The year is off to a record start in tornadic activity, and more is on the way. Another three-day tornado outbreak is currently underway, with day 1 having brought close to 40 tornado reports and over 250 hail and wind reports.
The system is expected to bring more severe weather to the eastern part of the U.S. through Tuesday and Wednesday. At the time of this writing, a squall line has developed in the Mississippi River Delta that is expected to push east overnight, bringing strong winds, rain, lightning, and the risk of embedded tornadoes.
This scenario is more complicated than the setup for the last big outbreak, which had a single powerful system to generate the intense weather. A closed low located over Arkansas is responsible for the day 1 activity. This low formed today from a shortwave kink in an upper trough. This low is expected to be blocked by a strong high pressure system off the Atlantic coast, causing it to move north and eventually northeast to die out over the Great Lakes area. However, a second shortwave kink is expected to enter the mid-South and undergo cyclogenesis on Tuesday afternoon or evening.
Surface low at 36 hours
The combination of the current cyclone’s development, the upper-atmospheric jet that is causing all this shortwave activity (see below), and the next cyclone’s appearance on the scene will result in there being significant sources of uplift.
250 mb jet stream
700 mb upward vertical velocities
The highest values of instability in the event are currently prognosticated by the models to occur around midday tomorrow. The NAM and GFS generally agree on the areas of high CAPE, with each model forecasting at least 3000 J/kg (and it should be noted that models do not do well with CAPE and have a tendency to underforecast. Keep an eye on observations such as soundings).
Surface-based CAPE at 18 hours, NAM
Surface-based CAPE at 18 hours, GFS
For Wednesday’s event, the highest CAPE values are expected to be over Mississippi. The NAM and GFS agree on the maximum values but have the location and orientation of the high CAPE axis different.
Surface-based CAPE at 45 hours, NAM
Surface-based CAPE at 45 hours, GFS
The Energy Helicity Index (EHI) values for 18 hours and 45 hours (midday Tuesday and afternoon Wednesday) indicate the areas that the models forecast are most conducive for tornadic supercell development. Here is what the NAM indicates for the two times:
EHI at 18 hours, NAM
EHI at 45 hours, NAM
I am especially concerned about the middle part of Tennessee on Wednesday if that is accurate. Those EHI values are almost off the scale, and they coincide with an area of at least 3000 J/kg CAPE. That area has a history of tornadoes, and between the strong instability, powerful uplift, and helical pattern to the winds, I think it is quite likely that the Tennessee valley may be in the worst part of this outbreak.
The Storm Prediction Center has already put up a Moderate risk for Tuesday and Wednesday. It is thought highly likely that at least one, if not both, of these days will see that risk upgraded to High.