Date: 201307
Local time of day: 0001-0600
Location: ZZZ.ARTCC, US
Flight conditions: IMC
Weather elements / visibility: Thunderstorm; Turbulence
Light: Night
Aircraft: EMB ERJ 145 ER/LR
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: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Time Pressure; Fatigue; Distraction; Workload
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural MEL / CDL; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Diverted; Flight Crew Took Evasive Action; General Flight Cancelled / Delayed
Contributing factors: Company Policy; Aircraft; Weather
Primary problem: Ambiguous

Narrative: Our flight was already running late from the first leg that afternoon so we had been playing catch up all day long but inevitably left delayed on this last leg; over an hour late after sunset. With the autopilot MEL'ed (the aircraft prohibited with RVSM and CAT II) and new routing to our overnight destination due to weather and having a distant airport as ALT; we already had several threats prior to departure. Toward the middle of the flight we were once again rerouted by ATC direct to our destination which placed us in a good position to enter through gap in a line of active thunderstorms that had stayed on the right of our course. We decided to fly through that narrowing gap in the line of TSRA but ATC queried us if we could climb since a previous aircraft flew above the weather at around FL350 just north of the line in front of us and did OK. I responded we were unable due to our RVSM limitation.  ATC said to standby as he made a phone call. He then came back said that we can climb if we want to FL330 to which we said yes and we then executed a good rate of climb. ATC made a note that he had to declare an emergency to allow us to climb beyond our RVSM limitation and I thanked him for that. At this point; we climbed to FL330 while on northeasterly heading as we crossed that opening in the line of TSRA having occasional light to moderate turbulence with little to no precipitation. The ATIS for our destination still showed VFR WX at 060BKN; 140/12 KTS for [landing] runway.  By the time we cleared the line of TSRA we turned left on a northerly heading but was less than 30 miles south of our destination still at FL330. We then requested to go lower to which we had pilot's discretion to 10;000 FT while now talking to Approach who said we can expect ILS. We kept descending and were still at FL250 less than 10 miles south of the airport on an approximately 10 mile downwind. This time; the cells started to show up drastically and were continually worsening to a Level 4 around the IAF of ILS to the right of us and also around the FAF. We turned left to a northwesterly heading to avoid that cell while at this time we leveled off at FL180 beginning to concentrate on TSRA avoidance and discussing our options since the line of TSRA we just successfully crossed behind us was closing in on the our path going toward our left and our front. All this time; the frequency of lightning flashes [became] very close and intensied and the ride became more and more turbulent most of which severe as we were experiencing moderate to heavy precipitation.  My First Officer who has been hand flying the aircraft all this time verbalized he is near his limits due to the extreme conditions that are unraveling and quickly deteriorating around us. At the same time Approach just gave the weather which was gusty winds 26-27 KTS and heavy rain on the field. We then agreed it is time to divert to our filed alternate but the left turn we took to avoid the cell in the inbound final and hopeful [to] cut across a very narrow gap through marching line of TSRA was closing up and [was] taking us to another cell. So we made a [turn to a] more northerly heading to stay away from the line of TSRA to our left and from the cells popping along the approach to our right. Approach turned us over to our destination Approach after we declared our intent to divert but we were then cautioned that we would need to fly over 100 miles north to go around the line of TSRA before coming back south to the airport. The only available opening toward our alternate [was] behind us now and had become virtually impassable. At this time the line extended from around south through northeast for hundreds of miles. We were suggested an airport further east [that] was a good airport which I knew was the next closer airport to the northeast; so we decided to divert as it was the closest most suitable airport. We were then turned over to Center who vectored us to the divert airport. We prepared forthe approach and landed with no issues. All this time I was communicating with Dispatch on our decision to divert; who initially questioned why we went to the divert airport instead of our filed alternate. After conversing with our Dispatcher later; he realized after looking at the radar it was the prudent decision due to the extensive length of the line of TSRA trailing from the northwest to southeast out of the locations mentioned above. The divert airport ground personnel graciously assisted us as well as they could considering we were not expected at their station. We blocked in shortly before midnight and were immediately in contact with Dispatch. We felt that it was not safe to try to fly back to our original destination even after 30 min due to the continually persistent unfavorable weather radar outlook which [was] trailing from the northwest into the southeast with our destination in the center. At around an hour after midnight we decided to not fly back to our filed destination after observing the WX for the final time before a final go/no go decision was to be made. Although the field was already VFR; the radar showed the same trend but with some improvement. We did not feel safe completing the flight because it was the same VFR WX conditions that we saw back then before got stuck in the middle of weather and diverted. The radar continued to show a trailing line of WX from the northwest to the southeast with our filed destination still in the center. In lieu of the threats listed below; we felt the safer and conservative option was to stay in the ground and try it again tomorrow. Due to multiple threats of ensuing fatigue; uncontrolled destination airport; persistent bad weather; MEL'ed aircraft equipment that required hand flying; night conditions that made it difficult to see WX other than via the radar; and the time being late in the wee hours of the morning while almost 13 hours into our duty day; we decided that it was the safe decision to stay on the ground and not fly to our filed destination. If we stuck to our altitude of FL270 and rejected ATC's offer to climb to FL330; we would have been low enough to fly the approach without having to waste too much time descending to a lower altitude. But in hindsight; climbing seemed to best option available to us at that time due to the rapidly deteriorating weather.Realizing how potentially overloaded the flying pilot [was] when hand flying the aircraft will help me anticipate how overseeing and managing the flight; while monitoring him/her can sometimes be overwhelming and distracting from setting priorities. Therefore; a more conscious proactive management of the flight is essential as well as eliminating the applicable threats as early as possible. If a dispatcher had more time to overlook the flight; some inputs on the severity of the cells behind the line of TSRA could have helped us make the decision to hold and maybe divert before penetrating that opening and being subjected to the deleterious effects of bad weather. Ultimately; a timely and earlier decision on my part not to cross the opening and just hold in front and away from it while brainstorming what to do with the help of ATC and Dispatch may have saved us from having to use emergency authority of diverting.


Second narrative: A very large threat in my mind is the fact that the aircraft had an MEL'ed autopilot. That increased our workload tremendously and made it more difficult for me to stay in the decision making process and assist the CA in evaluating our situation. During a few times while maneuvering the weather; altitude; as well as course deviated slightly due to the moderate turbulence experienced as well as the increasing intensity of the storms cells that were growing. I am very proud of how the Captain and I worked together to get through this situation. I think we were put in a difficult situation in being placed in an aircraft with an inoperative piece of equipment when even on a severe clear weather day can be tiring for the crew to operate. We operated the flight professionally and feel that we made good decisions in the safe operation of the flight.


Synopsis: An EMB-145 was dispatched with an inoperative autopilot and encountered severe weather which required flight into RVSM airspace and ultimately a diversion to an airport clear of the thunderstorms.