Summary

Description

Atmospheric blocking is commonly referred to as the situation when the normal zonal flow is interrupted by strong and persistent meridional flow. The normal eastward progression of synoptic disturbances is obstructed leading to episodes of prolonged extreme weather conditions. On intraseasonal time scales the persistent weather extremes can last from several days up to a few weeks, often accompanied by significant temperature and precipitation anomalies. Numerous definitions of blocking exist in the literature and all involve a level if subjectivity. We use the blocking index of Tibaldi and Molteni (1990) modified from that of Lejenas and Okland (1983). This product displays the daily observed blocking index for the past 3 months up to and including the present day. Additionally, outlookss of the blocking index and the 500 hPa geopotential height field anomalies are generated form the NCEP Global Forecast System (GFS) model. The outlooks mare displayed for days 1-9 for both the northern and southern Hemispheres. The product is not archived so a historical time series is not available. To produce a time series it is recommended that the methodology of Tibaldi and Molteni (1990) be applied using the reanalysis dataset of choice. Blocking frequency over the 1950-2000 time period and composites as a function of ENSO phase are also provided.