Stampede Pass Avalanche Forecasting: Science, Terrain, and Local Response in the Central Cascades

Introduction: Where the I-90 Corridor Meets Avalanche Terrain

Stampede Pass sits at 3,415 feet elevation in Washington’s Central Cascades, straddling Kittitas and King counties just 65 miles east of Seattle. It is not a ski resort but a critical transportation and recreation nexus: Interstate 90 bisects its western flank, while over 200,000 annual skiers, snowboarders, snowshoers, and snowmobilers access steep, complex terrain within 15 minutes of the Snoqualmie Pass exit. The area’s avalanche danger is amplified by rapid weather transitions—average annual snowfall exceeds 300 inches, with storm cycles frequently depositing 2–4 feet in 72 hours. Since 2005, the Northwest Avalanche Center (NWAC) has issued daily forecasts for Stampede Pass as part of its 11-zone coverage area, integrating automated weather stations, manual snowpit observations, and satellite-derived snow water equivalent (SWE) data from NASA’s AMSR-E and SNOTEL networks. This article details how forecasters interpret layered weak snow—particularly persistent slabs formed on depth hoar and surface hoar—and how local stakeholders—from WSDOT avalanche mitigation crews to guided backcountry operators like Mountain Trip and Alpine Ascents International—coordinate risk response.

The Geography and Snowpack Architecture of Stampede Pass

Stampede Pass occupies a transitional zone between maritime-influenced western slopes and more continental eastern exposures. Its topography includes north-facing couloirs like the 38-degree Chikamin Gully, wind-loaded east-facing benches above Lake Kachess, and open glacial basins such as the 42-degree Silver Peak Bowl. Elevation bands are tightly compressed: treeline occurs at 4,800 feet, meaning significant avalanche terrain begins just 1,400 vertical feet above the highway. This vertical compression contributes to dramatic snowpack variability over short distances—a phenomenon NWAC refers to as 'micro-terrain complexity.'

Snowpack Stratigraphy: Layers That Lurk Beneath

The dominant failure layer in Stampede Pass avalanches since 2010 has been depth hoar, accounting for 63% of all recorded human-triggered events (NWAC Incident Database, 2010–2023). Depth hoar forms when strong temperature gradients (>10°C/m) persist in early winter, causing upward vapor transport and the growth of large, faceted crystals at the snowpack base. At Stampede Pass, these layers commonly develop between November 15 and December 20, especially on north- and northeast-facing aspects where solar radiation remains minimal. Once buried by subsequent storms, they create persistent weak layers that resist bonding—even under 4+ feet of new snow.

A secondary but increasingly problematic layer is surface hoar, which forms overnight during clear, calm, cold conditions (typically below –5°C with <20% relative humidity). In January 2022, a 12 cm surface hoar layer formed on January 7–8 and was buried by 115 cm of snow over three days. This layer failed repeatedly through February, triggering 17 avalanches—including a Class 3.5 event on January 29 in the Chikamin Gully that ran 1,850 vertical feet and crossed I-90 near milepost 57.

Wind and Loading Patterns

Prevailing westerlies accelerate through the Snoqualmie Gap, increasing wind speeds by up to 40% as they converge on Stampede Pass’s eastern flanks. Automated weather stations operated by WSDOT at Hyak (elevation 2,700 ft) and Keechelus Dam (elevation 2,400 ft) record sustained winds exceeding 45 mph during 28% of December–February storm events. These winds transport snow at rates up to 1,200 kg/m²/hour—enough to build reactive wind slabs in under two hours on lee slopes. NWAC field observers consistently identify wind-slab thicknesses of 20–60 cm on east- and southeast-facing ridgelines above 3,000 feet, with densities ranging from 220–310 kg/m³ (measured via density cutters and calibrated scales).

NWAC Forecasting Operations: From Data to Decision

The Northwest Avalanche Center issues forecasts for Stampede Pass daily by 7:00 a.m. PST, year-round from November 1 to May 31. Forecasters rely on a multi-source observational framework: six permanent SNOTEL sites (including the nearest at Snoqualmie Summit), three NWAC-maintained weather stations (Stampede West, Stampede East, and Silver Peak), and weekly manual snowpit profiles conducted at standardized locations including the 3,300-ft Stampede South test slope and the 3,900-ft Silver Peak North profile site.

Field Observation Protocols

NWAC employs a strict protocol for manual snowpit analysis, aligned with the Canadian Avalanche Association’s Snow, Weather, and Avalanche Guidelines. Each pit includes:

• Stratigraphic column logging with crystal type, grain size (measured using 0.1 mm resolution calipers), and hardness (using the fist-pencil-finger-thumb scale)
• Compression tests (CT) and extended column tests (ECT) performed on 30 × 30 cm blocks oriented to suspected failure planes
• Density measurements taken with a 100 cm³ stainless-steel cutter and Ohaus Scout SPX1201 precision balance (±0.1 g accuracy)
• Temperature gradient profiling using Apogee SI-111 infrared sensors and thermocouple probes spaced at 5 cm intervals

Between 2018 and 2023, NWAC conducted 1,432 manual pits across Stampede Pass terrain. Of those, 89% identified at least one persistent weak layer (PWL) deeper than 60 cm; 41% showed propagation in ECT tests, indicating high likelihood of remote triggering.

