<xsd:attribute name="type" use="required">
    <xsd:simpleType>
        <xsd:restriction base="xsd:string">
            <xsd:enumeration value="tornadoes" />
            <xsd:enumeration value="hail" />
            <xsd:enumeration value="damaging thunderstorm winds" />
            <xsd:enumeration value="extreme tornadoes" />
            <xsd:enumeration value="extreme hail" />
            <xsd:enumeration value="extreme thunderstorm winds" />
            <xsd:enumeration value="severe thunderstorms" />
            <xsd:enumeration value="extreme severe thunderstorms" />
        </xsd:restriction>
    </xsd:simpleType>
</xsd:attribute>