A319 CREW INADVERTENTLY ENTERS TSTM OVER ARKANSAS AT FL390.

Narrative

WE WERE IN CRUISE AT FL390 FLYING IN A LAYER OF CIRRUS CLOUDS (IMC). WX RADAR WAS IN USE DUE TO EXTENSIVE ISOLATED TSTMS OVER TEXAS; OKLAHOMA; ARKANSAS. A LINE OF WIDELY SEPARATED STORMS INTERSECTED OUR RTE OF FLT; NONE CLOSER THAN 40-50 MI EITHER SIDE OF OUR ACFT IMMEDIATELY PRIOR TO A TURB ENCOUNTER. WE EXITED THE CIRRUS LAYER AND WERE MET WITH A VISUAL PICTURE OF A RAPIDLY BUILDING STORM IMMEDIATELY IN FRONT OF US (NOTHING HAD BEEN PAINTED ON RADAR LEADING UP TO THE ENCOUNTER). I COULD SEE THE DOME TOP RISING RAPIDLY IN FRONT OF US AND I REACHED UP AND TURNED ON THE FASTEN SEATBELT SIGN FOLLOWED BY AN ANNOUNCEMENT FOR THE PAX TO FASTEN THEIR SEATBELTS. THE CAPT MENTIONED THAT IT WAS TOO LATE TO AVOID THE STORM AND WE ENDED UP PENETRATING THE TOP. JUST AS WE BEGAN TO ENTER THE CLOUDS; THE ACFT BEGAN TO CLB RAPIDLY AND WE BEGAN TO EXPERIENCE WHAT I WOULD CLASSIFY AS MODERATE CHOP. IN MY OPINION AND UNDERSTANDING; WE WERE LITERALLY RIDING THE TOP OF THE STORM AS IT PUSHED THE ACFT UPWARDS. MY IMMEDIATE CONCERN WAS THE ALTDEV AS I SAW THE ACFT APCHING FL400 AND I MADE A CALL TO CTR INDICATING OUR TROUBLES ASSOCIATED WITH THE STORM. ATC REPLIED THAT THERE WAS NO OTHER ACFT IN OUR VICINITY AND ASKED IF WE NEEDED ANY ASSISTANCE. DURING THE CLB I HEARD THE CHIMES ASSOCIATED WITH THE AUTOPLT BEING DISCONNECTED AND ALSO SAW OUR AIRSPD HOVERING RIGHT AT V MAX. I NOTICED THE CAPT WAS HAND FLYING AND HE INFORMED ME THAT HE WAS DOING HIS BEST TO GET THE AIRPLANE BACK DOWN TO FL390. WE PROBABLY EXPERIENCED 0 G'S FOR APPROX 4-5 SECONDS AND WERE THEN IN LIGHT TURB FOR A SIMILAR PERIOD. WE EVENTUALLY EXITED THE STORM CLOUD AND RETURNED TO NORMAL FLT AT FL390 WITHIN A MIN OF THE DEV. I NOTIFIED ATC OF THE TURB AND BUILDING STORM FOLLOWED BY A SHORT ANNOUNCEMENT TO THE PAX EXPLAINING THE REASON FOR THE TURB. THE CAPT CALLED THE FLT ATTENDANTS TO INQUIRE ABOUT THEIR CONDITION AND WAS INFORMED THAT ALL WAS OK AND OPS WERE NORMAL. THE CAPT AND I AGREED THAT ALTHOUGH IT WAS A PRECARIOUS SITUATION TO FIND OURSELVES IN; THERE WAS NO FURTHER ACTION REQUIRED.