weather_forecast

Instructions

Implementation Reference

• src/index.js:148-170 (handler)

  Main handler function for weather_forecast tool that processes location and hours parameters, fetches forecast data from SMHI API, formats hourly forecast entries with temperature, conditions, wind, and precipitation data

  async ({ location, hours }) => {
    try {
      const loc = await getLocation(location);
      const data = await fetchForecast(loc.lat, loc.lon);
      const maxHours = hours || 24;
      const now = Date.now();
      const cutoff = now + maxHours * 3600000;
      const lines = [`## ${loc.name} — ${maxHours}h Forecast\n`];
      for (const entry of data.timeSeries) {
        const t = new Date(entry.validTime);
        if (t.getTime() > cutoff) break;
        if (t.getTime() < now - 3600000) continue;
        const time = t.toLocaleString("sv-SE", { timeZone: "Europe/Stockholm", weekday: "short", day: "numeric", month: "short", hour: "2-digit", minute: "2-digit" });
        const p = entry.parameters;
        const precip = getParam(p, "pmean");
        lines.push(`**${time}:** ${getParam(p, "t")}°C, ${WSYMB2[getParam(p, "Wsymb2")] || ""}, wind ${getParam(p, "ws")} m/s${precip > 0 ? `, ${precip} mm/h` : ""}`);
      }
      lines.push(`\n*SMHI Open Data*`);
      return { content: [{ type: "text", text: lines.join("\n") }] };
    } catch (err) {
      return { content: [{ type: "text", text: `Error: ${err.message}` }], isError: true };
    }
  }

• src/index.js:141-171 (registration)

  Tool registration with MCP server defining the weather_forecast tool name, description, input schema with location string and optional hours number, and the handler function

  server.tool(
    "weather_forecast",
    "Get hourly weather forecast for a location in Sweden using SMHI.",
    {
      location: z.string().describe("Swedish city name or lat,lon coordinates"),
      hours: z.number().min(1).max(72).optional().describe("Hours ahead (default 24, max 72)"),
    },
    async ({ location, hours }) => {
      try {
        const loc = await getLocation(location);
        const data = await fetchForecast(loc.lat, loc.lon);
        const maxHours = hours || 24;
        const now = Date.now();
        const cutoff = now + maxHours * 3600000;
        const lines = [`## ${loc.name} — ${maxHours}h Forecast\n`];
        for (const entry of data.timeSeries) {
          const t = new Date(entry.validTime);
          if (t.getTime() > cutoff) break;
          if (t.getTime() < now - 3600000) continue;
          const time = t.toLocaleString("sv-SE", { timeZone: "Europe/Stockholm", weekday: "short", day: "numeric", month: "short", hour: "2-digit", minute: "2-digit" });
          const p = entry.parameters;
          const precip = getParam(p, "pmean");
          lines.push(`**${time}:** ${getParam(p, "t")}°C, ${WSYMB2[getParam(p, "Wsymb2")] || ""}, wind ${getParam(p, "ws")} m/s${precip > 0 ? `, ${precip} mm/h` : ""}`);
        }
        lines.push(`\n*SMHI Open Data*`);
        return { content: [{ type: "text", text: lines.join("\n") }] };
      } catch (err) {
        return { content: [{ type: "text", text: `Error: ${err.message}` }], isError: true };
      }
    }
  );

• src/index.js:144-147 (schema)

  Input validation schema using zod for weather_forecast tool parameters, defining location as required string and hours as optional number between 1-72

  {
    location: z.string().describe("Swedish city name or lat,lon coordinates"),
    hours: z.number().min(1).max(72).optional().describe("Hours ahead (default 24, max 72)"),
  },

• src/index.js:97-102 (helper)

  Helper function that fetches weather forecast data from SMHI API for given latitude and longitude coordinates, returning JSON data

  async function fetchForecast(lat, lon) {
    const url = `${BASE_URL}/lon/${lon.toFixed(6)}/lat/${lat.toFixed(6)}/data.json`;
    const res = await fetch(url, { headers: { "User-Agent": USER_AGENT } });
    if (!res.ok) throw new Error(`SMHI API error (${res.status}): ${await res.text()}`);
    return res.json();
  }

• src/index.js:84-90 (helper)

  Helper function that resolves location input by first checking against known Swedish cities, then geocoding if needed, throwing error if location cannot be found in Sweden

  async function getLocation(input) {
    const loc = resolveLocation(input);
    if (loc) return loc;
    const geo = await geocode(input);
    if (geo) return geo;
    throw new Error(`Could not find location "${input}" in Sweden. Try a city name or lat,lon coordinates.`);
  }