How long is a Center Weather Advisory (CWA) valid?

A CWA is valid for a maximum of two hours from the time of issuance. If the hazardous condition persists beyond that window, the CWSU meteorologist may issue a follow-on CWA to maintain coverage. This two-hour valid period is significantly shorter than AIRMETs (up to six hours) or SIGMETs (up to four or six hours depending on the phenomenon).

Who issues Center Weather Advisories and where?

CWAs are issued by National Weather Service meteorologists assigned to a Center Weather Service Unit (CWSU), which is physically embedded within an Air Route Traffic Control Center (ARTCC). Each major ARTCC has its own CWSU, and the advisories cover only that ARTCC's area of jurisdiction. This is different from AIRMETs and SIGMETs, which are issued nationally by the Aviation Weather Center in Kansas City.

Does a Center Weather Advisory cancel or replace an AIRMET or SIGMET?

No. A CWA does not cancel, supersede, or replace any existing AIRMET or SIGMET. Both the national product and the CWA remain independently in effect at the same time. CWAs are a local supplement that adds timeliness and geographic precision for conditions that may not yet be reflected in — or may be more intense than — current national products. Pilots should review all applicable advisories together.

Center Weather Advisories (CWA) and the CWSU Role

When weather deteriorates faster than scheduled products can capture, pilots and air traffic controllers need an immediate, authoritative alert. That is the role of the Center Weather Advisory (CWA) and the specialists who issue them — the meteorologists of the Center Weather Service Unit (CWSU). Understanding both the product and the organization behind it is essential for any instrument-rated pilot or anyone planning operations in complex airspace.

CWAs occupy a unique niche in the family of inflight advisories. Unlike AIRMETs and SIGMETs, which originate at the Aviation Weather Center (AWC) in Kansas City and cover broad geographic regions, CWAs are produced locally, inside an Air Route Traffic Control Center (ARTCC), and are targeted at specific, immediate hazards affecting traffic in that ARTCC's area of jurisdiction. They are not a replacement for SIGMETs or AIRMETs — they are a real-time supplement that bridges the gap between national products and the rapidly evolving weather picture on a controller's radar scope.

The Center Weather Service Unit (CWSU)

Each major ARTCC in the United States hosts an embedded CWSU staffed by National Weather Service (NWS) meteorologists. These specialists work side-by-side with air traffic controllers, giving them an unparalleled view of both the radar picture and the flow of traffic. The close physical and operational relationship between CWSU meteorologists and ATC personnel is what makes the CWA so timely and tactically relevant.

The primary responsibilities of the CWSU are to support the safe and efficient flow of air traffic by providing weather guidance to ARTCC personnel, issuing CWAs when warranted, and preparing Meteorological Impact Statements (MIS) — planning forecasts that help traffic management units anticipate weather-related constraints hours in advance. The MIS is a collaborative, internal planning tool, while the CWA is the urgent, public-facing advisory product.

What Is a Center Weather Advisory?

A CWA is an unscheduled, short-fuse aviation weather warning issued for conditions that are occurring, or expected to occur, within the next two hours. It is designed to alert pilots and air traffic personnel to meteorological conditions that may not yet be reflected in current AIRMETs or SIGMETs, or that are more intense or geographically specific than those broader products convey.

CWAs are issued in plain-language text and are valid for a maximum of two hours. If conditions persist, follow-on CWAs can be issued. The advisory covers the ARTCC's area of jurisdiction and may address both en route and terminal weather hazards within that area. Because they are locally issued, CWAs can be extremely precise — describing a developing line of thunderstorms pushing through a specific fix, or rapidly building icing conditions along a commonly used arrival corridor.

When Are CWAs Issued?

A CWSU meteorologist issues a CWA when one or more of the following conditions exist or are anticipated within the ARTCC area during the next two hours:

Thunderstorms: Moderate or greater turbulence associated with thunderstorms, or any thunderstorm condition that is not adequately described by an existing convective SIGMET or other national product.
Icing: Moderate or severe icing not already covered by an AIRMET or SIGMET.
Turbulence: Moderate or severe turbulence not already covered by an AIRMET or SIGMET.
Low IFR conditions: Ceilings below 1,000 feet and/or visibility below 3 statute miles affecting a significant portion of the ARTCC area.
Surface winds or gusts: Surface winds or gusts of 30 knots or greater affecting a significant number of aircraft operations.
Any other condition the CWSU meteorologist determines is significant enough to warrant immediate notification to pilots and controllers — for example, a rapidly developing weather system or a localized phenomenon that national products have not yet captured.

The key operational trigger is imminence. A CWA is not a planning tool — it is a right now or within two hours product. If the event is forecast beyond that window, the MIS is the more appropriate instrument.

