An old boundary refers to a surface and low level boundary that was set down the previous day that has not completely dissolved and mixed out with the environmental air yet. There can be a variety of reasons why a boundary remains in place after many hours and continuing into the next day. Boundaries that slowly move or stall over several hours can be caused by old fronts (stationary front, dissolving cold front or warm front), an old outflow boundary set up from a complex of thunderstorms the day before that is gradually dissolving away, and any other boundary that remains present due to weak winds not mixing out a boundary that was established the previous day. An old boundary can have subtle moisture, wind and temperature differences on each side of it that can aid in convergence as the day progresses. If the environment has adequate instability and moisture but increased lift is needed, an old boundary can be just enough to provide that extra low level convergence that is needed to be the burst point for convection. Forecasters look for these boundaries and other mesoscale boundaries when doing a meso-analysis. Overlays of surface and low level temperature, dewpoint, wind direction, wind speed, etc. from a network of closely spaced weather stations can be used to find these subtle and non-subtle boundaries that can aid in the formation of thunderstorms. The website location that many forecasters use is the Storm Prediction Center’s Mesoscale Analysis Pages at the following link. A future Haby Hints series in the spring severe weather season will cover these mesoscale analysis updates and the weather information they provide along with example case studies: http://www.spc.noaa.gov/exper/mesoanalysis/ |