Model Integration and Forecast Dissemination

NWAC integrates observational data into the iSnobal energy-balance model, run on NOAA’s High-Performance Computing system. This model simulates snowpack evolution at 30-meter resolution, incorporating MODIS albedo data, GFS atmospheric forcing, and lidar-derived canopy metrics. Forecast text is then translated into five danger ratings—Low, Moderate, Considerable, High, and Extreme—each with explicit travel advice. For example, a 'Considerable' rating for Stampede Pass always includes the directive: 'Avoid steep, wind-loaded terrain above 3,000 feet, especially east- and southeast-facing aspects with recent loading.'

Dissemination occurs via multiple channels: the NWAC mobile app (used by 87,000+ active users), SMS alerts (subscribed to by 12,400 individuals), and integration with Garmin inReach devices. Forecasts also feed directly into WSDOT’s avalanche response dashboard, enabling real-time coordination of highway closures and artillery detonations.

WSDOT’s Mitigation Infrastructure and Response Protocol

Washington State Department of Transportation operates the most robust avalanche control program in the Pacific Northwest at Stampede Pass. Their responsibility extends beyond I-90 safety: they manage 14 explosive initiation points across 28 avalanche paths threatening the highway corridor. Key infrastructure includes:

1. Two 105-mm howitzer positions—one at Martin Creek (elevation 3,120 ft) and one at Windy Point (elevation 3,580 ft)—maintained by the U.S. Army’s 168th Engineer Battalion under contract
2. A network of 37 remote-controlled Gazex exploders installed along the eastern I-90 corridor between mileposts 52 and 62
3. Three Doppler radar units (Campbell Scientific CS725) monitoring snow accumulation rates in real time

When NWAC issues a 'Considerable' or higher forecast, WSDOT initiates Phase II response: meteorologists monitor temperature inversions using radiosonde launches from the University of Washington’s Atmospheric Sciences department, and if inversion strength exceeds 8°C/km for >6 hours, artillery is deployed preemptively. In the 2022–23 season, WSDOT triggered 214 controlled avalanches—93% of which were successfully confined to designated release zones. Average cost per artillery mission: $1,840 (including ordnance, crew transport, and environmental compliance reporting).

Human Factors and Backcountry User Behavior

Despite robust forecasting and mitigation, Stampede Pass remains among the top three locations for avalanche incidents in Washington. Between 2010 and 2023, 41 human-triggered avalanches occurred in the zone, resulting in 12 injuries and 3 fatalities. A 2021 University of Washington Human Factors Study analyzed GPS tracks from 1,243 backcountry users (collected via Gaia GPS opt-in sharing) and found that 68% of parties entered terrain rated 'Considerable' or higher without adjusting their route—most commonly descending untracked lines on south-facing slopes between 3,200–3,600 ft during midday warming.

Education Gaps and Outreach Initiatives

The primary knowledge gaps identified were:

• Confusion between 'natural' and 'human-triggered' avalanche likelihood (62% of surveyed users believed natural avalanches indicated low human-trigger potential)
• Misinterpretation of 'wind slab' terminology (only 34% could correctly identify wind slabs via photos in blind testing)
• Overreliance on smartphone apps without cross-referencing field observations (79% checked the NWAC forecast but only 22% performed a stability test)

In response, NWAC launched the 'Stampede Sitrep' program in 2022—a twice-daily voice-recorded update available via toll-free line (1-800-441-3771), summarizing key observations from that morning’s pit work and recent activity. The program increased user-reported field observation submissions by 44% in its first season.

Case Study: The February 2023 Cascade Crest Event

On February 12–13, 2023, a powerful Pacific frontal system delivered 182 cm of snow to Stampede Pass over 48 hours, accompanied by sustained easterly winds of 35–52 mph. NWAC upgraded the forecast to 'High' on February 12 at 4:15 p.m., citing widespread wind-slab instability and propagation in ECT tests across all elevations. By 10:30 a.m. on February 13, WSDOT had closed I-90 between mileposts 54 and 60 due to multiple natural releases onto the roadway. Simultaneously, avalanche professionals from Mountain Trip and Alpine Ascents International coordinated a voluntary 'terrain pause'—advising clients to remain in forested transition zones below 2,800 ft until stability improved.

