Check out L.A. Weatherman say Happy Birthday to Hugh Janus on air. The J is silent.

Someone ripped a page outta the Bart Simpson playbook this week to prank an L.A. weatherman ... asking for a birthday shout-out for "Hugh Janus" ... and it went down PERFECTLY!!!

Ya gotta see this ... KTLA weatherman Henry DiCarlo was giving his usual TV shout-outs for birthdays, anniversaries, etc. ... when he read the name of a supposed 10-year-old boy named "Hugh Janus."

That's when the two anchors LOST IT ... instantly realizing DiCarlo had been pranked and laughing their faces off.


CLICK HERE for more from TMZ