B757-200
Aircraft Equipment Problem Critical
Flight Crew Returned To Departure Airport; General Declared Emergency
Aircraft
During climb out while deviating for weather; WX WEAK displayed on both Captain and First Officer HSI screens which soon changed to WX FAIL.  Because of night conditions with scattered to broken thunderstorms enroute; we declared an emergency and returned to [departure airport].  Normal landing weight.
[Narrative 2 had no additional information]
B757-200 crewmember reported WX radar failed during climb out. Since thunderstorms were forecast enroute the flight declared an emergency and returned to departure airport.