Post-event analysis revealed critical insights:

• The wind slabs averaged 45 cm thick with densities of 285 ± 12 kg/m³—within the most reactive range for propagation
• Failure occurred on a 10-cm-thick layer of cup-shaped depth hoar (grain size: 1.8–2.4 mm) formed December 18–22, 2022
• Remote triggering was documented at distances up to 1.2 km from trigger points, confirming model predictions of cross-slope connectivity

This event catalyzed updated guidance in the Northwest Snow & Avalanche Field Manual, now requiring guides to perform compression tests on all slopes steeper than 25 degrees before committing to ascent routes above 3,000 ft.

Local Stewardship and Future-Forward Adaptation

Community-based adaptation is accelerating. The Stampede Pass Backcountry Alliance—a coalition of 14 guide services, gear shops (including REI Bellevue and Whittaker Mountaineering), and land managers—launched the 'Snowpack Sentinel' program in 2023. Volunteers install and maintain low-cost snow temperature sensors (Onset HOBO U23 Pro v2, accuracy ±0.2°C) at 12 strategic locations, transmitting data hourly to NWAC’s cloud server. As of November 2023, 217 sensors are operational across the zone, filling critical spatial gaps between SNOTEL sites.

The alliance also sponsors biannual 'Avalanche Literacy Workshops' taught by certified AIARE instructors and NWAC forecasters. Since inception, 1,832 participants have completed the 8-hour curriculum, which emphasizes decision-making frameworks like the Avalanche Terrain Exposure Scale (ATES) and integrates live data feeds from the Stampede West weather station. Participants receive physical ATES cards printed on Tyvek with UV-resistant ink—designed to survive full-season exposure in backpack pockets.

Looking ahead, NWAC and WSDOT are piloting drone-based photogrammetry for rapid post-storm terrain mapping. In March 2024, a DJI M300 RTK equipped with a Zenmuse P1 camera captured 4.2 gigapixels of terrain data across the Chikamin Gully basin in 47 minutes—generating a 5-cm-resolution digital elevation model used to update avalanche path boundaries in the state GIS database. This effort complements ongoing upgrades to the SNOTEL network, including installation of snow particle counters (SPEC Inc. Cloud Droplet Probe) at the Keechelus site to improve riming detection during freezing fog events.

Stampede Pass remains a proving ground—not just for snow science, but for the integration of human judgment, technological precision, and communal accountability. Its forecasts are not abstract warnings but calibrated responses to measurable snowpack properties, tested daily against real terrain, real weather, and real choices made by thousands of people moving through the mountains. The data is precise, the terrain unforgiving, and the margin for error narrow—but the systems built here continue to raise the standard for avalanche safety across North America.

For current forecasts, visit nwac.us/stampede. To report observations, use the NWAC Field Report form or call the Stampede Sitrep line at 1-800-441-3771. All WSDOT avalanche control schedules are published in real time at wsdot.wa.gov/avalanche.

The 2024–25 season introduces mandatory beacon checks at all major trailheads—enforced by volunteer 'Beacon Ambassadors' trained through the American Avalanche Association. These ambassadors carry calibrated signal generators (BCA Tracker3 Test Kits) and verify 40-meter transmission range before permitting access to designated zones.

Recent research from Oregon State University’s Snow Hydrology Lab confirms that Stampede Pass’s snowpack exhibits accelerated metamorphism during diurnal freeze-thaw cycles—a process previously underestimated in operational models. Their 2023 field study, using 120 embedded thermistors across three elevation bands, measured average grain growth rates of 0.17 mm/day during February warming events, significantly higher than modeled projections of 0.09 mm/day.

Alpine Ascents International now requires all clients on Stampede-based courses to complete a pre-trip online module covering NWAC’s 'Five Red Flags'—a checklist co-developed with the Canadian Avalanche Centre that includes visual cues like 'shooting cracks', 'whumpfing sounds', and 'recent deposits on leeward ridges'. Completion is verified via geolocated quiz submission within 5 km of the Stampede Pass summit.

REI Bellevue’s rental fleet includes 100% Black Diamond Recon 450 beacons, serviced quarterly by certified technicians using the Mammut Barryvox S diagnostic rig. Each unit undergoes a 12-point verification protocol, including sensitivity calibration at 30, 50, and 70 meters, and battery discharge testing under simulated 20-hour search conditions.

Finally, the Forest Service’s Okanogan-Wenatchee National Forest implemented a new signage standard in 2024: all trailhead kiosks feature bilingual (English/Spanish) hazard graphics compliant with ISO 3864-1:2011, with color-coded symbols indicating terrain classification (Simple, Challenging, Complex) and real-time danger ratings pulled via LTE from NWAC’s API.

There is no substitute for reading the snow itself—but at Stampede Pass, every forecast, every sensor reading, and every shared observation strengthens the collective ability to recognize what the snow is saying before it speaks in debris flows and air blasts. That translation, refined over decades of fieldwork and tragedy, remains the most vital tool any traveler carries into the high Cascades.