Date: 201707
Local time of day: 1201-1800
Location: ORD.Airport, IL
Flight conditions: IMC
Weather elements / visibility: Thunderstorm; Windshear
Light: Daylight
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B ORD
ATC / advisory: TRACON C90
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Distraction; Human-Machine Interface
Second reporter function: Captain; Pilot Flying
Anomaly: Deviation - Altitude Overshoot; Inflight Event / Encounter Weather / Turbulence; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Regained Aircraft Control; Flight Crew FLC Overrode Automation; Flight Crew Executed Go Around / Missed Approach; Flight Crew Diverted
Contributing factors: Weather
Primary problem: Weather

Narrative: This event took place on a flight to ORD. The Captain was the PF. We acknowledged building weather down final when cleared the approach to 9L and switched to tower. We intercepted the glideslope and; at around 2500 ft MSL; encountered a sudden tailwind. With AP (autopilot) engaged; the nose pitched down; throttles went to idle; and the jet was accelerating. The Captain recognized this as a potential microburst situation and called a go-around. FO called the go-around and tower told us to track the localizer and maintain 3000 ft MSL. We maintained airspeed; went flaps 15; and maintained 3000 ft. Tower then directed a left turn to 040; which was a turn into the weather. We delayed the turn and focused on flying the aircraft; though in the process tracked slightly right of course to remain clear of the weather. Tower again gave us the left turn to 040 and we headed that way. Tower then directed a climb to 4000 ft. Some confusion ensued with our automation during this process; though positive aircraft control was always maintained by the Captain. HDG SEL would not initially engage; we believe; because the approach was still armed. Our FDs also did not assist in our climb to 4000 ft because; as memory serves; our command for VS on the MCP did not take. The Captain hand flew the jet but was unable to level off at 4000 ft due to a significant updraft. Both crew members repeatedly acknowledged the impending and actual climb through the assigned altitude and FO reported a 'significant updraft' with ATC. We believe that; at most; the jet reached 5000 ft. ATC responded with a quick; respectful; and deferential 'okay;' understanding that we were aviating first. At this point we were indeed still dealing with the effects of the weather and focused on controlling the aircraft. We finished cleaning up the aircraft and requested a heading to our divert airport. Either due to yoke inputs while flying through the weather; or due to automation difficulties; the Captain's AP would not engage for some time. The Captain told ATC that we were low on fuel and were unwilling 'to fly through that thunderstorm again.' After receiving our heading to ZZZ we noticed the Captain's FMC was frozen. The FO's FMC was functional and we are not sure when the left FMC froze. Our divert to ZZZ was otherwise uneventful; as was our delayed but successful flight to ORD later that evening.We are certain a go-around was the right decision and are happy we kept the jet at a safe altitude above the ground. We could have performed a cleaner go-around but were focused on flying the aircraft around the weather and not exceeding any speed limitations. Our impression was that ATC understood that we were busy dealing with significant weather. Their 'okay' had a tone of deference and understanding; and we doubt they will mark this as a pilot deviation. In hindsight; we believe the best decision would have been to not accept the approach clearance and ask for a safe heading away from the weather. We did not have situational awareness on whether traffic behind us was cleared the ILS to 9L; but we hope not.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737 flight crew reported a windshear encounter while on approach to ORD. They executed a missed approach and elected to divert due to the weather and their fuel situation.