Hazardous Inflight Weather Advisory Service (HIWAS) is a continuous broadcast of hazardous weather information which is transmitted over selected VORs. This hazardous weather includes AIRMETs, SIGMETs, Convective SIGMETs, Center Weather Advisories (CWAs), Severe Alert Weather Watches (AWWs), and urgent PIREPs.
The presence of HIWAS information on a VOR is indicated on a sectional or terminal area chart by an "H" in the upper-right corner of the box surrounding the NAVAID frequency.
References
Hazardous Inflight Weather Advisory Service Wikipedia(Text) CC BY-SA