Hi guys,
A 2M Yagi will NOT perform properly on VHF WX frequencies or any other outside the band it's designed for. ONLY when the elements are cut to proper length and spaced properly acording to the frequency it's intended for all of it's electrical characteristics, most importantly gain and directional pattern become extremely unpredictable.
Commercial Yagis made for the purpose are available at a price (;->) so your best bet is to get a Radio Amateur Handbook and do the math. There are several construction projects in the antenna section and in the VHF Antenna Handbook but they're for the ham bands, what you want to look at are the formuli for calculating the proper dimensions. I'll bet you can make one with a wooden dowel and some coat hangers, it's been done before. Since the band you want is extremely narrow you don't have to use tubing to broaden it. (The larger the diameter the broader the bandwidth.)
Then again you can always use the Internet and get all the info on the weather you want, every TV station, the NWS, NOAA, weather.com and many others give you the current outlook, forecast and radar images. On the other hand they won't sound an alarm like a Weatheradio does so then you'll need a good antenna.
Now here's a thought, why don't you have a transmitter within range of an ordinary receiver and antenna? NOAA has been filling the holes in the coverage areas with more transmitters on more VHF frequencies, some at the top of the AM broadcast band and various frequencies in the standard FM broadcast band too. Maybe you should tell the boffins at NOAA that your area is without proper coverage.
