AN ACR SMT GOT CAUGHT IN HVY TSTM WX REQUIRING MANY DEVS AND A BLOCK ALT.

Narrative

ON THIS FLT WE WERE CRUISING AT 15000 FT MSL; MOSTLY IMC; WHEN WE APCHED AN AREA OF A CLUSTER OF BUILDUPS JUST W OF FWA. WE OBSERVED OUR RADAR FOR A POSSIBLE PATH THROUGH THE AREA AND REQUESTED/RECEIVED PERMISSION TO DEVIATE FROM ZAU. VERY RAPIDLY WE FOUND OURSELVES IN A LITERAL MINE FIELD OF BUILDUPS THAT WERE EITHER NOT REGISTERING ON OUR WX RADAR OR WERE MASKED BY OTHER BUILDUPS OR ATTENUATED BY THE HVY PRECIPITATION WE ENCOUNTERED. WE STARTED TO EXPERIENCE ALTDEVS PLUS OR MINUS 300 FT. ZAU OFFERED US A BLOCK ALT IF WE WISHED. WE REQUESTED 14000- 16000 FT AND IT WAS IMMEDIATELY GIVEN. OUR DEVS REGAINED CONSTANTLY CHANGING HDGS FROM APPROX 020 DEGS TO 180 DEGS AS WE PROGRESSED EBOUND TO REESTABLISH ON THE DTW MIZAR 1 ARR VIA THE FWA TRANSITION. ZAU CONFERRED WITH FWA APCH RADAR AND ADVISED THAT IF WE PROCEEDED NE FOR 10 MI WE WOULD EXIT THE AREA OF TSTMS. WE ADVISED WE WERE 'WORKING OUR WAY' IN THAT DIRECTION. WE FINALLY CLRED THE AREA OF TSTMS AND ADVISED WE COULD NOW MAINTAIN 15000 FT AND TAKE A HDG TO REJOIN THE MIZAR 1 ARR. WE WERE CLRED TO MAINTAIN 15000 FT AND GIVEN A HDG TO INTERCEPT THE FWA RADIAL. I BELIEVE WE COMPLIED WITH ATC CLRNCS BUT WE WERE NOT AWARE THAT THIS WX AREA; THAT APPEARED AS MANY WE HAVE ROUTINELY TRANSITTED; WAS SO SEVERE. HINDSIGHT MIGHT ASK WHY WE DIDN'T TURN BACK TO IND; BUT WE HAD JUST COME THROUGH VERY SIMILAR CONDITIONS TO THAT POINT; AND HAD NO REASON TO SUSPECT THE EXTREME CHANGE -- ESPECIALLY SINCE THE RADAR DIDN'T DEPICT ANY MORE THAN IT DID.