The TWEB (Transcribed Weather Broadcasts) route forecast is similar to the Area Forecast (FA) except information is contained in a route format. Forecast sky cover (height and amount of cloud bases), cloud tops, visibility (including vertical visibility), weather, and obstructions to vision are described for a corridor 25 miles either side of the route. Cloud bases and tops are always Mean Sea Level (MSL) unless noted. Ceilings are always above ground level (AGL).
The Synopsis is a brief statement of frontal and pressure systems affecting the route during the forecast valid period.
The TWEB route forecasts are prepared by National Weather Service Forecast Offices (WFOs) for more than 300 selected short-leg and cross-country routes over the contiguous U.S. WFOs prepare synopses for the routes in their areas.
The TWEB route forecasts and synopses are issued by the WFOs three times per day. Route forecasts are valid for 12 hours. This schedule provides 24-hour coverage with most frequent updating during the hours of greatest aviation activity.
This is an example for a TWEB synopsis:
BIS SYNS 251402. LO PRES TROF MVG ACRS ND TDA AND TNGT.
HI PRES MVG SEWD FM CANADA INTO NWRN ND BY TNGT AND OVR
MST OF ND BY WED MRNG.
The above message decodes as follows:
The rest of messages decodes as follows:
Low pressure trough moving across North Dakota today and tonight. High pressure moving southeastward from Canada into northwestern North Dakota by tonight and over most of North Dakota by Wednesday morning.
Examples of TWEB route forecasts:
249 TWEB 131102 ISN-MOT-GFK. ALL HGTS AGL XCP TOPS.
ISN-DVL TIL 16Z CIGS BLO 10 VSBYS BLO 1F...AFT 16Z CIGS
10-20 VSBYS 3-5F. DVL-GFK TIL 16Z CLR VSBYS 3-5F...AREAS
CIGS BLO 10 VSBYS AOB 1F...16-00Z 25 SCT-BKN...AFT 00Z
CIGS 20-30 VSBYS 3-5F.
All heights above ground level except tops. Williston to Devils Lake portion: until 1600Z, ceilings below 1,000 feet and visiblities below 1 mile in fog. After 1600Z ceilings 1,000 to 2,000 feet and visbilities 3 to 5 miles in fog. Devils Lake to Grand Forks portion: until 1600Z clear with visibilities 3 to 5 miles in fog. Areas with ceilings below 1,000 feet and at or below 1 mile in fog. From 1600Z to 0000Z, cloud bases 2,500 feet scattered occassional broken. After 0000Z, ceilings 2,000 to 3,000 feet with visibilities 3 to 5 miles in fog.
365 TWEB 131304 PDT-BKE-BOI. ALL HGTS MSL XCP CIGS. TIL 18Z
70-90 SCT-BKN 100 BKN TOPS 140...AFT 18Z 100 SCT. BKE-BOI TIL
18Z AREAS CIGS 25-30 RW- MTNS OBSCD...INVOF BOI TIL 20Z LCL
All heights mean sea level except ceilings. Until 1800Z cloud base 7,000 to 9,000 feet scattered occassional broken. Second cloud layer 10,000 broken with cloud tops 14,000 feet. After 1800Z, scattered clouds with 10,000 foot bases. Baker to Boise portion: Until 1800Z areas with ceilings 2,500 to 3,000 feet with light rain showers and mountains obscured. In vicinity of Boise, until 2000Z local areas of ceilings 1,000 to 1,500 feet.
When visibility is not stated it is implied to be greater than 6 miles. Information provided by the Aviation Weather Center, Kansas City, Missouri