- Boulder County Sheriff’s Office — origin and cause, 8 June 2023
- Boulder County — Operational After-Action Report
- NOAA/NWS Boulder — High Winds and Marshall Fire
- Incident management team — 3 January status
- FEMA/USFA — building performance assessment overview
- Boulder County — recovery and later property dataset
Version 1.0 · 28 September 2026. Permanent record ID TB-WBB-USA-2021-MARSHALL. Quality: DOCUMENTED, with parcel-level progression, receipt-level warning, full loss taxonomy and numerical evidence scoring pending a reproducible review. Revisit if an official investigative or damage dataset materially changes.
Tecnobosque (2026). Marshall Fire 2021 — Wildfire Black Box Record. TB-WBB-USA-2021-MARSHALL, version 1.0, TB-WBB 2.0-beta.1.