DISCUSSION: The image above (courtesy Unisys Weather and edited by Dr. Ken Leppert) shows sea level pressure (black contours) and 500-hPa heights (color) over North America valid 12Z 4 December 2016. A cut-off low (low 500-hPa heights) can be seen over Northwest Mexico (left of the black arrow), and the arrow shows flow from the tropical East Pacific into the southern U.S. The tropical East Pacific is a rich source of moisture, and this particular weather pattern results in abundant transport of moisture and rain into the southern U.S. that persists for many days (because of the slow moving cut-off low). A very similar pattern occurred in March 2016 that resulted in 22+ inches of rain falling in north Louisiana over 3 days and devastating flooding. Fortunately, only ~3 inches of rain have fallen in north Louisiana so far during this current event. It is interesting that the same weather pattern can result in such different outcomes at different times of year. The reason for this is that temperatures were much warmer in March (~70s F) compared to what they are now (~40s F). Warmer air can "hold" much more water vapor and, therefore, can deliver much more rain than colder air. This is good news for parts of the southern U.S. who are still trying to clean up from flooding earlier this year.
To learn more about other high-impact events over North America, be sure to click here!
~Meteorologist Dr. Ken Leppert