A Fractional pilot reported severe turbulence while in cruise resulted in a temporary loss of control.

Narrative

I was serving as the Second-in-Command and pilot monitoring on a flight from Chicago Executive Airport (PWK) in Pewaukee; Illinois; to ZZZ. Prior to departure from PWK; we were advised of an active ground stop for all eastbound traffic due to severe thunderstorms in the area. With passengers already on board; we remained on the ground for approximately 1.5 to 2 hours before receiving a clearance that routed us around the convective activity.We departed PWK en route to ZZZ. While in cruise flight at FL450; I observed an echo on the radar directly ahead; with a red core indicative of intense precipitation. Simultaneously; ATC advised us of significant weather in our flight path and inquired whether we wished to deviate. After conferring with the Captain; we requested a deviation to the left in an effort to avoid the weather cell. Unfortunately; the turn was initiated too late; and we inadvertently entered the storm cell.Upon entering; we encountered heavy precipitation and moderate to severe turbulence. The turbulence disrupted the aircraft's flight path; and a stall warning was annunciated on the Primary Flight Display (PFD); resulting in autopilot disengagement. The Captain instructed me to request FL430 in an attempt to reach denser air and regain stability. I contacted ATC to request FL430; but it was denied due to traffic separation.During this time; we experienced a deviation of approximately 500 feet below our assigned altitude. ATC promptly advised us to maintain FL450. I informed ATC that the altitude deviation was a result of moderate to severe turbulence and confirmed that we had exited the weather cell and were able to re-establish level flight at FL450. ATC was professional and empathetic; asking if there were any injuries or aircraft damage. I checked on the passengers and conducted a brief cabin inspection. I reported to ATC that there were no injuries and no visible damage to the aircraft.It is important to note that; following our communication with ATC; there was no indication that a deviation report would be filed. However; given the stall warning and temporary altitude deviation; I am submitting this report in the interest of transparency and safety.