気象庁の半分公式のようなAPIを使って、気象データを監視し、降水確率が50%以上のときには通知をするというサンプルコードです。
通知の部分はLINEAPIなどの通知を使ってください。

import json
import requests
import time

def weatherMan():
    # 気象庁データの取得
    jma_url = 'https://www.jma.go.jp/bosai/forecast/data/forecast/170000.json'
    data = requests.get(jma_url).json()
    
    # 時間帯の取得
    pop_0_time = data[0]["timeSeries"][1]["timeDefines"][0]
    pop_1_time = data[0]["timeSeries"][1]["timeDefines"][1]
    pop_2_time = data[0]["timeSeries"][1]["timeDefines"][2]
    pop_3_time = data[0]["timeSeries"][1]["timeDefines"][3]
    pop_4_time = data[0]["timeSeries"][1]["timeDefines"][4]

    # 地域の取得
    pop_0_name = data[0]["timeSeries"][1]["areas"][0]["area"]["name"]
    pop_1_name = data[0]["timeSeries"][1]["areas"][0]["area"]["name"]
    pop_2_name = data[0]["timeSeries"][1]["areas"][0]["area"]["name"]
    pop_3_name = data[0]["timeSeries"][1]["areas"][0]["area"]["name"]
    pop_4_name = data[0]["timeSeries"][1]["areas"][0]["area"]["name"]

    # 降水確率の取得
    pop_0 = data[0]["timeSeries"][1]["areas"][0]["pops"][0]
    pop_1 = data[0]["timeSeries"][1]["areas"][0]["pops"][1]
    pop_2 = data[0]["timeSeries"][1]["areas"][0]["pops"][2]
    pop_3 = data[0]["timeSeries"][1]["areas"][0]["pops"][3]
    pop_4 = data[0]["timeSeries"][1]["areas"][0]["pops"][4]

    # 表示
    print(pop_0_name + "：" + str(pop_0_time) + "：" + str(pop_0))
    print(pop_1_name + "：" + str(pop_1_time) + "：" + str(pop_1))
    print(pop_2_name + "：" + str(pop_2_time) + "：" + str(pop_2))
    print(pop_3_name + "：" + str(pop_3_time) + "：" + str(pop_3))
    print(pop_4_name + "：" + str(pop_4_time) + "：" + str(pop_4))
    
    if (int(pop_0) >= 50):
        print("今雨が降っているようです" + "　降水確率:" + str(pop_0))
    if (int(pop_1) >= 50):
        print("数時間後に雨が降るようです！" + "　降水確率:" + str(pop_1))
        sendNotify()
    time.sleep(5)
    weatherMan()
    
def sendNotify():
    print("雨が降る確率が高いです")

weatherMan()