Changes to the High Level and Medium Level Significant Weather (SIGWX) Forecasts

     In response to approved ICAO requirements, the World Area Forecast Centers (WAFCs) will enhance jet stream forecasts on all significant weather (SIGWX) forecasts. WAFC Washington, Aviation Weather Center, and WAFC London will add the vertical depth of the jet stream information to the jet element. This change will be implemented with the 00 UTC forecast valid November 25, 2004.

This jet forecast enhancement was presented and approved at the seventh meeting of the World Area Forecast System Study Group in September 2002.

Affected WMO Products

The jet vertical depth forecasts will be available on all WAFC Washington SIGWX high T4 charts and BUFR data. These additional forecast elements will also be available on the WAFC Washington North Atlantic SIGWX medium chart.

The affected WMO headers include:

Forecast Strategy

The jet stream flight level text will be placed along the wind barb shaft on the opposite side of the wind barb pennants and barbs. The vertical depths to the 80 knot wind field above and below the jet steam will be determined. This new forecast information will be added directly below the flight level text.

Two critical forecasts elements will be added. First, the vertical depth forecast above the jet maximum which will be depicted as "+XXX". Second, the vertical depth forecast below the jet maximum will be depicted as "-XXX".

EXAMPLE:

FL350

+300/-020

At this maximum jet wind speed the distance to the 80 knot wind field is 30,000 feet above flight level 35,000 feet and 2000 feet below flight level 35,000 feet. These values are altitude distance values and are not flight level values.

On rare occasions when jet streams cross from one hemisphere to the other, the wind flags and associated text placement will conform to the method used in the hemisphere containing the majority of the jet stream line.

The jet stream vertical depth forecast will adhere to these guidelines:

• Jet stream vertical depth forecasts will be added when the maximum speed is 120 knots or more.
• Jet stream height information should be placed at the maximum speed point only, normally at one point on each jet stream.
• When the jet stream is very long and there are several wind maxima, then each maxima should include forecasts of the vertical depth.

Wind change bars

Wind change bars occur at intervals of 20 knots from the spot of the maximum wind.

The jet stream vertical depth forecast is omitted when wind change bars are used.

Jet Stream Vertical Depth Depiction

Please contact: ncep.awc.avwx@noaa.gov for further information.

Reference:

MET/02-WP/18, CAeM-XII/Doc 18, 21/5/02

Montreal, 9 to 27 September 2002

A JOINT WAFC PROPOSAL FOR THE DEPICTION OF JET STREAM

INFORMATION ON SIGNIFICANT WEATHER CHARTS

(Presented by the United States and the United Kingdom)