8.3 PIREPs, AIRMET, SIGMET, Convective SIGMET, and Winds Aloft

ACS PA.I.C.K2a includes PIREPs. PA.I.C.K2g is inflight advisories: AIRMET, SIGMET, and Convective SIGMET. PA.I.C.K2e is the winds and temperatures aloft forecast (FB). These products sit between the airport (METAR/TAF) and the route (GFA).

PIREPs: UA versus UUA

A PIREP is a pilot weather report. UA is routine. UUA is urgent — the kinds of conditions AIM and the Aviation Weather Handbook treat as a hazard to all aircraft: tornado, funnel cloud, or waterspout; severe or extreme turbulence; severe icing; hail; low-level wind shear; volcanic ash; and other urgent items. ATC facilities solicit PIREPs when ceilings are at or below 5,000 feet, visibility is at or below 5 miles, and when thunderstorms, icing, turbulence, and similar hazards are reported or forecast. File good weather as well as bad. A “smooth, no ice” UA at your altitude is a planning fact.

Example: UA /OV OKC090025 /TM 1522 /FL080 /TP C172 /SK BKN030-TOP045 /WX FV03SM RA /TA M04 /WV 27025KT /TB LGT /IC LGT RIME /RM LLWS 015

Routine PIREP at 1522Z, 25 NM east of OKC, 8,000 feet, Cessna 172, broken 3,000 tops 4,500, flight visibility 3 SM in rain, −4 °C, wind 270°/25, light turbulence, light rime, low-level wind shear reported at 1,500. That /IC and /RM are why Maya does not climb through the layer “to see.” Temperature is already below freezing in cloud.

AIRMET Sierra, Tango, Zulu — and G-AIRMET

An AIRMET describes en-route weather that may affect safety at intensities below SIGMET criteria. AIM 7-1-6: AIRMETs are for all pilots, especially VFR pilots and operators of sensitive aircraft. CONUS AIRMETs are issued on a six-hour schedule (0245, 0845, 1445, 2045 UTC) and updated as needed. Alaska is every eight hours; Hawaii every six. Valid times run out to 12 hours on the graphical product in 3-hour snapshots (00, 03, 06, 09, 12), with optional extra hours in the first six.

The three hazard families are still how PAR names the product:

• Sierra — IFR (ceiling less than 1,000 feet and/or surface visibility less than 3 SM) and/or mountain obscuration.
• Tango — moderate turbulence; sustained surface winds of 30 knots or greater; nonconvective low-level wind shear (AIM: potential below 2,000 feet AGL).
• Zulu — moderate icing and freezing-level heights.

Memorize those three lines. Sierra is not turbulence. Tango is not ice. Zulu is not IFR. Mixing the names is one of the cheapest ways to lose a weather-products item.

Over the contiguous U.S., the official advisory is the G-AIRMET (graphical AIRMET) on aviationweather.gov, 1800WXBRIEF, and FIS-B. The legacy textual TAC AIRMET for CONUS was retired in January 2025. Alaska and Hawaii still issue text AIRMETs that still say SIERRA, TANGO, and ZULU. A briefing that leaves CONUS for Alaska or Hawaii will include both G-AIRMETs and text AIRMETs for the appropriate legs. G-AIRMET snapshots are valid at the plotted hour. If the 00-hour chart is clean and the 03-hour chart shows the hazard, AIM says assume the hazard develops in between unless a start/stop time is defined.

SIGMET and Convective SIGMET

A SIGMET is for phenomena expected to affect the safety of all aircraft. Nonconvective SIGMETs over the contiguous U.S. are issued for:

1. Severe icing not associated with thunderstorms.
2. Severe or extreme turbulence or CAT not associated with thunderstorms.
3. Widespread dust or sand lowering surface visibility below 3 miles.
4. Volcanic ash.

They are unscheduled, normally valid 4 hours (6 hours for tropical cyclones and volcanic ash), and use letter names from November through Yankee excluding Sierra and Tango (those letters belong to AIRMETs). Alaska and some oceanic SIGMETs also cover tornadoes, lines of thunderstorms, embedded thunderstorms, and hail ≥ 3/4 inch — because Convective SIGMETs are a CONUS product.

Convective SIGMETs (AIM 7-1-6) are issued in the conterminous U.S. for any of:

• Severe thunderstorm due to surface winds ≥ 50 knots, hail ≥ 3/4 inch, or tornadoes.
• Embedded thunderstorms.
• A line of thunderstorms.
• Thunderstorms producing heavy or greater precipitation affecting 40 percent or more of an area of at least 3,000 square miles.

Any Convective SIGMET implies severe or greater turbulence, severe icing, and low-level wind shear. Bulletins are issued for the western (W), central (C), and eastern (E) U.S. hourly at H+55, valid up to 2 hours. Specials can go out at any time. If none of the criteria are met, the area gets CONVECTIVE SIGMET...NONE. They are not issued for Alaska or Hawaii. Do not treat a Convective SIGMET as “just a thunderstorm watch.” It is the inflight convective advisory. The Convective Outlook told you the day might cook; the Convective SIGMET says the storm is here or about to be.

A Center Weather Advisory (CWA) is a short-fuse nowcast (generally conditions beginning within about two hours). It is not a flight-planning product. Use it in flight or just before departure, not as yesterday’s briefing.

Winds and temperatures aloft (FB)

The FB winds-and-temperatures-aloft forecast is coded true direction and knots, temperatures in Celsius. Levels are typically 3,000, 6,000, 9,000, 12,000, 18,000, 24,000, 30,000, 34,000, and 39,000 feet (no 3,000-foot group when that level is too close to the surface). A four-digit group is wind only; six digits add temperature.

• 2412+10 — wind 240° at 12 knots, temperature +10 °C.
• 9900 — wind light and variable (forecast less than 5 knots).
• Above 24,000 feet, temperatures are below freezing and the minus sign is omitted. 295537 at 30,000 is 290° at 55 knots, −37 °C, not +37.
• Winds 100 knots or more: 50 is added to the coded direction and 100 is subtracted from the coded speed. 731960 decodes as direction 73 − 50 = 230°, speed 19 + 100 = 119 knots, temperature −60 °C.

Interpolate between levels and stations for your cruise altitude. Direction is true, not magnetic — convert before you draw the wind triangle.

Scenario: three products, one Saturday

Jordan’s G-AIRMET Sierra covers the ridges along his route (mountain obscuration). That is an IFR/obscuration problem, not an ice problem. Maya sees a G-AIRMET Zulu with freezing level at 8,000 and a UA reporting light rime at 8,000 in cloud — that is known icing for a typical training airplane, even though it is “only” an AIRMET. A Convective SIGMET 12C for a line of thunderstorms is not something either of them threads using cockpit NEXRAD. They land or deviate while the line is still a line.