CWA Format

CWAs follow a standardized format so that both controllers and pilots can quickly parse the critical information. A typical CWA includes:

Issuing ARTCC identifier — the three-letter identifier of the center (e.g., ZDC for Washington ARTCC).
Advisory number — a sequential number for that ARTCC for the day, allowing easy reference and tracking.
Valid time — stated in UTC, covering the period of concern up to two hours from issuance.
Affected area — described in terms of fixes, airways, or coordinates within the ARTCC's jurisdiction.
Phenomenon — described in plain language, including intensity, altitude, and movement where applicable.
Trend — whether conditions are expected to intensify, weaken, or remain steady.

An example CWA might read: ZDC CWA 01 VALID UNTIL 2200Z. FROM HCM TO SBY TO ORF, AREA OF MODERATE TO OCCASIONALLY SEVERE TURB REPORTED BTN FL180 AND FL280. CONDITIONS CONTG AND MOVG NE 25KT. This compact format gives a pilot or controller everything needed to make an immediate routing or altitude decision.

CWAs vs. AIRMETs and SIGMETs

Understanding where the CWA fits relative to other inflight advisories prevents confusion on the flight deck and on knowledge tests.

SIGMETs are issued by the AWC and cover severe or extreme hazards (severe icing, severe turbulence, volcanic ash, tropical cyclones) over large geographic areas, with valid periods up to four hours (six hours for hurricane-related SIGMETs).
Convective SIGMETs are issued by the AWC for convective hazards meeting specific intensity thresholds, on a scheduled and unscheduled basis, and are valid for up to two hours.
AIRMETs are issued by the AWC on a six-hour schedule and cover moderate hazards (moderate icing, moderate turbulence, IFR conditions, mountain obscuration, non-convective low-level wind shear) across broad regions.
CWAs are locally issued, valid for two hours or less, and can describe hazards that fall below SIGMET thresholds but are still operationally significant — or that represent emerging conditions not yet reflected in national products.

A critical distinction: a CWA does not supersede or cancel an AIRMET or SIGMET. Both products coexist. The CWA adds local precision and timeliness to the national picture. Pilots should review all applicable products — SIGMETs, AIRMETs, convective SIGMETs, and CWAs — when evaluating weather along a route.

Operational Significance for Pilots

CWAs are available through standard preflight and inflight weather channels. Pilots can access them through weather briefings from Flight Service, through cockpit weather data link systems, and via ATC. Because CWSU meteorologists work directly with controllers, ATC may proactively relay CWA information to pilots as part of a weather advisory during flight. This integration is especially valuable in high-density airspace where routing decisions are being made on compressed timescales.

For instrument-rated pilots planning IFR flights within or across an ARTCC's area, checking for active CWAs during preflight is a best practice. They represent the most current, locally aware weather intelligence available and may highlight a hazard that has emerged since the last scheduled AIRMET issuance. In-flight, if ATC mentions a CWA, pilots should treat that information as actionable and reassess routing, altitude, or holding options accordingly.

Key Numbers and Rules

Maximum valid period: 2 hours. Follow-on CWAs may be issued if conditions persist.
Issuing authority: CWSU meteorologists embedded within an ARTCC facility.
Coverage area: Limited to the issuing ARTCC's area of jurisdiction.
Icing threshold for issuance: Moderate or greater icing not covered by existing national products.
Turbulence threshold: Moderate or greater turbulence not covered by existing national products.
Ceiling/visibility threshold: Ceilings below 1,000 feet and/or visibility below 3 statute miles affecting a significant portion of the ARTCC area.
Surface winds threshold: Surface winds or gusts of 30 knots or greater affecting a significant number of aircraft operations.
CWAs do NOT replace SIGMETs or AIRMETs — all applicable products remain in effect simultaneously.

Common Test Traps

Confusing valid periods: Students often mix up the two-hour CWA valid period with AIRMET (six hours) or SIGMET (four hours for most phenomena, six hours for volcanic ash and tropical cyclones) valid periods. CWAs are the shortest-duration advisory product.
Assuming CWAs replace SIGMETs or AIRMETs: A CWA does not cancel or supersede any existing national product. All applicable advisories remain independently valid.
Confusing MIS with CWA: The Meteorological Impact Statement is a planning product for ATC traffic flow purposes, not a pilot advisory. The CWA is the public-facing product pilots need to monitor.
Misidentifying the issuing authority: SIGMETs and AIRMETs come from the AWC in Kansas City. CWAs are issued by the CWSU inside the local ARTCC — a completely different facility and organizational structure.
Overlooking CWAs in preflight planning: Because CWAs are not part of the standard scheduled-product cycle that students learn first, it is easy to omit checking them. Any comprehensive weather briefing should include current CWAs for all ARTCCs along the planned route.