NEXRAD Product Access

NEXRAD Product Access selects and retrieves the correct NEXRAD data for exactly one radar site, UTC time, and requested product or Level II moment, keeping archive volumes, real-time chunks, and Level III products distinct because each has different availability and decoding contracts. It documents the current public NEXRAD-on-AWS resources in us-east-1: s3://unidata-nexrad-level2/ for completed historical Level II volumes, s3://unidata-nexrad-level2-chunks/ for real-time chunks that must be assembled and validated, and s3://unidata-nexrad-level3/ for selected real-time Level III data - noting the former noaa-nexrad-level2 bucket was deprecated after September 1, 2025, and that the live registry should be checked rather than assumed.

It starts by resolving an access contract: the four-character site code (such as KTLX), the requested UTC instant or interval, a nearest-at-or-before/either-direction/interval-overlap time policy, the data level (Level II, Level III, or an explicit mixed workflow), the product or moment and its sweep requirements, and the source (archive, real-time, or operational feed).

For historical Level II volumes, it validates the site, resolves the exact documented key prefix rather than inferring a layout from an example filename, lists only that prefix, selects by the stated time policy, requires a nonzero stable object before download, verifies received bytes (with a SHA-256 hash when provenance requires it), and confirms the decoded site, volume time, sweeps, moments, and quality fields before the file is treated as valid. For real-time Level II, it treats each chunk as a fragment with ordering and completion dependencies - discovering the full chunk sequence, verifying byte identities, assembling into a new file without overwriting source chunks, and rejecting or quarantining a sequence that is late, duplicated, discontinuous, or from another site.

For Level II moment selection, it records the exact moment name, calibrated unit and scale/offset, elevation-angle policy, and quality fields for range folding, clutter, dealiasing, or invalid gates - refusing to assume a moment is present in every volume or substitute a different moment for a missing one. For Level III, it resolves the official current product code and source rather than guessing one from a display name like "base reflectivity," and verifies site/coverage, product code, scan time, projection, units, and whether the result is single-site, sector, or multi-site.

{
  "site": "KTLX",
  "requested_time_utc": "2026-09-25T12:00:00Z",
  "selection_policy": "nearest-at-or-before",
  "selected_time_utc": "2026-09-25T11:59:54Z",
  "data_level": "Level II",
  "product_or_moment": "REF",
  "source": "s3://unidata-nexrad-level2/EXACT_DOCUMENTED_KEY",
  "object_identity": {
    "etag": "OPAQUE_IDENTITY",
    "last_modified": "2026-09-25T12:01:02Z",
    "size_bytes": 12345678
  },
  "single_site_required": true
}

Every access returns a structured record like this before decoding begins, plus rejected candidates, chunk dependencies, and the rule that prevented cross-site or cross-product substitution.

Use it to fetch a completed NEXRAD Level II volume from the historical archive, follow or assemble real-time Level II chunks for a named site, retrieve selected real-time Level III products, select a specific product (reflectivity, velocity, spectrum width, echo top, VIL, or a supported dual-polarization product), or restrict access to one radar site while rejecting composite or neighboring-site data.

Do not use it to convert a Level II moment name into a Level III product code by assumption - the two levels have different processing and distribution contracts. Use nexrad-mosaic-access instead when the requested result is a precomputed multi-radar composite, and noaa-radar-satellite-fetching for general NOAA archive discovery mechanics when available.

Input is a radar site code, a UTC time or interval, a time-selection policy, and the requested data level and product/moment. Output is a structured access record identifying the exact selected object, its identity (etag, last-modified, size), the data level and product/moment, and whether single-site access was enforced - plus a list of rejected candidates and the reason cross-site or cross-product substitution was prevented.

Uses noaa-radar-satellite-fetching for general NOAA archive discovery mechanics, hands off to nexrad-mosaic-access for precomputed multi-radar composites, and to radar-satellite-analysis for scientific interpretation once data is retrieved.

Atmospheric scientists and data engineers who need exactly the right NEXRAD volume or product for one radar site and time - avoiding the common failure of silently substituting a nearby site, an unverified chunk, or a guessed product code for what was actually requested.