A → Z 15 DEFINED TERMS

Anvil Hail Sentinel’s in-house ML scoring system: A LightGBM-based gradient boosting model trained on 5+ years of MRMS radar history aligned with ground-truth hail reports. Anvil produces a hail probability per storm object every scan and is the primary signal that gates an alert. Trained on 83 canonical radar/environment features.
MRMS Multi-Radar Multi-Sensor: A NOAA system that fuses observations from the entire NEXRAD radar network with satellite imagery, lightning detection, and surface observations into a single 1-km nationwide grid updated every 2 minutes. The foundation layer for all our radar-derived products.
MESH Maximum Estimated Size of Hail: A radar-derived diagnostic in millimeters that estimates the largest hailstone a storm cell could produce, computed from vertically integrated reflectivity above the freezing level. We use MESH as one input but never present it raw to users — Anvil ML replaces MESH for end-user sizing.
ProbSevere NOAA/CIMSS short-fuse severe-weather model: A statistical model from NOAA/CIMSS that combines radar, lightning, and environmental fields to produce a 0–60 minute probability of any severe convective hazard. Hail Sentinel’s Anvil shares many features with ProbSevere v3 but is specialized for hail and trained on Hail Sentinel’s ground-truth dataset.
Dual-pol Dual-polarization radar: A radar mode that transmits and receives both horizontally and vertically polarized waves. The shape and orientation information makes it possible to distinguish hail from rain, snow, or debris. ZDR and ρhv are the two key dual-pol variables we use.
Sub-severe Hail smaller than the NWS severe threshold: Hail under 25 mm (1 inch) — below the National Weather Service severe-thunderstorm criterion. Most insurance and operational damage actually comes from the 19–25 mm band, so we explicitly include sub-severe (≥ 19.05 mm / 0.75″) in detection and alerts.
NEXRAD Next Generation Weather Radar: The U.S. national network of 159 WSR-88D Doppler radar stations operated by the National Weather Service, FAA, and DOD. Each radar covers ~230 km and updates every 4–10 minutes. NEXRAD is the upstream feed for MRMS.
HRRR High-Resolution Rapid Refresh: A NOAA convection-allowing numerical weather prediction model with 3-km grid spacing, run hourly with an 18-hour forecast horizon. We use HRRR for environmental fields (CAPE, shear, freezing level) that condition our nowcast and 0–18h hail risk forecast.
SPC Storm Prediction Center: The NOAA office in Norman, OK that issues outlooks, watches, and mesoscale discussions for severe convective weather across the U.S. SPC convective outlooks (Day 1–8) are a useful benchmark to compare against our hail-specific risk forecasts.
IEM LSR Iowa Environmental Mesonet — Local Storm Reports: A near-real-time feed of NWS Local Storm Reports curated by the Iowa Environmental Mesonet. Reports include hail size, time, and location from trained spotters, law enforcement, and the public. One of three ground-truth sources we use for training and verification.
CoCoRaHS Community Collaborative Rain, Hail and Snow Network: A volunteer precipitation observation network covering all 50 states. Volunteers submit measured hail diameters (sometimes from hail pads), generally higher quality than estimates. We use CoCoRaHS as one of three ground-truth sources, weighted higher than visual estimates.
Geohash Spatial encoding of latitude/longitude: A short alphanumeric string representing a rectangular grid cell. Useful for fast spatial prefix queries — every cell is a substring of cells that contain it. We use geohash prefixes for nearby-location lookups in Firestore and to discretize stored locations as a privacy measure.
GPC Grid Point Cell: A discrete cell on the underlying 0.01° MRMS grid (~1 km on a side). Hail Sentinel resolves detections to a single GPC over a monitored location, giving alerts address-level rather than county-level scope.
ZDR Differential reflectivity: A dual-pol variable measuring the difference in reflectivity between horizontal and vertical polarizations, in dB. Large hail tumbles in free fall and tends toward ZDR ≈ 0 dB even in regions of very high reflectivity — a strong hail signature when paired with high reflectivity above the freezing level.
ρhv (rhoHV) Correlation coefficient: A dual-pol variable measuring the consistency of returns from horizontal and vertical polarizations, ranging 0–1. Pure rain is ~0.99; mixtures (hail + rain, debris, melting ice) drop ρhv. A reflectivity-rich region with low ρhv is a strong hail or debris signature.