CORP ACFT ENCOUNTERS SEVERE WX. ALTDEV AND LOSS OF NAV EQUIP RESULT.

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ASSIGNED TO FLY THE ACFT TO SAN MARCOS; TX; FOR A POSSIBLE SALE I CONDUCTED A PREFLT AND WX BRIEFING BEING ADVISED THAT ONLY LEVEL 2 SHOWERS WERE RPTED IN EASTERN PA. AFTER WHICH THE REMAINDER OF THE FLT WAS VFR. IN THE VICINITY OF MXE VOR. I ENCOUNTER SEVERE TURB; HVY RAIN; MAKING IT VIRTUALLY IMPOSSIBLE TO MAINTAIN ALT AND DIRECTIONAL CTL. I REQUESTED A DEV FROM DEP BUT DUE TO WX CONDITIONS IN AREA AND ACFT HOLDING THEY WERE UNABLE TO ACCOMMODATE THE REQUEST. DURING THIS ENCOUNTER I ALSO LOST NAV #1. WHICH CAUSED ME TO DEVIATE FROM MY CLRNC AS I HAD ALL I COULD DO TO KEEP THE ACFT SOMEWHAT UNDER CTL. I WAS FINALLY CLRED TO DEVIATE TO THE N STILL EXPERIENCING SEVERE TURB AND HVY RAIN; I OVERHEARD A COMMUTER ACFT TALKING WITH HARRISBURG APCH RPTING LOSS OF ALT OF 600 FT DUE TO TURB. I WAS AWARE AFTER THE FACT THAT MY COURSE FROM MXE VOR WAS OFF DUE TO NAV #1 OUTAGE AND VIOLENT WX CONDITIONS. I FEEL DURING MY PREFLT BRIEFING THE BRIEFER COULD HAVE OR SHOULD HAVE KNOWN OF THESE VIOLENT CONDITIONS.