On October 5th, 2020, the 24th named storm of the 2020 Atlantic Hurricane season, Tropical Storm Delta, was named. It was named in the Caribbean as it approached the Yucatan Peninsula and the Gulf of Mexico. Within 24 hours of being a tropical storm, Delta rapidly intensified into a Category 4 hurricane with sustained winds of 145 mph. Thankfully the storm weakened into a Category 2 hurricane before making its first landfall near Cancun, Mexico around 6:30 EDT on October 7th. Due to frictional forces from the land below, Delta weakened further into a Category 1 hurricane. However, as it moved into the Gulf of Mexico, it rapidly intensified again into a Category 3 hurricane. Delta was now on track to make landfall almost exactly where Hurricane Laura did a month prior, and this would be a record 4th time in one year that Louisiana was hit by a named storm. On Friday October 9th, Delta made landfall near Creole, Louisiana. To fully understand why Delta intensified so rapidly, the synoptic scale environment, or large-scale atmosphere setting, must be looked at.
Figure 1: This loop shows the 5 day forecast and watches and warnings for Hurricane Delta. (Source Link)
When looking at the synoptic level setting regarding tropical cyclone development, there are 3 major things to consider: Sea surface temperature, moisture content, and upper level wind shear. Tropical cyclones are warm core systems that strengthen over very warm water.
A sea surface temperature of 27°C (80°F) or higher will be what is needed to fuel these cyclones. The sea surface temperature of the Caribbean and the Gulf of Mexico was around 30°C (Figure 2) which is more than enough to strengthen a tropical cyclone. In regard to moisture, tropical cyclones thrive in a moisture rich environment. The relative humidity in the atmosphere over the Gulf while Delta was moving though the region was 90% and higher (Figure 3). When looking at the wind shear environment, the key for tropical cyclone development and strengthening is little to no upper level wind shear. If it was present, it would not allow the storm to properly form. In the case of Delta, there was little shear over the waters it traveled, but some upper level shear was present over the Southeastern United States due to a jet streak just north of Mississippi, Alabama, and Georgia (Figure 4). Due to this synoptic setup, Hurricane Delta was able to rapidly intensify twice in its lifetime.
Figure 2: Global sea surface temperature at 07 Z October 7th. Warmer colors indicate warmer temperatures. (Source Link)
Figure 3: Mid to upper atmospheric moisture content measured at 18 Z October 7th. (Source Link)
Figure 4: Wind speed measured at the 200 mb level on 06 Z October 8th. Allows to see how jet is behaving. (Source Link)
As Hurricane Delta made landfall in Louisiana it was longer over the warm, moist, shear free environment it had been and began to weaken and fall apart. As tropical cyclones fall apart and weaken over the mid-latitudes, they can begin to show extratropical cyclone characteristics. This was exactly the case with Hurricane Delta due to dry air being entrained into the system over land. As Delta went more extratropical, it lost its classic symmetrical tropical cyclone look and became asymmetrical as it exhibited frontal boundaries. Now post-tropical cyclone Delta will be much more interesting to look at on the mesoscale level.
The cold front associated with post-tropical cyclone Delta (Figure 5) caused severe weather for much of metro Atlanta as it moved through Saturday evening into Sunday. Looking at a sounding diagram (Figure 6) from the region during this time, about 848 J/kg of CAPE were observed. This value matched the most-unstable CAPE which means the atmosphere as a whole was very unstable at this time. This high CAPE value along with a strong frontal system that provides a lifting mechanism can brew convective activity for severe weather. That is exactly what happened in the state of Georgia on Saturday/Sunday as the National Weather Service confirrmed 7 tornadoes in the state that day. In addition to this, there were 6 reports of hail and 36 reports of wind damage (Figure 7). Over the span of a week, Delta ranged from a category 4 hurricane bringing life threatening flooding, to a post-tropical system causing severe weather outbreaks in the Southeast.
Figure 5: Surface analysis map from the Weather Prediction Center of Atlanta at 15 Z on October 11th. (Source Link)
Figure 6: This is a sounding diagram of the metro Atlanta area at 18 Z October 11th. (Source Link)
Figure 7: This is a storm report of the state of Georgia from the severe weather over the weekend. (Source Link)