National Volcano Early Warning and Monitoring System Act (Sec. 3) This bill directs the United States Geological Survey (USGS) to establish the National Volcano Early Warning and Monitoring System to monitor, issue warnings of, and protect U.S. citizens from undue and avoidable harm from, volcanic activity. The purposes of the system are to: (1) organize, modernize, standardize, and stabilize the monitoring systems of U.S. volcano observatories; and (2) unify such systems into a single interoperative system. The objective of the system is to monitor all U.S. volcanoes at a level commensurate with the threat posed by the volcanoes by: (1) upgrading existing networks on monitored volcanoes, (2) installing new networks on unmonitored volcanoes, and (3) employing geodetic and other components when applicable. The system shall include: (1) a national volcano watch office that is operational 24 hours a day and 7 days a week, (2) a national volcano data center, (3) an external grants program to support research in volcano monitoring science and technology, and (4) modernization activities including the comprehensive application of emerging technologies. The USGS must: (1) submit to Congress a five-year management plan for establishing and operating the system, (2) establish an advisory committee to assist in implementing the system, and (3) report to Congress annually describing the activities carried out under this bill. The USGS may enter into cooperative agreements designating institutions of higher education and state agencies as volcano observatory partners for the system. (Sec. 4) The bill authorizes appropriations for FY2019-FY2023.