Date: 201510
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: IMC
Aircraft: Large Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Human factors: Communication Breakdown; Confusion; Situational Awareness
Second reporter function: Pilot Flying
Anomaly: Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew FLC Overrode Automation; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Clearance
Contributing factors: Airspace Structure; Chart Or Publication; Procedure; Weather
Primary problem: Procedure

Narrative: Enroute to ZZZ. ZZZ was reporting marginal weather (low ceilings/gusty cross-winds/ rain) with three possible easterly approaches to include the potential of Precision Radar Monitored approaches. We had thoroughly reviewed and briefed the RNAV arrival into ZZZ along with each of the approaches; to include CATEGORY II procedures. During [an] RNAV Arrival; we were in level flight at 24;000 feet; in VNAV Path mode. Just past [ZZZZZ Intersection]; ATC cleared us to 'descend via the [Arrival]'. The captain (flying) reset the MCP altitude window to 11;000 feet; the bottom altitude of the arrival at [ZZZZZ intersection] waypoint. We both verbally confirmed the altitude. Prior to the 'top of descent point' we requested a right deviation for weather which was approved by ATC. ATC also re-cleared us 'direct [ZZZZZ intersection] when able' but did not confirm an altitude for the off course deviation. Neither of us thought to ask for confirmation of the assigned altitude. Since 11;000 feet was still set in the MCP; and we were in VNAV path; the jet began a descent shortly after we began the weather deviation; abeam the TOD. I immediately understood the error and ask ATC for a lower altitude. ATC cleared us to 'descend to 13;000 feet' but that clearance wasn't received until we were descending through 23;600 feet. I believe the fact that we had not reached the 'top of descent point;' and still in level flight at the time of the deviation; contributed to our oversight in not prompting ATC to confirm the off course altitude assignment. Bottom line - any off course deviation / vector from an RNAV Arrival; whether descending or in level flight should be an immediate trigger for ATC and cockpit crews to confirm assigned altitudes. Lesson learned. ATC training would be helpful. It is not unusual to get an off course clearances during RNAV arrival / 'descend via' without an ATC altitude assignment. Obviously; all pilots must be alert for this oversight and insist on a specific altitude assignment. As noted above; I don't think we would have taken the bait IF we were actually descending at the time of the off course clearance. I have reset my habit pattern on this.


Second narrative: Any off course deviation/vector from an RNAV arrival; should trigger a query to ATC for altitude clearance confirmation.


Synopsis: An aircraft on an RNAV Arrival with published descents deviated for weather. The aircraft continued descending even though it was no longer on the published route.