High Altitude Downbursts over Grand Teton National Park
Disclosure: this article includes text generated with the assistance of an AI writing program.
During the early afternoon of 27 August 2026, a Snake River float cruise, as led by Triangle X Ranch in Grand Teton National Park, Wyoming, evolved from a recreational experience to a convective storm field campaign when strong, downburst-producing convective storms developed and intensified over the Teton Range.
On 27 August 2026, a high-altitude convective weather event unfolded over Grand Teton National Park and the surrounding Jackson Hole region. Around midday (1215 MDT / 1815 UTC), prominent virga shafts, which signify rain evaporating before reaching the ground, were observed descending from convective storm clouds over the Teton Range. While virga often appears harmless, it frequently signals the localized onset of severe convective downbursts in high-elevation, arid mountain environments as depicted in the visual image, satellite and radar imagery, and information graphic shown below.
1. Midday Virga Signatures: The Deceptive Onset
Around midday (1815 UTC / 1215 MDT), as shown in Figures 1 and 2 below, cloud-base virga shafts became visually prominent beneath convective cells over the high peaks of the Teton Range, including Inspiration Peak and Grand Teton.
Visual Signatures vs. Environmental Reality: While descending virga shafts often appear benign to ground observers, in high-elevation sub-cloud environments they signal localized hydrometeor evaporation and melting aloft.
Radar Corroboration: Pocatello (KSFX) radar base reflectivity around 1813–1823 UTC captured localized cores of 40–50 dBZ developing directly over high terrain.
Downdraft Genesis: Evaporative cooling beneath elevated cloud bases initiated localized, dense downdrafts, marking the immediate onset of convective downbursts in the high-mountain landscape.
2. Satellite Thermodynamic Assessment: GOES-18 ABI MBR Evolution
The regional environment was characterized by strong boundary layer heating and downstream destabilization through the early afternoon. A comparison of GOES-18 Advanced Baseline Imager (ABI) Microburst Risk (MBR) three-channel Brightness Temperature Difference (BTD) fields between 1800 UTC and 2030 UTC in Figure 3 captured this rapid thermodynamic transition:
Jackson Hole Sector (1800 UTC): Modest MBR BTD signatures corresponded to wind gust potentials of 35–45 knots.
Riverton Sector (2030 UTC): Downstream boundary layer destabilization drove microburst risk potentials upward to 40–60 knots.
Convective Cluster Propagation: Multi-Radar Multi-Sensor (MRMS) reflectivity overlaid on ABI BTD fields confirmed convective expansion eastward along strong surface insolation gradients.
3. High-Resolution Soundings: Inverted-V Profiles and Downdraft Instability
Vertical thermodynamic profiles over the Gros Ventre Range (18 km east of Jackson, WY) at 1800 UTC contrasted the retrieved state with numerical guidance:
MetOp-C NUCAPS Retrieval (43.48°N, 110.53°W)
Superadiabatic surface contact layer; classic inverted-V / “hourglass” signature:
MU-CAPE: 788 J/kg, MWPI: 4.40
MWPI/DCAPE Wind Gust Potential (WGP): 46.4/28 knots
HRRR Analysis Sounding (KJAC Airport)
Deep, well-mixed dry-adiabatic sub-cloud layer
DCAPE: 942 J/kg, MWPI: 3.82
MWPI/DCAPE Wind Gust Potential (WGP): 43.9/84 knots
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The MetOp-C NUCAPS sounding profile, while most closely fitting the Wakimoto (1985) Type B pattern, captured a pronounced surface-based superadiabatic layer and classic inverted-V structure. This profile, shown in Figure 4, confirmed relatively deep sub-cloud dry air capable of supporting substantial negative parcel buoyancy via evaporation and melting, priming the Grand Teton region for severe outflow winds.
4. ABI 8.4 µm — 11.2 µm Microphysical Transitions and Surface Mesonet Outflows
Multi-spectral infrared difference products (GOES-18 ABI 8.4 µm — 11.2 µm BTD) in Figures 5–7 tracked convective microphysical evolution over the Bridger-Teton National Forest Avalanche Center mesonetwork:
Initial Outflow Episode (1810 UTC):
ABI 8.4–11.2 µm BTD exhibited slightly negative values (purple to magenta shading, roughly -1 K to 0 K), indicative of developing ice/mixed-phase cloud particles. Local mesonet stations recorded peak wind gusts of 34–37 knots.
Secondary, Severe Episode (1920 UTC):
ABI BTD transitioned to positive values (yellow shading, > 0 K), signaling an expanding dense volume of mixed-phase precipitation and hydrometeor loading. KSFX radar reflectivity peaked above 50 dBZ over high terrain.
Mesonet stations observed severe downburst wind gusts reaching 44–56 knots.
5. Downstream Deep Convection: Riverton Destabilization and AMSR-3 Indices
As the convective system propagated eastward into the Wind River Basin, downstream thermodynamic and microwave observations tracked further storm intensification as depicted in Figure 8:
Riverton Radiosonde (RIW) vs. MetOp-C NUCAPS (1800 UTC):
NUCAPS retrieved severe DCAPE of 1056 J/kg (maximum downdraft velocity of 46.0 m/s) and a WGP of 61.7 knots.
RIW RAOB observed a DCAPE of 1177 J/kg with a deeply destabilized convective boundary layer.
AMSR-3 Microwave Sounder Diagnostics (2032 UTC):
As shown in Figures 9 and 10, High-frequency window channel (165.5 GHz) brightness temperature dropped to < 200 K, while the 165 GHz Scattering Index exceeded 40–50 K. Water vapor absorption sounding channels (183.31±7 GHz) displayed deep scattering depressions (BTD < -15 K), confirming robust updrafts supporting dense volumes of graupel, aggregates, and small hail aloft.
