A large storm system, commonly known as a mid-latitude cyclone, began its path on Saturday, January 27th from the Gulf to northeastern US, showering many states with rain or snow over the next few days.
Figure 1: A shortwave IR image (at night) mixed with enhanced color visible imagery (during the day) of the storm, taken at 10 AM EST Saturday, January 27th. (RAMMB CIRA)
The potential severity of this low pressure system didn’t bypass the eyes of the NWS, prompting widespread warnings and watches for a large swathe of the East Coast.
Figure 2: NWS’ national severe weather outlook and warnings for Saturday, Jan. 27th. Much of the East Coast and Midwest were under monitoring, following the storm’s path. (X)
It takes quite a few “ingredients” to form these cyclones; a collaboration between upper air, lower air, and temperature dynamics. As for last weekend, a smaller system around the Texas/Oklahoma border rode on the upper air currents – jet streams where air flows much faster – on its southward trajectory, joining with the warm-air storms forming in the Gulf of Mexico. The meeting of these two systems then grew rapidly in size, developing into the cyclone of interest.
Additionally, with the sharp dip in the jet stream, the dynamics of wind-flow direction, also known as vorticity, assisted in forming and maintaining the storm structure. With the U-shape the jet stream forms, the air follows along, wrapping around in a circular motion. With such, the circulation sustains upward airflow, the main proponent of cloud and precipitation formation.
The satellite imagery below helps to exemplify its development, where the warmer colors signify higher, cooler cloud tops – a typical trademark of high-rising storm clouds.
Figure 3: Storm development between 11 AM Saturday, the 26th, to 5 PM Sunday, the 27th (EST), using longwave IR satellite imagery from the GOES-16 satellite. Warmer colors indicate higher-altitude cloud tops that rise through the atmosphere. (College of DuPage)
Though the majority missing Atlanta, the most intense weather gave states spanning Alabama and Florida, to North Carolina inches of rain in a matter of hours, and eventually heavy snow in the northern states as it grew in strength. Additionally, a stray thunderstorm split off from the structure, forming an EF-1 tornado in South Carolina.
With the warmer colors representing increasing precipitation intensity, the radar system demonstrates another facet of the storm. It showcases the sheer scale of the precipitation spanning across many states in an organized “leaf” structure. A line of yellow and green-colored thunderstorms near the center, called the squall line, also happened to form in the system along the cold front, providing an additional chance for severe weather following the main system. With the complexities of these atmospheric interactions, each storm proves to be an interesting one for the many states these cyclones span.
Figure 4: A composite radar image of the storm’s precipitation structure and distribution at 7 PM EST on the 27th. Rain intensity spans green to red, with warmer colors being more intense. Snow is primarily cyan and darker blues, with the deeper colors meaning heavier snowfall. (MRMS)