Tropical Storm Erika: Short Term Certainty, Long Term Uncertainty

Good afternoon.

As expected, Tropical Storm Erika formed last night, although it isn’t looking very impressive today with a poor cloud pattern. Winds remain near 45 mph at this time, although it is possible it has weakened somewhat in the last few hours. The conditions at this time are marginal for intensification with moderate wind shear and some dry air in the area, so it should remain a weak to moderate tropical storm for the next couple days. By Thursday or Friday an opportunity may open up for Erika to intensify to hurricane intensity, although if land interaction takes its toll or if shear or dry air increase, dissipation is still possible.

Short Term: Good Consensus

There is good consensus on the short term track (through Friday). Virtually all models and ensembles bring Erika through the Leeward Islands and near or just north of Puerto Rico and Hispaniola en route to the southeastern Bahamas. Depending on land interaction, Erika may strengthen to a hurricane by Thursday or Friday as mentioned previously. However, if Erika tracks along the southern edge of this forecast, it may dissipate by late this week and no longer be a threat for anyone downstream. Tropical storm conditions are likely in the Leeward Islands tomorrow into early Thursday and possible in Puerto Rico and Hispaniola Thursday into Friday. Confidence in the short term: High for track, Moderate for intensity.

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Short term forecast area for potential tropical storm impacts through Friday. Note that the cone does NOT represent the forecast center of circulation. Impacts extend well outside of the core of the storm. (My map)

Long Term: High Uncertainty, So Many Possibilities

By the time we get past Friday, with the center of Erika likely in the SE Bahamas or near the coast of Hispaniola, the models diverge dramatically. At this time it is very difficult to forecast intensity, since anything from dissipation to a major hurricane is possible. However, my best guesses can be made on six possible scenarios. Each of them will be explained in detail below.

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Track possibilities for Erika for Saturday and beyond into next week and the percentage possibilities. There is VERY high uncertainty. (My map)

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Complete forecasts of all models and ensembles over the long term. Map from South Florida Water Management District.

Scenario 1: Out to sea quickly, near Bermuda

This scenario has little model support, but is possible if the trough comes through quickly and the ridge does not rebuild at all (notice the 06Z HWRF tries to lean in that direction). This is most likely with a storm that intensifies fairly quickly and/or tracks on the northern envelope of the model ensembles to begin with. A stronger storm would be more likely to feel the trough sooner, especially if the storm is located closer to the recurving trough to begin with allowing the Bermuda High to temporarily break. Bermuda would need to closely watch Erika early next week as it would be a close call for them with impacts likely.  Likelihood: 10% chance.

Scenario 2: Out to sea after Bahamas along Gulf Stream

This scenario has about half of the models and ensembles agreeing with it. Erika would likely come dangerous close to – or even over – the central and northern Bahamas before the trough comes in and recurves it. Given the ridge nearby providing an outflow channel, Erika may very well be a hurricane in this situation. Fortunately, the strong trough axis would allow a safe recurve off the US East Coast, with high surf the main threat. Likelihood: 20% chance.

Scenario 3: Near or along the East Coast

Unlike in Scenario 2, this scenario involves a weaker and slower trough. The ridge does not rebuild, however, and it does allow Erika to recurve, but it would be too late to avoid land impacts next week. As a result, Erika would likely track very close to – or along – the East Coast (similar to storms like Arthur last year, Floyd in 1999, Bertha in 1996, Bob in 1991 and the 1944 hurricane, among others). The strength would be dictated by land interaction and time over water. If it stays just offshore, it would likely hit Atlantic Canada. The exact impacts would be highly dependent on track and intensity, but the Northeast would likely see at least some impacts no matter what here, with the greatest impacts near the coast and exposed locations most likely. Few models suggest this will happen (as most either rebuild the ridge or bring Erika out to sea sooner), but this is still a very plausible scenario. Likelihood: 15% chance.

Scenario 4: Over Florida, near or into the Eastern Gulf of Mexico

The other half of scenarios that didn’t agree with Scenario 2 largely support this scenario. The trough largely misses Erika, although the storm may be able to move northward for a while before the ridge rebuilds. However, a rebuilding ridge of high pressure off the Carolina coast and inland is modeled by many ensembles and operational models including the ECMWF (a very reliable model) and earlier HWRF runs. In this scenario, Erika then is forced westward from the Bahamas into the east coast of Florida early next week. Depending on the strength of the ridge, the storm would start turning northward after landfall, possibly into the eastern Gulf of Mexico making a second landfall. The intensity would be uncertain, but some models show a very strong hurricane (possibly a major hurricane) approaching. It is certainly not the only possible scenario, but one worth watching. Likelihood: 25% chance.

Scenario 5: Southerly track into the Central Gulf of Mexico

Few ensembles and no operational models support this scenario, but it is not entirely impossible. In this scenario, Erika remains on the southern end of the short-term envelope of possibilities, while avoiding land interaction and remaining intact, although likely as a weak tropical storm through Saturday. The weaker storm misses the trough entirely and the ridge rebuilds on it, allowing Erika to track just north of Cuba and into the central Gulf of Mexico. Anywhere from Texas to Alabama enters play there as Erika rounds the ridge which is able to build farther. The 00Z CMC (not a very reliable model) is the only one showing something similar to this. As Erika would likely be weak initially, it is unclear if it will be able to intensify in the Gulf of Mexico. Likelihood: 5% chance.

Scenario 6: Dissipation

Probably the best case scenario for the continental US, but not for the islands. Several models, including the 12Z GFS, actually dissipate Erika before Saturday. This is a very plausible scenario if Erika tracks along the southern edge of the ensembles over high terrain in Hispaniola and Puerto Rico, and that area also has the greatest amount of shear and dry air which helped to decapitate Danny. Shear is much greater in the NE Caribbean as well, and Erika would feel a lot of it on that track. However, the slow movement and close track would mean heavy rainfall would be likely in the islands, but there would be no circulation to move northward. Likelihood: 25% chance.

Conclusion

If there is one thing clear, we all need to be watching Erika. So many possibilities exist, which will be largely linked to a strong Bermuda High to the north and an incoming trough. In addition, the short term intensity will also play a role in the future track and situation with Erika. It is important to note, though, that any impacts to the continental US are at least 6 to 8 days away, and any impact to the Northeast would be at least 8 to 10 days away. The probabilities of impact look significant but at this time are about equal to a slight chance of precipitation on any given day. There will be lots of time to refine the forecast.

Forecaster Craig Ceecee (@EternalWeather1)

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