All colored regions in the hovmoller plot below depict regions where the flow is blocked according to the blocking index (Tibaldi and Molteni 1990). The color scheme denotes the strength of the blocked flow as determined from the 500 hPa geopotential height gradient measured from the blocking ridge equatorward (denoted GHGS in blocking index definition). The presence of an intense cutoff low equatorward of the blocking ridge can act to enhance the blocking strength. (Text after NOAA, <a rel="nofollow" class="external text" href="http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/block.shtml">Blocking</a>),
Illustrates early 2015 blockings; heat waves in India, Pakistan, U.S. West coast and Europe,
NOAA  / National Weather Service / National Centers for Environmental Prediction / Climate Prediction Center Internet Team
NOAA NCEP CPC &gt; Climate &amp; Weather Linkage &gt; Blocking: <a rel="nofollow" class="external text" href="http://www.cpc.ncep.noaa.gov/products/precip/CWlink/blocking/real_time_nh/1panel_ghgs_obs_nh_nrm.gif">1panel_ghgs_obs_nh_nrm.gif</a>