On 18 March, AVO lowered the Alert Level for Redoubt to Advisory and Aviation Color Code to Yellow because seismicity declined to levels recorded prior to the ash emission on 15 March. Seismicity remained low the next day. Shallow earthquake activity was noted again on 20 March. On 21 March, a steam plume rose to an altitude of 3.4 km (11,200 ft) a.s.l. Later that day, the rate of seismic events continued to increase, prompting AVO to raise the Alert Level to Watch and the Aviation Color Code to Orange. On 22 March, an explosive eruption began at 2238, prompting AVO to raise the Alert Level to Warning and the Aviation Color Code to Red, the highest levels. Four more explosive eruptions occurred, with each lasting between 4-30 minutes. Ash plumes rose to an altitude of 18.2 km (60,000 ft) a.s.l., with the bulk of the ash volume between 7.6-9.1 km (25,000-30,000 ft) a.s.l. Ashfall was reported in areas 190-250 km NE. The last explosion ended at 0500 on 23 March. No ash was detected afterwards utilizing radar data, suggesting that if ash emissions were occurring, plumes would be below approximately 4 km (13,000 ft) a.s.l. and/or contain particles too fine to be detected. Poor weather hindered visual observations of the volcano. The sixth explosion began on 23 March at 1941, lasted about 17 minutes, and produced an ash plume that rose to an altitude of 15.2 km (50,000 ft) a.s.l. and drifted W and N. Pyroclastic flows were visible traveling down the N flank in web camera images. According to news articles, Alaska Airlines cancelled 19 flights on 23 March due to ash plumes.
On 23 March, AVO staff visited areas around Redoubt and saw large lahar and flood deposits in the Drift River valley. The eruptions caused melting of the Drift Glacier and greatly increased discharge down the Drift River, causing lahars that traveled more than 35 km, reaching the Cook Inlet. In the middle to upper Drift River valley, high-water marks reached 6-8 m above the valley floor. At the AVO hut (roughly 11 km NNW of the summit), a 6-cm-thick fall deposit was observed. On 24 March, AVO reported that a steam plume rose to an altitude of 6.1 km (20,000 ft) and drifted about 65 km NW.
Sources: Alaska Volcano Observatory (AVO), The Associated Press – “Reports provided courtesy of the Smithsonian’s Global Volcanism Program and the US Geological Survey’s Volcano Hazards Program.”