Weather Story

Weekend Weather

New Probability Rules in Aviation Forecasts

Dry conditions will continue through the weekend with warming temperatures, making it a nice weekend to get some outdoor work done in preparation for the winter and holiday season. A cold front will move through the region Monday bringing an increasing in rain chances (20-40%) and cooler temperatures. When the cold front moves through the area will impact how warm areas get on Monday. If the front moves through early in the day temperatures will stay very cool (40s), but if it moves through later in the day temperatures will be able to warm up similar to Sunday (upper 50s to low 60s).

Attention aviation interests! The rules governing how the National Weather Service writes Terminal Aerodrome Forecasts (TAFs) have been updated. A 30 percent chance (PROB30) for a significant weather type (i.e. thunderstorms, rain, snow, etc…) will now be allowed at any time period in the TAF. This changes a long-standing rule that PROB30 could not be used within the first 9 hours of the beginning of the TAF. This change was made in part due to the inconsistent use of vicinity thunderstorms (VCTS) to express uncertainty. It also brings U.S. TAFs more in line with international TAFs, allows forecasters to express uncertainty in the short-term, and allows TAFs to be more consistent with other forecast products issued by the NWS. The use of VCTS is now discouraged to forecast a low probability of thunderstorms in the TAFs. Please direct any questions or comments to nws.stlouis@noaa.gov