US Weather MCP Server

Instructions

Implementation Reference

• src/index.ts:86-132 (registration)

  Registration of the 'get_alerts' MCP tool, including name, description, input schema, and inline handler function.

  server.tool(
      "get_alerts",
      "Get weather alerts for a state",
      {
        state: z.string().length(2).describe("Two-letter state code (e.g. CA, NY)"),
      },
      async ({ state }) => {
        const stateCode = state.toUpperCase();
        const alertsUrl = `${NWS_API_BASE}/alerts?area=${stateCode}`;
        const alertsData = await makeNWSRequest<AlertsResponse>(alertsUrl);
    
        if (!alertsData) {
          return {
            content: [
              {
                type: "text",
                text: "Failed to retrieve alerts data",
              },
            ],
          };
        }
    
        const features = alertsData.features || [];
        if (features.length === 0) {
          return {
            content: [
              {
                type: "text",
                text: `No active alerts for ${stateCode}`,
              },
            ],
          };
        }
    
        const formattedAlerts = features.map(formatAlert);
        const alertsText = `Active alerts for ${stateCode}:\n\n${formattedAlerts.join("\n")}`;
    
        return {
          content: [
            {
              type: "text",
              text: alertsText,
            },
          ],
        };
      },
    );

• src/index.ts:92-131 (handler)

  The core handler logic for the 'get_alerts' tool: fetches alerts from NWS API using makeNWSRequest, handles errors and empty results, formats alerts with formatAlert, and returns markdown text content.

  async ({ state }) => {
    const stateCode = state.toUpperCase();
    const alertsUrl = `${NWS_API_BASE}/alerts?area=${stateCode}`;
    const alertsData = await makeNWSRequest<AlertsResponse>(alertsUrl);
  
    if (!alertsData) {
      return {
        content: [
          {
            type: "text",
            text: "Failed to retrieve alerts data",
          },
        ],
      };
    }
  
    const features = alertsData.features || [];
    if (features.length === 0) {
      return {
        content: [
          {
            type: "text",
            text: `No active alerts for ${stateCode}`,
          },
        ],
      };
    }
  
    const formattedAlerts = features.map(formatAlert);
    const alertsText = `Active alerts for ${stateCode}:\n\n${formattedAlerts.join("\n")}`;
  
    return {
      content: [
        {
          type: "text",
          text: alertsText,
        },
      ],
    };
  },

• src/index.ts:89-91 (schema)

  Zod input schema for the 'get_alerts' tool, validating the 'state' parameter as a 2-character string.

  {
    state: z.string().length(2).describe("Two-letter state code (e.g. CA, NY)"),
  },

• src/index.ts:48-58 (helper)

  Helper function used by 'get_alerts' to format individual alert features into multi-line text strings.

  function formatAlert(feature: AlertFeature): string {
    const props = feature.properties;
    return [
      `Event: ${props.event || "Unknown"}`,
      `Area: ${props.areaDesc || "Unknown"}`,
      `Severity: ${props.severity || "Unknown"}`,
      `Status: ${props.status || "Unknown"}`,
      `Headline: ${props.headline || "No headline"}`,
      "---",
    ].join("\n");
  }

• src/index.ts:19-35 (helper)

  Shared helper function for making authenticated fetch requests to the National Weather Service (NWS) API, used by 'get_alerts'.

  async function makeNWSRequest<T>(url: string): Promise<T | null> {
      const headers = {
        "User-Agent": USER_AGENT,
        Accept: "application/geo+json",
      };
    
      try {
        const response = await fetch(url, { headers });
        if (!response.ok) {
          throw new Error(`HTTP error! status: ${response.status}`);
        }
        return (await response.json()) as T;
      } catch (error) {
        console.error("Error making NWS request:", error);
        return null;
      }
    }