HIWAS

HIWAS (Hazardous In-Flight Weather Advisory Service) is a continuous broadcast of in-flight weather advisories including AIRMETs (WA), SIGMETS (WS), Convective SIGMETS (WST), Center Weather Advisories (CWA), and Urgent PIREPS (UUA). Pilots can tune in to selected VOR stations that broadcast HIWAS. These stations are identified on a VFR Sectional Chart by a letter H located in the upper righthand corner of the VOR identification box (see picture above).

AIRMETs (abbreviated "WA") are advisories of significant weather phenomena that describe hazardous conditions at intensities lower than those which require the issuance of SIGMETs. AIRMET bulletins are issued every 6 hours beginning at 0245 UTC. There are 3 different types of AIRMETS: Sierra, Tango, and Zulu. AIRMET Sierra describes IFR conditions and/or extensive mountain obscurations. AIRMET Tango describes moderate turbulence, sustained surface winds of 30 knots or greater, and/or non-convective low-level wind shear. AIRMET Zulu describes moderate icing and provides freezing level heights.

A SIGMET advises of non-convective weather that is potentially hazardous to all aircraft. The maximum forecast period is four hours, or six hours in conditions associated with hurricanes. In the coterminous United States, SIGMETS are issued when the following phenomena occur or are expected to occur: severe icing not related to a thunderstorm; severe or extreme turbulence or clear air turbulence (CAT) not associated with thunderstorms, dust storms or sandstorms lowering surface or in-flight visibilities to below 3 miles, and volcanic ash.

Convective SIGMETs are issued hourly (beginning at H+55) for the following thunderstorm-related hazards: 

  • severe or greater turbulence

  • low-level wind shear

  • surface winds greater than or equal to 50 knots

  • hail greater than or equal to 3/4 inches in diameter

  • tornados

  • embedded thunderstorms

  • lines of thunderstorms

  • thunderstorms greater than or equal to VIP level 4 (colored yellow on radar charts) that affect 40% or more of an area of at least 3000 square miles

  • severe icing 

A Center Weather Advisory (CWA) is an aviation warning for use by aircrews to anticipate and avoid adverse weather conditions falling just below severe criteria in the en route and terminal environments. These include thunderstorms, turbulence, icing, and IFR conditions. A CWA is not a flight planning product; instead, it reflects current conditions expected at the time of issuance, and/or is a short-range forecast for conditions expected to begin within 2 hours from that time. CWAs are valid for a maximum of 2 hours. If conditions are expected to continue beyond that period, a statement will be included in the CWA.