PLTS OF AN SMT TWIN MOMENTARILY LOST CTL OF THE ACFT ALT DUE TO SEVERE TURB OF UP-AND DOWNDRAFTS DURING A NIGHT IFR FLT.

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ON THE NIGHT OF THE OCCURRENCE WE HAD BEEN GIVEN A VECTOR TO THE SALEM ARPT FROM THE TROUTDALE ARPT. THE FREEZING LEVEL WAS RPTED TO BE AT 7000 FT OR 8000 FT. OUR ENTIRE TRIP FROM TROUTDALE AND BEFORE HAD BEEN SOLID IMC WITH NO INDICATION OF TURB. CHKING WX PRIOR TO THE FLT REVEALED NO CONVECTIVE ACTIVITY IN THE AREA AND NONE FORECAST. SHORTLY BEFORE THE OCCURRENCE; CTR CLRED US TO DSND FROM 5000 FT TO 3000 FT. AT 4500 FT MSL WE ENCOUNTERED HVY RAIN AND A SUDDEN UPDRAFT WHICH OUR VSI INDICATED WE WERE ASCENDING AT +3000 FPM. WITH PWR BACK AND 1 NOTCH OF FLAPS WE ASCENDED WITH THE VSI PEGGED OUT AT A 3000 FPM CLB FOR 20-30 SECONDS. NEXT WE WERE IN A DOWNDRAFT -3000 FPM INDICATED ON THE VSI. THE DOWNDRAFT LASTED APPROX 20- 30 SECONDS ALSO. 2 MI FROM SALEM WE WERE ABLE TO DSND TO 3100 FT; WE BROKE OUT OF THE CLOUDS AND WERE CLRED FOR A VISUAL APCH. ATIS INFO RECEIVED BEFORE THE OCCURRENCE SHOWED WINDS 190 DEGS AT 14 KTS; 10 MI VISIBILITY. WHEN WE WERE CLRED FOR THE VISUAL TWR ADVISED WINDS 270 DEGS GUSTING 24-30 KTS; FLT VISIBILITY IN HVY RAIN WAS 2-3 MI. THROUGHOUT THE OCCURRENCE WE ENCOUNTERED HVY TURB WITH AIRSPD FLUCTUATIONS FROM 20-30 KTS. BEFORE LNDG WINDS ON THE SURFACE SWUNG TO APPROX 290 DEGS 24 KTS. A CLOSER TRACK OF WX AND THE MOVEMENT OF COLD FRONT COULD HAVE GIVEN US AN INDICATION OF WHAT TO EXPECT. A COLD FRONT HAD BEEN RPTED MOVING THROUGH THE AREA AT THE SAME TIME. WX RADAR ON BOARD DID NOT INDICATE THE HVY RAIN UNTIL WE WERE IN IT AND GAVE NO WARNING OF THE CELL OR MICROBURST WE FLEW THROUGH. 2 OTHER ACFT ON IFR CLRNCS DID NOT SEEM TO BE EXPERIENCING THE SAME CONDITIONS. VARIATIONS IN ALT/AIRSPD RPTED TO CTR.