Effective on or about September 18, 2002, guidance for visibility (VIS) and obstruction to vision (OBV) will be added to the AVN MOS alphanumeric and BUFR messages produced from the 0600 and 1800 UTC runs of the NWS Global Forecast System (GFS). Guidance for VIS and OBV will be available in the alphanumeric message for forecast projections valid every 3 hours from 6 to 60 hours after 0600/1800 UTC, as well as 66 and 72 hours after 0600 and 1800 UTC. Categorical forecasts in the message denote predicted conditions of visibility (1 = < 1/4 mi; 2 = > 1/4 mi to 1/2 mi; 3 = > 1/2 mi to < 1 mi; 4 = 1 to < 3 mi; 5 = 3 to 5 mi; 6 = 6 mi; 7 = > 6 mi) and expected non-precipitating obstructions to vision (N = none; HZ = haze, smoke, dust; BR = mist (fog with visibility > 5/8 mi); FG = fog or ground fog (visibility < 5/8 mi); BL = blowing dust, sand, snow) at specific locations. In AWIPS, the alphanumeric messages are available as the MAVxxx product where xxx denotes the station identifier (3 characters). See Technical Procedures Bulletin No. 481 at http://www.nws.noaa.gov/om/tpb/481body.htm for further details.
The BUFR messages contain probabilistic forecasts of the categories of visibility and obstruction to vision as well as the categorical guidance. Guidance is valid for projections every 3 hours from 6 to 78 hours after 0600/1800 UTC.
Finally, note that when this change is made, the MOS equations used to predict visibility and obstruction to vision during the 0000 and 1200 UTC forecast cycles will also be revised; additional model and observational data were used to redevelop the forecast equations.
Effective on or about September 18, 2002, the probability of precipitation occurrence (PoPO) at a specific hour and the probability of precipitation occurrence over a 3-h period (PoPO3) will be added to the AVN MOS BUFR messages produced from all four runs of the NWS Global Forecast System (GFS). Guidance for PoPO and PoPO3 will be available for forecast projections valid every 3 hours from 6 to 78 hours after 0000, 0600, 1200, and 1800 UTC. New Probability of Precipitation Occurrence Forecasts
The BUFR messages are used primarily to support the Interactive Forecast Preparation System (IFPS). The PoPO denotes the likelihood of precipitation in any form occurring at a specific time. The PoPO3 indicates the likelihood of precipitation in any form occurring within a 3-h period ending at the specified hour. In both instances, the definition of the event differs from that used for the probability of precipitation (PoP). For PoPO, the event is defined as occurring if the METAR record observation at the hour reports precipitation. For the PoPO3, the event is defined as occurring if any one of the four METAR record observations for the 3-h period ending at the specified time reports precipitation. For example, the PoPO3 forecast for a period ending at 1200 UTC indicates whether precipitation is expected in any of the record observations taken at 0900, 1000, 1100, or 1200 UTC. Unlike PoP which predicts measurable precipitation over some interval of time, PoPO predicts precipitation occurrence at specific hours.
Please see MOS Guidance in BUFR Format for additional details on the MOS BUFR messages. For a complete summary of the AVN MOS products, including graphical displays of the probabilities, please view the MOS Forecast Products Page