National Weather Service United States Department of Commerce

NWS Schemas

Please note: These pages are not intended to support operations directly. Details here.

IWXXM-US Overview

Goals of IWXXM-US

IWXXM-US was developed to support the encoding of US products (specifically, METAR, SPECI, TAF, and SIGMET) in XML. IWXXM-US builds directly off the work of the World Meteorological Organization (WMO) and the International Civil Aviation Organization (ICAO) that created data models and schemas for Aviation XML (AvXML). One of the packages within AvXML is named IWXXM, and this package supports the four products mentioned above.

Over the years, the US has filed a number of differences with ICAO in the formatting of METAR, SPECI, TAF, and SIGMET products in Traditional Alphanumeric Code (TAC) forms. These filed differences support an important set of user requirements inside the US. In many cases, US TAC products are converted from US formats to international formats before they are transmitted internationally.

IWXXM-US is intended to support products for the US, not just the National Weather Service (NWS). The data model, XML schema, and code tables were coordinated among representatives of the NWS, Federal Aviation Administration (FAA), and the Department of Defense (DoD).

Relationship Between IWXXM-US and IWXXM

Since IWXXM was developed by WMO and ICAO, it only supports the needs of those organizations. IWXXM-US extends IWXXM and it supports in XML the differences that been filed by the US for TAC. IWXXM-US uses the methods and constructs of IWXXM and AvXML wherever possible.

It is likely that meteorological services will need to convert documents encoded in IWXXM-US into IWXXM documents. To that end, IWXXM-US uses the availible constructs with the IWXXM schemas to faciliate that process.

Use of Code Lists in IWXXM-US

One of the powerful techniques employed by AvXML is the use of code lists and vocabularies instead of enumerations within XML schemas. This gives the agencies that encode documents in AvXML substantial flexibility in the evolution of their data encoding. Typically new values can be added to a code list with minimal operational disruption. Deploying updated schemas typically creates substantial work for both data producers and data consumers.

The WMO is hosting code lists in a data repository that can be found at The US is hosting code lists at