As with everything else being extra in 2020, the hurricane season has thrown yet another storm at the Gulf Coast. This time it placed our very own metro Atlanta under a tropical storm warning for early Thursday (29 October 2020) morning. And it certainly packed a punch as it moved through the region. Fig. 1 above shows some of the damage sustained in the metro Atlanta area with the main issues being downed trees and power lines. Although the metro area fared well in comparison, around 1 million Georgians were without power causing many counties to cancel school the following day. Zeta made landfall as a Category 2 storm over New Orleans, LA, strengthening more than the early forecasts expected (again this season). A great look at Zeta’s structure is shown in Fig. 2. The cross sections show the intensity of the v-component winds on the eastern side of the storm. These winds are stronger due to the strong cyclonic circulation and storm motion contribute to rapid wind speeds, which also contribute to the larger storm surge threat being where these winds are occurring. At one point in Boluxi, MS, which was right in this sweet spot of being just east of the eye at landfall, the forecast storm surge was 7’-11’. The cross sections to the far right also show a great look at the stability and sinking air in the eye of the storm, as the potential temperature contours bow down and have slightly less space between them indicating larger stability in the eye.
Fig. 3 shows the current forecast cone for now Tropical Storm Zeta as it moves through our region and its wind field, as well as the wind history, showing that the hurricane force winds sustained all the way into central Alabama. That, along with the narrow width of the forecast cone, indicate how quickly this storm is moving. At the time of that forecast issuance, Zeta was moving NNE at 39 mph. Contributing to this rapid motion are upper level flow and the setup of a high pressure region’s anticyclonic circulation off the East Coast and an extratropical cyclone’s cyclonic circulation just west of Zeta. Like a pitching machine, Hurricane Zeta was steered right between these two “cogs” and sent flying through the region. Fig. 4 shows the upper level jet contributing to Zeta’s path (Fig. 4a), as well as the height anomalies of the high region to the east and the cut off low feature to the west (Fig. 4b). Fig. 4c-d show a comparison of cyclonic vorticity between 500 mb and 700 mb, which shows the increasing vorticity with height of the cold core extratropical cyclone over Texas and Oklahoma (which can indicate upward vertical motions through the QG-Omega equation). These panels also show the symmetrical, compact region of extremely strong cyclonic vorticity associated with the rotation of Zeta (Zeta has a lot of zeta! – Zak).
While the majority of Georgia north of I-85 was under a tropical storm warning, with winds forecast in the 60-70 mph range and flash flood watches, we noticed a lack of hurricane related tornadoes throughout the region. That’s one distinct difference between when the remnants Delta rolled through and this time around with Zeta is the associated tornadoes (or lack thereof). One potential reason for this is the rapid speed that Zeta is moving through the region. Most of what could have potentially formed tornadoes was here and gone before having the chance to set up shop. Another potential reason is the lack of significant CAPE in the region. Fig. 5 shows the MLCAPE in the region as well as the surface to 6 km wind shear. While the wind shear is between 40-50 knots at 13Z 29 October 2020, there is only very modest MLCAPE in the region. The lack of CAPE, and presence of CIN there as well, could be another factor representing the lack of energy available to aid the formation of tornadoes. Overall, the CAPE doesn’t seem to be sufficient and the rapid speed at which Zeta is moving makes it difficult for it to produce many tornadoes.
Now, going back to the strengthening of Zeta to a Category 2 storm before landfall. We must begin to wonder why that rapid strengthening just before landfall has been a common theme this hurricane season (other than just talking it up to being 2020). Keeping in mind that hurricanes require low shear environments to strengthen due to the symmetrical nature of the storm’s structure, the presence of some modest vertical wind shear in Fig. 6a shows a potential dissuading variable to Zeta’s strengthening. Fig. 6b shows that water temperatures in the Gulf of Mexico just before Zeta’s landfall were definitely warm and able to provide warm moist air to invigorate the storm, but nothing that out there concerning the typical warmth of these waters. So why do the models keep underpredicting this season? Zak suggested in our discussion that maybe something along the lines of an anomalously warm loop current in the Gulf has been able to consistently provide warmer waters to give the storms a little bit more juice just before they make landfall, and the models just haven’t been able to capture it well this season. But that will definitely be something interesting and worth looking into in the future to improve our forecast models to provide more accurate information to the public regarding the dangerous conditions heading there way.
