Record-Strength Upper Low Brings Extreme Rains to South U.S., Thundersnow to Mexico

This upper low originated from energy that moved across southern California late in the weekend, producing heavy thunderstorms. Rather than barreling across the southeast U.S., the powerful subtropical jet stream carved out a progressively deeper trough into Mexico that cut off from the jet stream, forming a slow-moving closed low...

This upper low featured a large 558-decameter contour (the 558 dm, which is 5580 meters, refers to the height at which the atmospheric pressure is 500 mb, or about half of the typical surface pressure). Such a large, strong upper low appears to be an unprecedented event in modern weather observations for Mexico; upper-air analyses dating back to 1948 from the NCEP/NCAR reanalysis projectsuggest that no previous upper low in this region has been strong enough to generate a 558-dm contour