Last night, the National Hurricane Center upgraded TD-29 to Tropical Storm Eta, declaring it the 28th named storm of the season. Those well versed in meteorological history will understand the significance of that: the 2020 Atlantic season has officially tied 2005 for the most named storms in one season, just over two months early. One of the most famous records in meteorology has fallen. Unfortunately, Eta won’t just be a meteorological curiosity. The storm seems likely to cause potentially significant impacts centered over Nicaragua and Honduras. Before we can start examining these impacts, it’s important to figure out where Eta wants to go. To forecast steering flow for this still rather weak tropical system, it helps to look at the 500mb level. Initially, a zone of east->west flow south of a broad Bahaman ridge will maintain westerly motion for Eta. However, as a powerful East Coast trough noses through the Southeast, a ridge over Texas will spill into the northwest Gulf. This will increase the southerly component of flow over Eta from Monday as it approaches the coast. There is pretty cohesive, tight track guidance spread through ~50 hours as a result, with largely predictable steering influences making the approach to landfall rather forecastable. To see what sort of storm Eta will be as it nears northeast Nicaragua, it’s necessary to look at the conditions over the storm through these first 50 hours. Now, after examining steering, we know where we need to look. The first consideration with a storm like this is sea surface temperatures. Will Eta be moving over water that stores enough energy to be conducive to tropical development? As a rule of thumb, conditions over 26.5°C are favorable for tropical storms to not weaken, while sea temperatures of 28-29+°C are favorable for intensification. This sea surface temperature analysis from the National Hurricane Center shows that much of the western Caribbean is still at least 28°C, which means thermodynamically, conditions will likely be favorable for intensification through landfall. Additionally, with warm water at or greater than 20°C extending deeper than 100m for much of the track, upwelling due to slow forward motion won’t be much of a threat to intensification. Next, let’s look at upper level winds over the storm. These can be a real double edged sword for intensification- flow oriented unfavorably to a system can shred it, while flow oriented favorably can enhance outflow and help the storm deepen.

Determining whether storm-relative flow is favorable or not can be tricky. Here, it looks like interactions will be positive for intensification, as Eta itself will be in a zone of relatively weak flow, with arcing jet streaks to the north and south helping remove air from the column, deepening the storm. Eta is a tropical storm likely to move through conditions thermodynamically and kinematically favorable for development and intensification. So what does this all mean? In all likelihood, Eta will strengthen at at least a steady clip over the next 50 hours, with a fair shot at one or more periods of rapid intensification during that time depending on how well the storm can organize. The NHC has the storm getting to 85mph before landfall, while the HWRF has the storm reaching low-end category 3 intensity. I think the most likely scenario is something between these two intensities, and will depend a lot on watching how the storm structure improves over the next 24 hours. Either way, impactful winds and surge look likely in northeast Nicaragua Tuesday. Around and following landfall, the forecast gets significantly messier. Eta could get stuck in a weak flow regime between westerly flow over South America and easterly flow over the northern Caribbean. This means there’s a good shot for a sloooow down, with models showing slow looping, curving, or simply stalling. Regardless of exact path, it’s looking increasingly likely that Honduras, Nicaragua, and other parts of Central America will see a significant, multi day onslaught of heavy rain as deep moisture flow meets persistent convergence over either a slow moving tropical storm or topographical features. There’s a reasonable chance some parts of the region will see 20-30+” of rain over the next week as the storm meanders, which would cause potentially devastating mudslides and flooding reminiscent of Mitch in 1998, one of the deadliest storms in the history of the basin.  Tropical storm Eta is, by its very existence, a record-setting system. It’s looking increasingly likely that the storm will produce significant wind and surge over northeast Nicaragua before dropping very high-end rainfall over Central America that could prove devastating. Stay tuned- we’ll be sure keep you posted here and on twitter. Of course, for those potentially in the path of Eta, please follow local government information and advice.