Microphysical-Thermodynamic Coupling:
In accordance with high-plains and intermountain convective studies (e.g., Srivastava 1987), substantial CAPE coupled with a deep, dry convective boundary layer promotes vigorous updrafts capable of generating high concentrations of small, mixed-phase and ice-phase precipitation particles (graupel, snow pellets, and small hail) aloft. As this dense core of frozen hydrometeors descends through the melting level into the deeply destabilized, dry-adiabatic sub-cloud layer over the Wind River Basin, two synergistic diabatic cooling processes occur simultaneously: the rapid latent heat absorption via melting of small hail and graupel, followed immediately by the rapid evaporation of the resulting liquid droplets into the subsaturated boundary layer. Because smaller hydrometeors possess a significantly larger surface-area-to-volume ratio than larger drops, evaporative cooling rates are maximized. Combined with initial downward precipitation drag (hydrometeor loading), this intense diabatic cooling creates a localized, high-density, negatively buoyant air parcel that accelerates rapidly toward the surface.
Downstream Wind Reports: Driven by these microphysical and thermodynamic processes, severe outflow gusts of 49–53 knots struck the Riverton area between 2230 and 2240 UTC.
6. Orographic Influence: The Venturi Effect and Gravitational Acceleration
While thermodynamic instability (DCAPE and MWPI) provided the energy source for convective downdrafts, terrain kinematics over the Grand Teton Range amplified local surface wind speeds. Thermodynamic evaporation and hydrometeor melting produced dense, negatively buoyant air parcels aloft. As this cooled, dense air mass descended the steep relief of the Teton Range, downslope gravitational acceleration converted negative buoyant energy into concentrated forward kinetic energy. Concurrently, as outflow penetrated narrow mountain cols, canyons, and passes, cross-sectional flow boundaries contracted. Governed by mass continuity and Bernoulli’s principle, the Venturi effect induced localized pressure drops and forced horizontal jet acceleration through terrain constrictions. The superimposition of negative thermal buoyancy, gravitational acceleration down steep alpine topography, and Venturi channeling explains why high-elevation passes and mountain bases recorded severe downburst wind gusts exceeding baseline thermodynamic potential.
Explanatory Note on DCAPE vs. MWPI Wind Gust Potential Accuracy: A critical takeaway from this event is the marked operational superiority of the Microburst Windspeed Potential Index (MWPI) over theoretical Downdraft CAPE (DCAPE) calculations (W=2DCAPE) in diagnosing realistic surface wind gust potential. As illustrated in the 1800 UTC Jackson Hole sector soundings (Section 3), DCAPE-derived wind gust potentials exhibited extreme sensitivity and wide divergence: the MetOp-C NUCAPS retrieval (DCAPE=105 J/kg) yielded a theoretical DCAPE WGP of only 28 knots compared to an MWPI WGP of 46.4 knots (MWPI=4.40), whereas the concurrent KJAC HRRR analysis sounding (DCAPE=942 J/kg) yielded an unadjusted DCAPE WGP of 84 knots compared to an MWPI WGP of 43.9 knots (MWPI=3.82). While the MWPI WGP values (43.9–46.4 knots) closely bracketed the actual severe convective outflow winds recorded by the Bridger-Teton mesonet (34–37 knots initially, intensifying to 44–56 knots), the DCAPE-derived values either severely underestimated the threat or overestimated wind speeds by nearly a factor of two. As documented by Gilmore and Wicker (1998) and Pryor (2015), unadjusted DCAPE calculations frequently fail in dry intermountain convective environments because fundamental parcel theory assumptions are violated: entrainment of ambient dry air dilutes the descending parcel, and the evaporative cooling rates required to maintain saturated parcel descent rarely occur in nature, resulting in substantially lower kinetic energy gains than DCAPE predicts. Conversely, by explicitly coupling convective updraft instability (CAPE, which governs precipitation loading and particle concentration) with sub-cloud temperature lapse rates and vertical moisture gradients, the MWPI provides a statistically robust, nearly unbiased prediction (mean error <1kt and MAE 4.6–5.5kt) of baseline thermodynamic downburst potential prior to orographic amplification.
Bibliography
Gilmore, M. S., and L. J. Wicker, 1998: The influence of midtropospheric dryness on supercell morphology and evolution. Mon. Wea. Rev.,126, 943–958.
Google, 2026: Gemini [Large language model]. Google LLC, accessed 2 November 2026, https://gemini.google.com.
Pryor, K. L., 2015: Progress and Developments of Downburst Prediction Applications of GOES. Wea. Forecasting, 30, 1182–1200, DOI: 10.1175/WAF-D-14–00106.1.
Pryor, K.L., and S.D. Miller, 2016: Downburst Prediction Applications of GOES over the Western United States. arXiv 2016, arXiv:1611.01407.
Pryor, K. L., 2022: Examination of the Physical Process of Severe Convective Windstorms (Order №29326600). Available From ProQuest Dissertations & Theses Global. (2758640654). Accessed August 2026. https:// www.proquest.com/dissertations-theses/examination-physical-process-severe-convective/docview/2758640654/se-2.
Srivastava, R.C., 1987: A model of intense downdrafts driven by the melting and evaporation of precipitation. J. Atmos. Sci., 44, 1752–1773.
Wakimoto, R. M., 1985: Forecasting dry microburst activity over the high plains. Mon. Wea. Rev., 113, 1131–1143.
Whiteman, C. D., 2000: Terrain-forced flows. In Mountain meteorology: Fundamentals and applications (pp. 141–170). Oxford University Press.
