Jun 21-26 2023 Rockies Hail Storms & Central/Eastern Severe Weather

Hail Vulnerability Model — v1.7 Physics

Damage probability is computed using a physically grounded stone-population model. Rather than mapping peak MESH directly to a damage ratio, the model distributes stone impacts across the realistic hail size spectrum — where the largest stones are rare outliers, not the norm.

Algorithm

1. Peak MRMS MESH is corrected for radar calibration bias (Ortega 2018: 0.75×) and storm type (supercell 1.15×) to estimate the effective largest stone diameter D_l.
2. Grieser & Hill (2019) power-law fits give average stone diameter D_a, storm duration T, and hit rate Hr from D_l, calibrated on 37,726 CoCoRaHS observations.
3. Stone size distribution: Gamma distribution (α = 1.75; Li et al. 2024) between 5 mm and D_l.
4. Each 1 mm bin is assigned a damage probability via a lognormal fragility curve for unrated asphalt shingle (θ = 46 mm, γ = 0.25).
5. Aggregate P_fail = 1 − ∏(1 − p_i)^N_i. Duration multiplier applied (cap 1.4×).
6. Ackermann et al. (2024) HDE: surface-occurrence sigmoid P_surface = 1/(1 + e^{−0.18(MESH−27)}). Corrects for hail melting aloft at swath edges.

Validation Benchmark (NCEI)

NCEI billion-dollar loss benchmark for this event: $5.5B. Model output represents roof-loss ground-up across the MRMS swath footprint. NCEI includes all perils, sectors, and loss layers; direct comparison requires applying sector/coverage ratios.

Key Parameters

Calibration Signal — Austin Permits

City of Austin re-roof permit filings (Socrata dataset 3syk-w9eu) show a pronounced post-event surge. Re-roof permits in Jun 2024 alone (80) were 3.8× the 2017–2023 baseline of ~21/month. Excess permits Jun–Dec 2024: ~238 above baseline.

Permit rate proxy ≈ 238 / (Austin structures in swath) — provides a weak but independent lower bound on the fraction of buildings with roof damage. Permits undercount actual damage by 3–10× (not all damaged roofs file permits immediately).

References